LAND SCULPTURE.
The Basin of the Colorado offers peculiar facilities for the study of
the origin of topographic forms, and its marvelous sculpture has excited
the interest of every observer. It has already made notable
contributions to the principles of earth sculpture[5], and its resources
are far from exhausted. The study of the Henry Mountains has not proved
entirely unfruitful, and for the sake of showing the bearing of its
peculiar features upon the general subject, I shall take the liberty to
restate certain principles of erosion which have been derived or
enforced by the study of the Colorado Plateaus.
Footnote 5:
Geology of the “Colorado Exploring Expedition”, by J. S. Newberry, p.
45.
“Exploration of the Colorado River of the West”, by J. W. Powell, p.
152.
“Geology of the Uinta Mountains”, by J. W. Powell, p. 181.
“Explorations West of the 100th Meridian”, Vol. III, Part I, by G. K.
Gilbert, pp. 67 and 554.
“The Colorado Plateau Region” in American Journal of Science for
August, 1876, by G. K. Gilbert.
A portion of the last paper is repeated, after modification, in the
first section of this chapter.
I.—EROSION.
The sculpture and degradation of the land are performed partly by shore
waves, partly by glaciers, partly by wind; but chiefly by rain and
running water. The last mentioned agencies only will be here discussed.
The erosion which they accomplish will be considered (A) as consisting
of parts, and (B) as modified by conditions.
A. PROCESSES OF EROSION.
All indurated rocks and most earths are bound together by a force of
cohesion which must be overcome before they can be divided and removed.
The natural processes by which the division and removal are accomplished
make up erosion. They are called disintegration and transportation.
Transportation is chiefly performed by running water.
Disintegration is naturally divided into two parts. So much of it as is
accomplished by running water is called corrasion, and that which is
not, is called weathering.
Stated in their natural order, the three general divisions of the
process of erosion are (1) _weathering_, (2) _transportation_, and (3)
_corrasion_. The rocks of the general surface of the land are
disintegrated by _weathering_. The material thus loosened is
_transported_ by streams to the ocean or other receptacle. In transit it
helps to _corrade_ from the channels of the streams other material,
which joins with it to be transported to the same goal.
_Weathering._
In weathering the chief agents of disintegration are solution, change of
temperature, the beating of rain, gravity, and vegetation.
The great solvent of rocks is water, but it receives aid from some other
substances of which it becomes the vehicle. These substances are chiefly
products of the formation and decomposition of vegetable tissues. Some
rocks are disintegrated by their complete solution, but the great
majority are divided into grains by the solution of a portion; and
fragmental rocks usually lose by solution the cement merely, and are
thus reduced to their original incoherent condition.
The most rigid rocks are cracked by sudden changes of temperature; and
the crevices thus begun are opened by the freezing of the water within
them. The coherence of the more porous rocks is impaired and often
destroyed by the same expansive force of freezing water.
The beating of the rain overcomes the feeble coherence of earths, and
assists solution and frost by detaching the particles which they have
partially loosened.
When the base of a cliff is eroded so as to remove or diminish the
support of the upper part, the rock thus deprived of support is broken
off in blocks by gravity. The process of which this is a part is called
cliff erosion or _sapping_.
Plants often pry apart rocks by the growth of their roots, but their
chief aid to erosion is by increasing the solvent power of percolating
water.
In general soft rocks weather more rapidly than hard.
_Transportation._
A portion of the water of rains flows over the surface and is quickly
gathered into streams. A second portion is absorbed by the earth or rock
on which it falls, and after a slow underground circulation reissues in
springs. Both transport the products of weathering, the latter carrying
dissolved minerals and the former chiefly undissolved.
Transportation is also performed by the direct action of gravity. In
sapping, the blocks which are detached by gravity are by the same agency
carried to the base of the cliff.
_Corrasion._
In corrasion the agents of disintegration are solution and mechanical
wear. Wherever the two are combined, the superior efficiency of the
latter is evident; and in all fields of rapid corrasion the part played
by solution is so small that it may be disregarded.
The mechanical wear of streams is performed by the aid of hard mineral
fragments which are carried along by the current. The effective force is
that of the current; the tools are mud, sand, and bowlders. The most
important of them is sand; it is chiefly by the impact and friction of
grains of sand that the rocky beds of streams are disintegrated.
Streams of clear water corrade their beds by solution. Muddy streams act
partly by solution, but chiefly by attrition.
Streams transport the combined products of corrasion and weathering. A
part of the _débris_ is carried in solution, and a part mechanically.
The finest of the undissolved detritus is held in suspension; the
coarsest is rolled along the bottom; and there is a gradation between
the two modes. There is a constant comminution of all the material as it
moves, and the work of transportation is thereby accelerated. Bowlders
and pebbles, while they wear the stream bed by pounding and rubbing, are
worn still more rapidly themselves. Sand grains are worn and broken by
the continued jostling, and their fragments join the suspended mud.
Finally the detritus is all more or less dissolved by the water, the
finest the most rapidly.
In brief, weathering is performed by solution; by change of temperature,
including frost; by rain beating; by gravity; and by vegetation.
Transportation is performed chiefly by running water. Corrasion is
performed by solution, and by mechanical wear.
Corrasion is distinguished from weathering chiefly by including
mechanical wear among its agencies, and the importance of the
distinction will be apparent when we come to consider how greatly and
peculiarly this process is affected by modifying conditions.
B. CONDITIONS CONTROLLING EROSION.
The chief conditions which affect the rapidity of erosion are (1)
declivity, (2) character of rock, and (3) climate.
_Rate of Erosion and Declivity._
In general _erosion is most rapid where the slope is steepest_; but
weathering, transportation, and corrasion are affected in different ways
and in different degrees.
With increase of slope goes increase in the velocity of running water,
and with that goes increase in its power to transport undissolved
detritus.
The ability of a stream to corrade by solution is not notably enhanced
by great velocity; but its ability to corrade by mechanical wear keeps
pace with its ability to transport, or may even increase more rapidly.
For not only does the bottom receive more blows in proportion as the
quantity of transient detritus increases, but the blows acquire greater
force from the accelerated current, and from the greater size of the
moving fragments. It is necessary however to distinguish the ability to
corrade from the rate of corrasion, which will be seen further on to
depend largely on other conditions.
Weathering is not directly influenced by slope, but it is reached
indirectly through transportation. Solution and frost, the chief agents
of rock decay, are both retarded by the excessive accumulation of
disintegrated rock. Frost action ceases altogether at a few feet below
the surface, and solution gradually decreases as the zone of its
activity descends and the circulation on which it depends becomes more
sluggish. Hence the rapid removal of the products of weathering
stimulates its action, and especially that portion of its action which
depends upon frost. If however the power of transportation is so great
as to remove completely the products of weathering, the work of
disintegration is thereby checked; for the soil which weathering tends
to accumulate is a reservoir to catch rain as it reaches the earth and
store it up for the work of solution and frost, instead of letting it
run off at once unused.
Sapping is directly favored by great declivity.
In brief, a steep declivity favors transportation and thereby favors
corrasion. The rapid, but partial, transportation of weathered rock
accelerates weathering; but the complete removal of its products retards
weathering.
_Rate of Erosion and Rock Texture._
Other things being equal, _erosion is most rapid when the eroded rock
offers least resistance_; but the rocks which are most favorable to one
portion of the process of erosion do not necessarily stand in the same
relation to the others. Disintegration by solution depends in large part
on the solubility of the rocks, but it proceeds most rapidly with those
fragmental rocks of which the cement is soluble, and of which the
texture is open. Disintegration by frost is most rapid in rocks which
absorb a large percentage of water and are feebly coherent.
Disintegration by mechanical wear is most rapid in soft rocks.
Transportation is most favored by those rocks which yield by
disintegration the most finely comminuted _débris_.
_Rate of Erosion and Climate._
The influence of climate upon erosion is less easy to formulate. The
direct influences of temperature and rainfall are comparatively simple,
but their indirect influence through vegetation is complex, and is in
part opposed to the direct.
Temperature affects erosion chiefly by its changes. Where the range of
temperature includes the freezing point of water, frost contributes its
powerful aid to weathering; and it is only where changes are great and
sudden that rocks are cracked by their unequal expansion or contraction.
All the processes of erosion are affected directly by the amount of
rainfall, and by its distribution through the year. All are accelerated
by its increase and retarded by its diminution. When it is concentrated
in one part of the year at the expense of the remainder, transportation
and corrasion are accelerated, and weathering is retarded.
Weathering is favored by abundance of moisture. Frost accomplishes most
when the rocks are saturated; and solution when there is the freest
subterranean circulation. But when the annual rainfall is concentrated
into a limited season, a larger share of the water fails to penetrate,
and the gain from temporary flooding does not compensate for the
checking of all solution by a long dry season.
Transportation is favored by increasing water supply as greatly as by
increasing declivity. When the volume of a stream increases, it becomes
at the same time more rapid, and its transporting capacity gains by the
increment to velocity as well as by the increment to volume. Hence the
increase in power of transportation is more than proportional to the
increase of volume.
It is due to this fact chiefly that the transportation of a stream which
is subject to floods is greater than it would be if its total water
supply were evenly distributed in time.
The indirect influence of rainfall and temperature, by means of
vegetation, has different laws. Vegetation is intimately related to
water supply. There is little or none where the annual precipitation is
small, and it is profuse where the latter is great—especially where the
temperature is at the same time high. In proportion as vegetation is
profuse the solvent power of percolating water is increased, and on the
other hand the ground is sheltered from the mechanical action of rains
and rills. The removal of disintegrated rock is greatly impeded by the
conservative power of roots and fallen leaves, and a soil is thus
preserved. Transportation is retarded. Weathering by solution is
accelerated up to a certain point, but in the end it suffers by the
clogging of transportation. The work of frost is nearly stopped as soon
as the depth of soil exceeds the limit of frost action. The force of
rain drops is expended on foliage. Moreover a deep soil acts as a
distributing reservoir for the water of rains, and tends to equalize the
flow of streams.
Hence the general effect of vegetation is to retard erosion; and since
the direct effect of great rainfall is the acceleration of erosion, it
results that its direct and indirect tendencies are in opposite
directions.
In arid regions of which the declivities are sufficient to give thorough
drainage, the absence of vegetation is accompanied by absence of soil.
When a shower falls, nearly all the water runs off from the bare rock,
and the little that is absorbed is rapidly reduced by evaporation.
Solution becomes a slow process for lack of a continuous supply of
water, and frost accomplishes its work only when it closely follows the
infrequent rain. Thus weathering is retarded. Transportation has its
work so concentrated by the quick gathering of showers into floods, as
to compensate, in part at least, for the smallness of the total rainfall
from which they derive their power.
Hence in regions of small rainfall, surface degradation is usually
limited by the slow rate of disintegration; while in regions of great
rainfall it is limited by the rate of transportation. There is probably
an intermediate condition with moderate rainfall, in which a rate of
disintegration greater than that of an arid climate is balanced by a
more rapid transportation than consists with a very moist climate, and
in which the rate of degradation attains its maximum.
Over nearly the whole of the earth’s surface there is a soil, and
wherever this exists we know that the conditions are more favorable to
weathering than to transportation. Hence it is true in general that the
conditions which limit transportation are those which limit the general
degradation of the surface.
To understand the manner in which this limit is reached it is necessary
to look at the process by which the work is accomplished.
_Transportation and Comminution._
A stream of water flowing down its bed expends an amount of energy that
is measured by the quantity of water and the vertical distance through
which it descends. If there were no friction of the water upon its
channel the velocity of the current would continually increase; but if,
as is the usual case, there is no increase of velocity, then the whole
of the energy is consumed in friction. The friction produces
inequalities in the motion of the water, and especially induces
subsidiary currents more or less oblique to the general onward movement.
Some of these subsidiary currents have an upward tendency, and by them
is performed the chief work of transportation. They lift small particles
from the bottom and hold them in suspension while they move forward with
the general current. The finest particles sink most slowly and are
carried farthest before they fall. Larger ones are barely lifted, and
are dropped at once. Still larger are only half lifted; that is, they
are lifted on the side of the current and rolled over without quitting
the bottom. And finally there is a limit to the power of every current,
and the largest fragments of its bed are not moved at all.
There is a definite relation between the velocity of a current and the
size of the largest bowlder it will roll. It has been shown by Hopkins
that the weight of the bowlder is proportioned to the sixth power of the
velocity. It is easily shown also that the weight of a suspended
particle is proportioned to the sixth power of the velocity of the
upward current that will prevent its sinking. But it must not be
inferred that the total load of detritus that a stream will transport
bears any such relation to the rapidity of its current. The true
inference is, that the velocity determines the size-limit of the
detritus that a stream can move by rolling, or can hold in suspension.
Every particle which a stream lifts and sustains is a draft upon its
energy, and the measure of the draft is the weight (weighed in water) of
the particle, multiplied by the distance it would sink in still water in
the time during which it is suspended. If for the sake of simplicity we
suppose the whole load of a stream to be of uniform particles, then the
measure of the energy consumed in their transportation is their total
weight multiplied by the distance one of them would sink in the time
occupied in their transportation. Since fine particles sink more slowly
than coarse, the same consumption of energy will convey a greater load
of fine than of coarse.
Again, the energy of a clear stream is entirely consumed in the friction
of flow; and the friction bears a direct relation to its velocity. But
if detritus be added to the water, then a portion of its energy is
diverted to the transportation of the load; and this is done at the
expense of the friction of flow, and hence at the expense of velocity.
As the energy expended in transportation increases, the velocity
diminishes. If the detritus be composed of uniform particles, then we
may also say that as the load increases the velocity diminishes. But the
diminishing velocity will finally reach a point at which it can barely
transport particles of the given size, and when this point is attained,
the stream has its maximum load of detritus of the given size. But fine
detritus requires less velocity for its transportation than coarse, and
will not so soon reduce the current to the limit of its efficiency. A
greater percentage of the total energy of the stream can hence be
employed by fine detritus than by coarse.
(It should be explained that the friction of flow is in itself a complex
affair. The water in contact with the bottom and walls of the channel
develops friction by flowing past them, and that which is farther away
by flowing past that which is near. The inequality of motion gives rise
to cross currents and there is a friction of these upon each other. The
ratio or coefficient of friction of water against the substance of the
bed, the coefficient of friction of water against water, or the
viscosity of water, and the form of the bed, all conspire to determine
the resistance of flow and together make up what may be called the
coefficient of the friction of flow. The friction depends on its
coefficient and on the velocity.)
Thus the capacity of a stream for transportation is enhanced by
comminution in two ways. Fine detritus, on the one hand, consumes less
energy for the transportation of the same weight, and on the other, it
can utilize a greater portion of the stream’s energy.
It follows, as a corollary, that the velocity of a fully loaded stream
depends (_ceteris paribus_) on the comminution of the material of the
load. When a stream has its maximum load of fine detritus, its velocity
will be less than when carrying its maximum load of coarse detritus; and
the greater load corresponds to the less velocity.
It follows also that a stream which is supplied with heterogeneous
_débris_ will select the finest. If the finest is sufficient in quantity
the current will be so checked by it that the coarser cannot be moved.
If the finest is not sufficient the next grade will be taken, and so on.
_Transportation and Declivity._
To consider now the relation of declivity to transportation we will
assume all other conditions to be constant. Let us suppose that two
streams have the same length, the same quantity of water, flow over beds
of the same character, and are supplied to their full capacities with
detritus of the same kind; but differ in the total amount of fall. Their
declivities or rates of fall are proportional to their falls. Since the
energy of a stream is measured by the product of its volume and its
fall, the relative energies of the two streams are proportional to their
falls, and hence proportional to their declivities. The velocities of
the two streams, depending, as we have seen above, on the character of
the detritus which loads them, are the same; and hence the same amount
of energy is consumed by each in the friction of flow. And since the
energy which each stream expends in transportation is the residual after
deducting what it spends in friction from its total energy, it is
evident that the stream with the greater declivity will not merely have
the greater energy, but will expend a less percentage of it in friction
and a greater percentage in transportation.
Hence declivity favors transportation in a degree that is greater than
its simple ratio.
There are two elements of which no account is taken in the preceding
discussion, but which need to be mentioned to prevent misapprehension,
although they detract in no way from the conclusions.
The first is the addition which the transported detritus makes to the
energy of the stream. A stream of water charged with detritus is at once
a compound and an unstable fluid. It has been treated merely as an
unstable fluid requiring a constant expenditure of energy to maintain
its constitution; but looking at it as a compound fluid, it is plain
that the energy it develops by its descent is greater than the energy
pertaining to the water alone, in the precise ratio of the mass of the
mixture to the mass of the simple water.
The second element is the addition which the detritus makes to the
friction of flow. The coefficient of friction of the compound stream
upon its bottom will always be greater than that of the simple stream of
water, and the coefficient of internal friction or the viscosity will be
greater than that of pure water, and hence for the same velocity a
greater amount of energy will be consumed.
It may be noted in passing, that the energy which is consumed in the
friction of the detritus on the stream bed, accomplishes as part of its
work the mechanical corrasion of the bed.
_Transportation and Quantity of Water._
A stream’s friction of flow depends mainly on the character of the bed,
on the area of the surface of contact, and on the velocity of the
current. When the other elements are constant, the friction varies
approximately with the area of contact. The area of contact depends on
the length and form of the channel, and on the quantity of water. For
streams of the same length and same form of cross-section, but differing
in size of cross-section, the area of contact varies directly as the
square root of the quantity of water. Hence, _ceteris paribus_, the
friction of a stream on its bed is proportioned to the square root of
the quantity of water. But as stated above, the total energy of a stream
is proportioned directly to the quantity of water; and the total energy
is equal to the energy spent in friction, plus the energy spent in
transportation. Whence it follows that if a stream change its quantity
of water without changing its velocity or other accidents, the total
energy will change at the same rate as the quantity of water; the energy
spent in friction will change at a less rate, and the energy remaining
for transportation will change at a greater rate.
Hence increase in quantity of water favors transportation in a degree
that is greater than its simple ratio.
It follows as a corollary that the running water which carries the
_débris_ of a district loses power by subdivision toward its sources;
and that, unless there is a compensating increment of declivity, the
tributaries of a river will fail to supply it with the full load it is
able to carry.
It is noteworthy also that the obstruction which vegetation opposes to
transportation is especially effective in that it is applied at the
infinitesimal sources of streams, where the force of the running water
is least.
A stream which can transport _débris_ of a given size, may be said to be
_competent_ to such _débris_. Since the maximum particles which streams
are able to move are proportioned to the sixth powers of their
velocities, competence depends on velocity. Velocity, in turn, depends
on declivity and volume, and (inversely) on load.
In brief, the capacity of a stream for transportation is greater for
fine _débris_ than for coarse.
Its capacity for the transportation of a given kind of _débris_ is
enlarged in more than simple ratio by increase of declivity; and it is
enlarged in more than simple ratio by increase of volume.
The competence of a stream for the transport of _débris_ of a given
fineness, is limited by a corresponding velocity.
The _rate_ of transportation of _débris_ of a given fineness may equal
the capacity of the transporting stream, or it may be less. When it is
less, it is always from the insufficiency of supply. The supply
furnished by weathering is never available unless the degree of fineness
of the _débris_ brings it within the competence of the stream at the
point of supply.
The chief point of supply is at the very head of the flowing water. The
rain which falls on material that has been disintegrated by weathering,
begins after it has saturated the immediate surface to flow off. But it
forms a very thin sheet; its friction is great; its velocity is small;
and it is competent to pick up only particles of exceeding fineness. If
the material is heterogeneous, it discriminates and leaves the coarser
particles. As the sheet moves on it becomes deeper and soon begins to
gather itself into rills. As the deepening and concentration of water
progresses, either its _capacity_ increases and the load of fine
particles is augmented, or, if fine particles are not in sufficient
force, its _competence_ increases, and larger ones are lifted. In either
case the load is augmented, and as rill joins rill it steadily grows,
until the accumulated water finally passes beyond the zone of
disintegrated material.
The particles which the feeble initial currents are not competent to
move, have to wait either until they are subdivided by the agencies of
weathering, or until the deepening of the channels of the rills so far
increases the declivities that the currents acquire the requisite
velocity, or until some fiercer storm floods the ground with a deeper
sheet of water.
Thus rate of transportation, as well as capacity for transportation, is
favored by fineness of _débris_, by declivity, and by quantity of water.
It is opposed chiefly by vegetation, which holds together that which is
loosened by weathering, and shields it from the agent of transportation
in the very place where that agent is weakest.
When the current of a stream gradually diminishes in its course—as for
example in approaching the ocean—the capacity for transportation also
diminishes; and so soon as the capacity becomes less than the load,
precipitation begins—the coarser particles being deposited first.
_Corrasion and Transportation._
Where a stream has all the load of a given degree of comminution which
it is capable of carrying, the entire energy of the descending water and
load is consumed in the translation of the water and load and there is
none applied to corrasion. If it has an excess of load its velocity is
thereby diminished so as to lessen its competence and a portion is
dropped. If it has less than a full load it is in condition to receive
more and it corrades its bottom.
A fully loaded stream is on the verge between corrasion and deposition.
As will be explained in another place, it may wear the walls of its
channel, but its wear of one wall will be accompanied by an addition to
the opposite wall.
The work of transportation may thus monopolize a stream to the exclusion
of corrasion, or the two works may be carried forward at the same time.
_Corrasion and Declivity._
The rapidity of mechanical corrasion depends on the hardness, size, and
number of the transient fragments, on the hardness of the rock-bed, and
on the velocity of the stream. The blows which the moving fragments deal
upon the stream bed are hard in proportion as the fragments are large
and the current is swift. They are most effective when the fragments are
hard and the bed-rock is soft. They are more numerous and harder upon
the bottom of the channel than upon the sides because of the constant
tendency of the particles to sink in water. Their number is increased up
to a certain limit by the increase of the load of the stream; but when
the fragments become greatly crowded at the bottom of a stream their
force is partially spent among themselves, and the bed-rock is in the
same degree protected. For this reason, and because increase of load
causes retardation of current, it is probable that the maximum work of
corrasion is performed when the load is far within the transporting
capacity.
The element of velocity is of double importance since it determines not
only the speed, but to a great extent the size of the pestles which
grind the rocks. The coefficients upon which it in turn depends, namely,
declivity and quantity of water, have the same importance in corrasion
that they have in transportation.
Let us suppose that a stream endowed with a constant volume of water, is
at some point continuously supplied with as great a load as it is
capable of carrying. For so great a distance as its velocity remains the
same, it will neither corrade (downward) nor deposit, but will leave the
grade of its bed unchanged. But if in its progress it reaches a place
where a less declivity of bed gives a diminished velocity, its capacity
for transportation will become less than the load and part of the load
will be deposited. Or if in its progress it reaches a place where a
greater declivity of bed gives an increased velocity, the capacity for
transportation will become greater than the load and there will be
corrasion of the bed. In this way a stream which has a supply of
_débris_ equal to its capacity, tends to build up the gentler slopes of
its bed and cut away the steeper. It tends to establish a single,
uniform grade.
Let us now suppose that the stream after having obliterated all the
inequalities of the grade of its bed loses nearly the whole of its load.
Its velocity is at once accelerated and vertical corrasion begins
through its whole length. Since the stream has the same declivity and
consequently the same velocity at all points, its capacity for corrasion
is everywhere the same. Its rate of corrasion however will depend on the
character of its bed. Where the rock is hard corrasion will be less
rapid than where it is soft, and there will result inequalities of
grade. But so soon as there is inequality of grade there is inequality
of velocity, and inequality of capacity for corrasion; and where hard
rocks have produced declivities, there the capacity for corrasion will
be increased. The differentiation will proceed until the capacity for
corrasion is everywhere proportioned to the resistance, and no
further,—that is, until there is an equilibrium of action.
In general, we may say that a stream tends to equalize its work in all
parts of its course. Its power inheres in its fall, and each foot of
fall has the same power. When its work is to corrade and the resistance
is unequal, it concentrates its energy where the resistance is great by
crowding many feet of descent into a small space, and diffuses it where
the resistance is small by using but a small fall in a long distance.
When its work is to transport, the resistance is constant and the fall
is evenly distributed by a uniform grade. When its work includes both
transportation and corrasion, as in the usual case, its grades are
somewhat unequal; and the inequality is greatest when the load is least.
It is to be remarked that in the case of most streams it is the flood
stage which determines the grades of the channel. The load of detritus
is usually greatest during the highest floods, and power is conferred so
rapidly with increase of quantity of water, that in any event the
influence of the stream during its high stage will overpower any
influence which may have been exerted at a low stage. That relation of
transportation to corrasion which subsists when the water is high will
determine the grades of the waterway.
_Declivity and Quantity of Water._
The conclusions reached in regard to the relations of corrasion and
declivity depend on the assumption that the volume of the stream is the
same throughout its whole course, and they consequently apply directly
to such portions only of streams as are not increased by tributaries. A
simple modification will include the more general case of branching
streams.
Let us suppose that two equal streams which join, have the same
declivity, and are both fully loaded with detritus of the same kind. If
the channel down which they flow after union has also the same
declivity, then the joint stream will have a greater velocity than its
branches, its capacity for transportation will be more than adequate for
the joint load, and it will corrade its bottom. By its corrasion it will
diminish the declivity of its bed, and consequently its velocity and
capacity for transportation, until its capacity is equal to the total
capacity of its tributaries. When an equilibrium of action is reached,
the declivity of the main stream will be less than the declivities of
its branches. This result does not depend on the assumed equality of the
branches, nor upon their number. It is equally true that in any river
system which is fully supplied with material for transportation and
which has attained a condition of equal action, the declivity of the
smaller streams is greater than that of the larger.
Let us further suppose that two equal streams which join, are only
partially loaded, and are corrading at a common rate a common rock. If
the channel down which they flow after union is in the same rock and has
the same declivity, then the joint river will have a greater velocity,
and will corrade more rapidly than its branches. By its more rapid
corrasion it will diminish the declivity of its bed, until as before
there is an equilibrium of action,—the branch having a greater declivity
than the main. This result also is independent of the number and
equality of the branches; and it is equally true that in any river
system which traverses and corrades rock of equal resistance throughout,
and which has reached a condition of equal action, the declivity of the
smaller streams is greater than that of the larger.
In general we may say that, _ceteris paribus, declivity hears an inverse
relation to quantity of water_.
(There is an apparent exception to this law, which is specially
noteworthy in the sculpture of bad-lands, and will be described in
another place).
II. SCULPTURE.
Erosion may be regarded from several points of view. It lays bare rocks
which were before covered and concealed, and is thence called
_denudation_. It reduces the surfaces of mountains, plateaus, and
continents, and is thence called _degradation_. It carves new forms of
land from those which before existed, and is thence called _land
sculpture_. In the following pages it will be considered as land
sculpture, and attention will be called to certain principles of erosion
which are concerned in the production of topographic forms.
_Sculpture and Declivity._
We have already seen that erosion is favored by declivity. Where the
declivity is great the agents of erosion are powerful; where it is small
they are weak; where there is no declivity they are powerless. Moreover
it has been shown that their power increases with the declivity in more
than simple ratio.
It is evident that if steep slopes are worn more rapidly than gentle,
the tendency is to abolish all differences of slope and produce
uniformity. The law of uniform slope thus opposes diversity of
topography, and if not complemented by other laws, would reduce all
drainage basins to plains. But in reality it is never free to work out
its full results; for it demands a uniformity of conditions which
nowhere exists. Only a water sheet of uniform depth, flowing over a
surface of homogeneous material, would suffice; and every inequality of
water depth or of rock texture produces a corresponding inequality of
slope and diversity of form. The reliefs of the landscape exemplify
other laws, and the law of uniform slopes is merely the conservative
element which limits their results.
_Sculpture and Structure; the Law of Structure._
We have already seen that erosion is influenced by rock character.
Certain rocks, of which the hard are most conspicuous, oppose a stubborn
resistance to erosive agencies; certain others, of which the soft are
most conspicuous, oppose a feeble resistance. Erosion is most rapid
where the resistance is least, and hence as the soft rocks are worn away
the hard are left prominent. The differentiation continues until an
equilibrium is reached through the law of declivities. When the ratio of
erosive action as dependent on declivities becomes equal to the ratio of
resistances as dependent on rock character, there is equality of action.
In the structure of the earth’s crust hard and soft rocks are grouped
with infinite diversity of arrangement. They are in masses of all forms,
and dimensions, and positions; and from these forms are carved an
infinite variety of topographic reliefs.
In so far as the law of structure controls sculpture, hard masses stand
as eminences and soft are carved in valleys.
_The Law of Divides._
We have seen that the declivity over which water flows bears an inverse
relation to the quantity of water. If we follow a stream from its mouth
upward and pass successively the mouths of its tributaries, we find its
volume gradually less and less and its grade steeper and steeper, until
finally at its head we reach the steepest grade of all. If we draw the
profile of the river on paper, we produce a curve concave upward and
with the greatest curvature at the upper end. The same law applies to
every tributary and even to the slopes over which the freshly fallen
rain flows in a sheet before it is gathered into rills. The nearer the
watershed or divide the steeper the slope; the farther away the less the
slope.
It is in accordance with this law that mountains are steepest at their
crests. The profile of a mountain if taken along drainage lines is
concave outward as represented in the diagram; and this is purely a
matter of sculpture, the uplifts from which mountains are carved rarely
if ever assuming this form.
[Illustration:
FIG. 54.—Typical profile of the Drainage Slopes of Mountains.
]
Under the _law of Structure_ and the _law of Divides_ combined, the
features of the earth are carved. Declivities are steep in proportion as
their material is hard; and they are steep in proportion as they are
near divides. The distribution of hard and soft rocks, or the geological
structure, and the distribution of drainage lines and watersheds, are
coefficient conditions on which depends the sculpture of the land. In
the sequel it will be shown that the distribution of drainage lines and
watersheds depends in part on that of hard and soft rocks.
In some places the first of the two conditions is the more important, in
others the second. In the bed of a stream without tributaries the grade
depends on the structure of the underlying rocks. In rock which is
homogeneous and structureless all slopes depend on the distribution of
divides and drainage lines.
The relative importance of the two conditions is especially affected by
climate, and the influence of this factor is so great that it may claim
rank as a third condition of sculpture.
_Sculpture and Climate._
The Henry Mountains consist topographically of five individuals,
separated by low passes, and practically independent in climate. At the
same time they are all of one type of structure, being constituted by
similar aggregation of hard and soft rocks. Their altitudes appear in
the following table.
Altitude above the sea.
Mount Ellen 11,250 feet.
Mount Pennell 11,150 feet.
Mount Hillers 10,500 feet.
Mount Ellsworth 8,000 feet.
Mount Holmes 7,775 feet.
The plain on which they stand has a mean altitude of 5,500 feet, and is
a desert. A large proportion of the rain which falls in the region is
caught by the mountains, and especially by the higher mountains. Of this
there is abundant proof in the distribution of vegetation and of
springs.
The vegetation of the plain is exceedingly meager, comprising only
sparsely set grasses and shrubs, and in favored spots the dwarf cedar of
the West (_Juniperus occidentalis_).
Mount Ellen, which has a continuous ridge two miles long and more than
11,000 feet high, bears cedar about its base, mingled higher up with
piñon (_Pinus edulis_), and succeeded above by the yellow pine (_P.
ponderosa_), spruce (_Abies Douglasii_), fir (_A. Engelmanni_), and
aspen (_Populus tremuloides_). The pines are scattering, but the cedars
are close set, and the firs are in dense groves. The upper slopes where
not timbered are matted with luxuriant grasses and herbs. The summits
are naked.
Mount Pennell sends a single peak only to the height of the Ellen ridge.
Its vegetation is nearly the same, but the timber extends almost to the
summit.
Mount Hillers is 650 feet lower. Its timber reaches to the principal
summit, but is less dense than on the higher mountains. The range of
trees is the same.
Mount Ellsworth, 2,500 feet lower than Mount Hillers, bears neither fir,
spruce, pine nor aspen. Cedar and piñon climb to the summit, but are not
so thickly set as on the lower slopes of the larger mountains. The
grasses are less rank and grow in scattered bunches.
Mount Holmes, a few feet lower, has the same flora, with the addition of
a score of spruce trees, high up on the northern flank. Its summits are
bare.
In a word, the luxuriance of vegetation, and the annual rainfall, of
which it is the index, are proportioned to the altitude.
Consider now the forms of the mountain tops.
In Figure 55 are pictured the summit forms of Mount Ellen. The crests
are rounded; the slopes are uniform and smooth. Examination has shown
that the constituent rocks are of varying degrees of hardness, trachyte
dikes alternating with sandstones and shales; but these variations
rarely find expression in the sculptured forms.
In Figure 56 are the summit crags of Mount Holmes. They are dikes of
trachyte denuded by a discriminating erosion of their encasements of
sandstone, and carved in bold relief. In virtue of their superior
hardness they survive the general degradation.
[Illustration:
FIG. 55.—The Crest of Mount Ellen, as seen from Ellen Peak.
]
[Illustration:
FIG. 56.—The Crest of Mount Holmes.
]
The other mountains are intermediate in the character of their
sculpture. Mount Pennell is nearly as smooth as Mount Ellen. Mount
Ellsworth is nearly as rugged as Mount Holmes. One may ride to the crest
of Mount Ellen and to the summit of Mount Pennell; he may lead his
sure-footed cayuse to the top of Mount Hillers; but Mounts Ellsworth and
Holmes are not to be scaled by horses. The mountaineer must climb to
reach their summits, and for part of the way use hands as well as feet.
In a word, the ruggedness of the summits or the differentiation of hard
and soft by sculpture, is proportioned inversely to the altitude. And
rainfall, which in these mountains depends directly on altitude, is
proportioned inversely to ruggedness.
The explanation of this coincidence depends on the general relations of
vegetation to erosion.
We have seen that vegetation favors the disintegration of rocks and
retards the transportation of the disintegrated material. Where
vegetation is profuse there is always an excess of material awaiting
transportation, and the limit to the rate of erosion comes to be merely
the limit to the rate of transportation. And since the diversities of
rock texture, such as hardness and softness, affect only the rate of
disintegration (weathering and corrasion) and not the rate of
transportation, these diversities do not affect the rate of erosion in
regions of profuse vegetation, and do not produce corresponding
diversities of form.
On the other hand, where vegetation is scant or absent, transportation
and corrasion are favored, while weathering is retarded. There is no
accumulation of disintegrated material. The rate of erosion is limited
by the rate of weathering, and that varies with the diversity of rock
texture. The soft are eaten away faster than the hard; and the structure
is embodied in the topographic forms.
Thus a moist climate by stimulating vegetation produces a sculpture
independent of diversities of rock texture, and a dry climate by
repressing vegetation produces a sculpture dependent on those
diversities. With great moisture the law of divides is supreme; with
aridity, the law of structure.
Hence it is that the upper slopes of the loftier of the Henry Mountains
are so carved as to conceal the structure, while the lower slopes of the
same mountains and the entire forms of the less lofty mountains are so
carved as to reveal the structure; and hence too it is that the arid
plateaus of the Colorado Basin abound in cliffs and cañons, and offer
facilities to the student of geological structure which no humid region
can afford.
Here too is the answer to the question so often asked, “whether the
rains and rivers which excavated the cañons and carved the cliffs were
not mightier than the rains and rivers of to-day.” Aridity being an
essential condition of this peculiar type of sculpture, we may be sure
that through long ages it has characterized the climate of the Colorado
Basin. A climate of great rainfall, as Professor Powell has already
pointed out in his “Exploration of the Colorado,” would have produced
curves and gentle slopes in place of the actual angles and cliffs.
_Bad-lands._
Mountain forms in general depend more on the law of divides than on the
law of structure, but their independence of structure is rarely perfect,
and it is difficult to discriminate the results of the two principles.
For the investigation of the workings of the law of divides it is better
to select examples from regions which afford no variety of rock texture
and are hence unaffected in their erosion by the law of structure. Such
examples are found in _bad-lands_.
Where a homogeneous, soft rock is subjected to rapid degradation in an
arid climate, its surface becomes absolutely bare of vegetation and is
carved into forms of great regularity and beauty. In the neighborhood of
the Henry Mountains, the Blue Gate and Tununk shales are of this
character, and their exposures afford many opportunities for the study
of the principles of sculpture. I was able to devote no time to them,
but in riding across them my attention was attracted by some of the more
striking features, and these I will venture to present, although I am
conscious that they form but a small part of the whole material which
the bad-lands may be made to yield.
If we examine a bad-land ridge, separating two drainage lines and
forming a divide between them, we find an arrangement of secondary
ridges and secondary drainage lines, similar to that represented in the
diagram, (Figure 58.)
[Illustration:
FIG. 57.—General view of the Plateaus lying East of the Henry
Mountains.
]
The general course of the main ridge being straight, its course in
detail is found to bear a simple relation to the secondary ridges.
Wherever a secondary joins, the main ridge turns, its angle being
directly toward the secondary. The divide thus follows a zigzag course,
being deflected to the right or left by each lateral spur.
The altitude of the main ridge is correspondingly related to the
secondary ridges. At every point of union there is a maximum, and in the
intervals are saddles. The maxima are not all equal, but bear some
relation to the magnitudes of the corresponding secondary ridges, and
are especially accented where two or more secondaries join at the same
point. (See profile in Figure 59.)
I conceive that the explanation of these phenomena is as follows: The
heads of the secondary drainage lines laid down in the diagram are in
nature tolerably definite points. The water which during rain converges
at one of these points is there abruptly concentrated in volume. Above
the point it is a sheet, or at least is divided into many rills. Below
it, it is a single stream with greatly increased power of transportation
and corrasion. The principle of equal action gives to the concentrated
stream a less declivity than to the diffused sheet, and—what is
especially important—it tends to produce an equal grade in all
directions upward from the point of convergence. The converging surface
becomes hopper-shaped or funnel-shaped; and as the point of convergence
is lowered by corrasion, the walls of the funnel are eaten back equally
in all directions—except of course the direction of the stream. The
influence of the stream in stimulating erosion above its head is thus
extended radially and equally through an arc of 180°, of which the
center is at the point of convergence.
Where two streams head near each other, the influence of each tends to
pare away the divide between them, and by paring to carry it farther
back. The position of the divide is determined by the two influences
combined and represents the line of equilibrium between them. The
influences being radial from the points of convergence, the line of
equilibrium is tangential, and is consequently at right angles to a line
connecting the two points. Thus, for example, if _a_, _b_, and _c_
(Figure 58) are the points of convergence at the heads of three drainage
lines, the divide line _ed_ is at right angles to a line connecting _a_
and _b_, and the divides _fd_ and _gd_ are similarly determined. The
point _d_ is simultaneously determined by the intersection of the three
divide lines.
[Illustration:
FIG. 58.—Ground plan of a Bad-land Ridge, showing its relation to
Waterways. The smooth lines represent Divides.
FIG. 59.—Profile of the same ridge.
]
Furthermore, since that point of the line _ed_ which lies directly
between _a_ and _b_ is nearest to those points, it is the point of the
divide most subject to the erosive influences which radiate from _a_ and
_b_, and it is consequently degraded lower than the contiguous portions
of the divide. The points _d_ and _e_ are less reduced; and _d_, which
can be shown by similar reasoning to stand higher than the adjacent
portion of either of the three ridges which there unite, is a local
maximum.
There is one other peculiarity of bad-land forms which is of great
significance, but which I shall nevertheless not undertake to explain.
According to the law of divides, as stated in a previous paragraph, the
profile of any slope in bad-lands should be concave upward, and the
slope should be steepest at the divide. The union or intersection of two
slopes on a divide should produce an angle. But in point of fact the
slopes do not unite in an angle. They unite in a curve, and the profile
of a drainage slope instead of being concave all the way to its summit,
changes its curvature and becomes convex. Figure 60 represents a profile
from _a_ to _b_ of Figure 58. From _a_ to _m_ and from _b_ to _n_ the
slopes are concave, but from _m_ to _n_ there is a convex curvature.
Where the flanking slopes are as steep as represented in the diagram,
the convexity on the crest of a ridge has a breadth of only two or three
yards, but where the flanking slopes are gentle, its breadth is several
times as great. It is never absent.
Thus in the sculpture of the bad-lands there is revealed an exception to
the law of divides,—an exception which cannot be referred to accidents
of structure, and which is as persistent in its recurrence as are the
features which conform to the law,—an exception which in some
unexplained way is part of the law. Our analysis of the agencies and
conditions of erosion, on the one hand, has led to the conclusion that
(where structure does not prevent) the declivities of a continuous
drainage slope increase as the quantities of water flowing over them
decrease; and that they are great in proportion as they are near
divides. Our observation, on the other hand, shows that the declivities
increase as the quantities of water diminish, up to a certain point
where the quantity is very small, and then decrease; and that
declivities are great in proportion as they are near divides, unless
they are _very_ near divides. Evidently some factor has been overlooked
in the analysis,—a factor which in the main is less important than the
flow of water, but which asserts its existence at those points where the
flow of water is exceedingly small, and is there supreme.
[Illustration:
FIG. 60.—Cross-profile of a Bad-land Divide.
]
_Equal Action and Interdependence._
The tendency to equality of action, or to the establishment of a dynamic
equilibrium, has already been pointed out in the discussion of the
principles of erosion and of sculpture, but one of its most important
results has not been noticed.
Of the main conditions which determine the rate of erosion, namely,
quantity of running water, vegetation, texture of rock, and declivity,
only the last is reciprocally determined by rate of erosion. Declivity
originates in upheaval, or in the displacements of the earth’s crust by
which mountains and continents are formed; but it receives its
distribution in detail in accordance with the laws of erosion. Wherever
by reason of change in any of the conditions the erosive agents come to
have locally exceptional power, that power is steadily diminished by the
reaction of rate of erosion upon declivity. Every slope is a member of a
series, receiving the water and the waste of the slope above it, and
discharging its own water and waste upon the slope below. If one member
of the series is eroded with exceptional rapidity, two things
immediately result: first, the member above has its level of discharge
lowered, and its rate of erosion is thereby increased; and second, the
member below, being clogged by an exceptional load of detritus, has its
rate of erosion diminished. The acceleration above and the retardation
below, diminish the declivity of the member in which the disturbance
originated; and as the declivity is reduced the rate of erosion is
likewise reduced.
But the effect does not stop here. The disturbance which has been
transferred from one member of the series to the two which adjoin it, is
by them transmitted to others, and does not cease until it has reached
the confines of the drainage basin. For in each basin all lines of
drainage unite in a main line, and a disturbance upon any line is
communicated through it to the main line and thence to every tributary.
And as any member of the system may influence all the others, so each
member is influenced by every other. There is an interdependence
throughout the system.
III.—SYSTEMS OF DRAINAGE.
To know well the drainage of a region two systems of lines must be
ascertained—the drainage lines and the divides. The maxima of surface on
which waters part, and the minima of surface in which waters join, are
alike intimately associated with the sculpture of the earth and with the
history of the earth’s structure; and the student of either sculpture or
history can well afford to study them. In the following pages certain
conditions which affect their permanence and transformations are
discussed.
THE STABILITY OF DRAINAGE LINES.
In corrasion the chief work is performed by the impact and friction of
hard and heavy particles moved forward by running water. They are driven
against all sides of the channel, but their tendency to sink in water
brings them against the bottom with greater frequency and force than
against the walls. If the rate of wear be rapid, by far the greater part
of it is applied to the bottom, and the downward corrasion is so much
more powerful than the lateral that the effect of the latter is
practically lost, and the channel of the stream, without varying the
position of its banks, carves its way vertically into the rock beneath.
It is only when corrasion is exceedingly slow that the lateral wear
becomes of importance; and hence as a rule the position of a stream bed
is permanent.
The stability of drainage lines is especially illustrated in regions of
displacement. If a mountain is slowly lifted athwart the course of a
stream, the corrasion of the latter is accelerated by the increase of
declivity, and instead of being turned aside by the uplift, it
persistently holds its place and carves a channel into the mountain as
the mountain rises. For example the deep clefts which intersect the
Wasatch range owe their existence to the fact that at the time of the
beginning of the uplift which has made the range, there were streams
flowing across the line of its trend which were too powerful to be
turned back by the growing ridge. The same relation has been shown by
Professor Powell where the Green River crosses the uplift of the Uinta
Mountains, and in many instances throughout the Rocky Mountain region it
may be said that rivers have cut their way through mountains merely
because they had established their courses before the inception of the
displacement, and could not be diverted by an obstruction which was
thrown up with the slowness of mountain uplift.
THE INSTABILITY OF DRAINAGE LINES.
The stability of waterways being the rule, every case of instability
requires an explanation; and in the study of such exceptional cases
there have been found a number of different methods by which the courses
of streams are shifted. The more important will be noted.
_Ponding._
When a mountain uplift crosses the course of a stream, it often happens
that the rate of uplift is too rapid to be equaled by the corrasion of
the stream, and the uprising rock becomes a dam over which the water
still runs, but above which there is accumulated a pond or lake.
Whenever this takes place, the pond catches all the _débris_ of the
upper course of the stream, and the water which overflows at the outlet
having been relieved of its load is almost powerless for corrasion, and
cannot continue its contest with the uplift unless the pond is silted up
with detritus. As the uplift progresses the level of the pond is raised
higher and higher, until finally it finds a new outlet at some other
point. The original outlet is at once abandoned, and the new one becomes
a permanent part of the course of the stream. As a rule it is only large
streams which hold their courses while mountains rise; the smaller are
turned back by ponding, and are usually diverted so as to join the
larger.
The disturbances which divert drainage lines are not always of the sort
which produce mountains. The same results may follow the most gentle
undulations of plains. It required a movement of a few feet only to
change the outlet of Lakes Michigan, Huron, and Superior from the
Illinois River to the St. Clair; and in the tilting which turned Lake
Winipeg from the Mississippi to the Nelson no abrupt slopes were
produced. If the entire history of the latter case were worked out, it
would probably appear that the Saskatchewan River which rises in the
Rocky Mountains beyond our northern boundary, was formerly the upper
course of the Mississippi, and that when, by the rising of land in
Minnesota or its sinking at the north, a barrier was formed, the water
was ponded and Lake Winipeg came into existence. By the continuance of
the movement of the land the lake was increased until it overflowed into
Hudson’s Bay; and by its further continuance, combined with the
corrasion of the outlet, the lake has been again diminished. When
eventually the lake disappears the revolution will be complete, and the
Saskatchewan will flow directly to Hudson’s Bay, as it once flowed
directly to the Gulf of Mexico. (See the “Physical Features of the
Valley of the Minnesota River,” by General G. K. Warren.)
_Planation._
It has been shown in the discussion of the relations of transportation
and corrasion that downward wear ceases when the load equals the
capacity for transportation. Whenever the load reduces the downward
corrasion to little or nothing, lateral corrasion becomes relatively and
actually of importance. The first result of the wearing of the walls of
a stream’s channel is the formation of a flood-plain. As an effect of
momentum the current is always swiftest along the outside of a curve of
the channel, and it is there that the wearing is performed; while at the
inner side of the curve the current is so slow that part of the load is
deposited. In this way the width of the channel remains the same while
its position is shifted, and every part of the valley which it has
crossed in its shiftings comes to be covered by a deposit which does not
rise above the highest level of the water. The surface of this deposit
is hence appropriately called the _flood-plain_ of the stream. The
deposit is of nearly uniform depth, descending no lower than the bottom
of the water-channel, and it rests upon a tolerably even surface of the
rock or other material which is corraded by the stream. The process of
carving away the rock so as to produce an even surface, and at the same
time covering it with an alluvial deposit, is the process of
_planation_.
It sometimes happens that two adjacent streams by extending their areas
of planation eat through the dividing ridge and join their channels. The
stream which has the higher surface at the point of contact, quickly
abandons the lower part of its channel and becomes a branch of the
other, having shifted its course by planation.
The slopes of the Henry Mountains illustrate the process in a peculiarly
striking manner. The streams which flow down them are limited in their
rate of degradation at both ends. At their sources, erosion is opposed
by the hardness of the rocks; the trachytes and metamorphics of the
mountain tops are carved very slowly. At their mouths, they discharge
into the Colorado and the Dirty Devil, and cannot sink their channels
more rapidly than do those rivers. Between the mountains and the rivers,
they cross rocks which are soft in comparison with the trachyte, but
they can deepen their channels with no greater rapidity than at their
ends. The grades have adjusted themselves accordingly. Among the hard
rocks of the mountains the declivities are great, so as to give
efficiency to the eroding water. Among the sedimentary rocks of the base
they are small in comparison, the chief work of the streams being the
transportation of the trachyte _débris_. So greatly are the streams
concerned in transportation, and so little in downward corrasion
(outside the trachyte region), that their grades are almost unaffected
by the differences of rock texture, and they pass through sandstone and
shale with nearly the same declivity.
The rate of downward corrasion being thus limited by extraneous
conditions, and the instrument of corrasion—the _débris_ of the hard
trachyte—being efficient, lateral corrasion is limited only by the
resistance which the banks of the streams oppose. Where the material of
the banks is a firm sandstone, narrow flood-plains are formed; and where
it is a shale, broad ones. In the Gray Cliff and Vermilion Cliff
sandstones flat-bottomed cañons are excavated; but in the great shale
beds broad valleys are opened, and the flood-plains of adjacent streams
coalesce to form continuous plains. The broadest plains are as a rule
carved from the thickest beds of shale, and these are found at the top
of the Jura-Trias and near the base of the Cretaceous. Where the streams
from the mountains cross the Blue Gate, the Tununk, or the Flaming Gorge
shale at a favorable angle, a plain is the result.
The plain which lies at the southern and western bases of Mount Hillers
is carved chiefly from the Tununk shale (see Figure 27). The plain
sloping eastward from Mount Pennell (Figure 36) is carved from the Blue
Gate and Tununk shales. The Lewis Creek plain, which lies at the western
base of Mount Ellen, is formed from the Blue Gate, Tununk, and Masuk
shales, and the planation which produced it has so perfectly truncated
the Tununk and Blue Gate sandstones that their outcrops cannot be traced
(Figures 61, 39, and 42). The plain which truncates the Crescent arch
(Figure 49) is carved in chief part from the Flaming Gorge shale. Toward
the east it is limited by the outcrops of the Henry’s Fork conglomerate,
but toward the mountain it cuts across the edge of the same conglomerate
and extends over Tununk shale to the margin of the trachyte.
[Illustration:
FIG. 61.—Cross-section of the Lewis Creek Plain. M, Masuk Shale. BG,
Blue Gate Group. T, Tununk Group. HF, Henry’s Fork conglomerate.
Scale, 1 inch = 4,000 feet.
]
The streams which made these plains and which maintain them, accomplish
their work by a continual shifting of their channels; and where the
plains are best developed they employ another method of shifting—a
method which in its proper logical order must be treated in the
discussion of alluvial cones, but which is practically combined in the
Henry Mountains with the method of planation. The supply of detritus
derived from the erosion of the trachyte is not entirely constant. Not
only is more carried out in one season than another and in one year than
another, but the work is accomplished in part by sudden storms which
create great floods and as suddenly cease. It results from this
irregularity that the channels are sometimes choked by _débris_, and
that by the choking of the channels the streams are turned aside to seek
new courses upon the general plain. The abandoned courses remain plainly
marked, and one who looks down on them from some commanding eminence can
often trace out many stages in the history of the drainage. Where a
series of streams emerge from adjacent mountain gorges upon a common
plain, their shiftings bring about frequent unions and separations, and
produce a variety of combinations.
[Illustration:
FIG. 62.—Ideal sketch to illustrate the Shifting of waterways on a
slope of Planation.
]
The accompanying sketch, Figure 62, is not from nature, but it serves to
illustrate the character of the changes. The streams which issue from
the mountain gorges _a_ and _b_ join and flow to _z_; while that which
issues at _c_ flows alone to _x_. An abandoned channel, _n_, shows that
the stream from _b_ was formerly united with that from _c_, and flowed
to _x_; and another channel, _m_, shows that it has at some time
maintained an independent course to _y_. By such shiftings streams are
sometimes changed from one drainage system to another; the hypothetical
courses, _x_, _y_, and _z_, may lead to different rivers, and to
different oceans.
An instance occurs on the western flank of the mountains. One of the
principal heads of Pine Alcove Creek rises on the south slope of Mount
Ellen and another on the northwest slope of Mount Pennell. The two unite
and flow southward to the Colorado River. They do not now cross an area
of planation, but at an earlier stage of the degradation they did; and
the portions of that plain which survive, indicate by the direction of
their slopes that one or both of the streams may have then discharged
its water into Lewis Creek, which runs northward to the Dirty Devil
River.
As the general degradation of the region progresses the streams and
their plains sink lower, and eventually each plain is sunk completely
through the shale whose softness made it possible. So soon as the
streams reach harder rock their lateral corrasion is checked, and they
are no longer free to change their ways. Wherever they chance to run at
that time, there they stay and carve for themselves cañons. Portions of
the deserted plains remain between the cañons, and having a durable
capping of trachyte gravel are long preserved. Such stranded fragments
abound on the slopes of the mountains, and in them one may read many
pages of the history of the degradation. They form tabular hills with
sloping tops and even profiles. The top of each hill is covered with a
uniform layer of gravel, beneath which the solid rock is smoothly
truncated. The slope of the hill depends on the grade of the ancient
stream, and is independent of the hardness and dip of the strata.
The illustration represents a _hill of planation_ on the north slope of
Mount Ellsworth. It is built of the Gray Cliff sandstone and Flaming
Gorge shale, inclined at angles varying from 25° to 45°; but
notwithstanding their variety of texture and dip the edges of the strata
are evenly cut away, so that their upper surface constitutes a plane.
The stream which performed this truncation afterward cut deeper into the
strata and carved the lower table which forms the foreground of the
sketch. It has now abandoned this plain also and flows through a still
deeper channel on the opposite side of the hill.
[Illustration:
FIG. 63.—A Hill of Planation.
]
The phenomena of planation are further illustrated in the region which
lies to the northwest of the Henry Mountains. Tantalus and Temple
Creeks, rising under the edge of the Aquarius Plateau, transport the
trachyte of the plateau across the region of the Waterpocket flexure to
the Dirty Devil River. Their flood-plains are not now of great extent,
but when their drainage lines ran a few hundred feet higher they appear
to have carved into a single plain a broad exposure of the Flaming Gorge
shale, which then lay between the Waterpocket and Blue Gate flexures.
At the Red Gate where the Dirty Devil River passes from a district of
trachyte plateaus to the district of the Great Flexures, it follows for
a few miles the outcrop of the Shinarump shale, and the remnants of its
abandoned flood-plains form a series of terraces upon each bank. Small
streams from the sides have cut across the benches and displayed their
structure. Each one is carved from the rock _in situ_, but each is
covered by a layer of the rounded river gravel. The whole are results of
planation; and they serve to connect the somewhat peculiar features of
the mountain slopes with the ordinary terraces of rivers.
[Illustration:
FIG. 64.—Ideal cross-section of a Terraced River Valley, after
Hitchcock. A, B, C, D, E, and F, Alluvial deposits. G, Indurated
rock, _in situ_.
]
River terraces as a rule are carved out, and not built up. They are
always the vestiges of flood-plains, and flood-plains are usually
produced by lateral corrasion. There are instances, especially near the
sea-coast, of river-plains which have originated by the silting up of
valleys, and have been afterward partially destroyed by the same rivers
when some change of level permitted them to cut their channels deeper;
and these instances, conspiring with the fact that the surfaces of
flood-plains are alluvial, and with the fact that many terraces in
glacial regions are carved from unconsolidated drift, have led some
American geologists into the error of supposing that river terraces in
general are the records of sedimentation, when in fact they record the
stages of a progressive corrasion. The ideal section of a terraced river
valley which I reproduce from Hitchcock (Surface Geology, Plate XII,
figure 1) regards each terrace as the remnant of a separate deposit,
built up from the bottom of the valley. To illustrate my own idea I have
copied his profile (Figure 65) and interpreted its features as the
results of lateral corrasion or planation, giving each bench a capping
of alluvium, but constituting it otherwise of the preëxistent material
of the valley. The preëxistent material in the region of the Henry
Mountains is always rock _in situ_, but in the Northern States it often
includes glacial drift, modified or unmodified.
There is a kindred error, as I conceive, involved in the assumption that
the streams which occupied the upper and broader flood-plains of a
valley were greater than those which have succeeded them. They may have
been, or they may not. In the process of lateral corrasion all the
material that is worn from the bank has to be transported by the water,
and where the bank is high the work proceeds less rapidly than where it
is low. A stream which degrades its immediate valley more rapidly than
the surrounding country is degraded (and the streams which abound in
terraces are of this character) steadily increases the height of the
banks which must be excavated in planation and diminishes the extent of
its flood-plain; and this might occur even if the volume of the stream
was progressively increasing instead of diminishing.
[Illustration:
FIG. 65.—Ideal cross-section of a Terraced River Valley, regarded as a
result of Planation. A, B, C, &c., Alluvial deposits. G, Preëxistent
material from which the valley was excavated.
]
Of the same order also is the mistake, occasionally made of ignoring the
excavation which a stream has performed, and assuming that when the
upper terraces were made the valley was as open as at present, and the
volume of flowing water was great enough to fill it.
_Alluvial Cones._
Wherever a stream is engaged in deposition instead of corrasion—wherever
it deposits its load—there is a shifting of channel by a third process.
The deposition of sediment takes place upon the bottom of the channel
and upon its immediate banks, and this continues until the channel
bottom is higher than the adjacent country. The wall of the channel is
then broken through at some point, and the water abandons its old bed
for one which is lower. Such occurrences belong to the histories of all
river deltas, and the devastation they have wrought at the mouths of
large rivers has enforced attention to their phenomena and stimulated a
study of their causes.
The same thing happens among the mountains. Wherever, as in Nevada and
Western Utah, the valleys are the receptacles of the detritus washed out
from the mountains, the foot-slopes of the mountains consist of a series
of alluvial cones. From each mountain gorge the products of its erosion
are discharged into the valley. The stream which bears the _débris_
builds up the bed of its channel until it is higher than the adjacent
land and then abandons it, and by the repetition of this process
accumulates a conical hill of detritus which slopes equally in all
directions from the mouth of the mountain gorge. At one time or another
the water runs over every part of the cone and leaves it by every part
of its base; and it sometimes happens that the opposite slopes of the
cone lead to different drainage systems.
An illustration may be seen in Red Rock Pass at the north end of Cache
Valley, Idaho. Lake Bonneville, the ancient expansion of Great Salt
Lake,[6] here found outlet to the basin of the Columbia, and the channel
carved by its water is plainly marked. For a distance of twelve miles
the bed of the channel is nearly level, with a width of a thousand feet.
Midway, Marsh Creek enters it from the east, and has built an alluvial
cone which extends to the opposite bank and divides it into two parts.
In the construction of the cone Marsh Creek has flowed alternately to
the north and to the south, being in one case a tributary to the Snake
and Columbia Rivers and to the Pacific Ocean, and in the other to the
Bear River and Great Salt Lake. So far as the creek is known to white
men it is a tributary of the Snake, but an irrigating ditch that has
been dug upon its cone carries part of its water to the Bear.
Footnote 6:
Lake Bonneville is described in volume III (Geology) of the “U. S.
Geog. Surveys West of the 100th Meridian,” pp. 88–104; and less fully
in the American Naturalist for November, 1876, and the American
Journal of Science for March, 1876, p. 228. See also Johnson’s
Cyclopedia, article “Sevier Lake.”
Another illustration exists at the mouth of the Colorado River. As has
been shown by Blake in the fifth volume of the Pacific Railroad Reports
(p. 236), the delta of the Colorado—or in other words the alluvial cone
which is built at its mouth—has extended itself completely across the
Gulf of California, severing the upper end from the lower and from the
ocean, and converting it into a lake. In continuing the upbuilding of
the delta the river has flowed alternately into the lower gulf and into
its severed segment. At the present day its mouth opens to the lower
gulf; but at rare intervals a portion of its water runs by the channel
known as “New River” to the opposite side of the delta. While it is
abandoned by the river the lake basin is dry, and it is known to human
history only as the Colorado Desert. Its bottom, which is lower than the
surface of the ocean, is strewn with the remains of the life its waters
sustained, and its beaches are patiently awaiting the cycle of change
which is slowly but surely preparing to restore to them their parent
waves.
[Illustration:
FIG. 66.—Cross-section of inclined strata, to illustrate Monoclinal
Shifting of waterways.
]
_Monoclinal Shifting._
In a fourth manner drainage lines are unstable.
In a region of inclined strata there is a tendency on the part of
streams which traverse soft beds to continue therein, and there is a
tendency to eliminate drainage lines from hard beds. In Figure 66, _S_
represents a homogeneous soft bed, and _H_ and _K_, homogeneous hard
beds. _A_ and _B_ are streams flowing through channels opened in the
soft rock, and in the hard. As the general degradation progresses the
stream at _a_ abrades both sides of its channel with equal force; but it
fails to corrade them at equal rates because of the inequality of the
resistance. It results that the channel does not cut its way vertically
into the hard rock, but works obliquely downward without changing its
relation to the two beds; so that when the degradation has reached the
stage indicated by the dotted line, the stream flows at _a_, having been
shifted horizontally by circumstances dependent on the dip and order of
the strata.
At the same time the stream at _B_, encountering homogeneous material,
cuts its way vertically downward to _b_; and a continuance of the
process carries it completely through the hard rock and into the soft.
Once in the soft it tends like the other streams to remain there; and in
the course of time it finds its way to the lower edge and establishes a
channel like that at _A_.
[Illustration:
FIG. 67.—Ground plan of outcrops of inclined strata, to illustrate the
results of Monoclinal Shifting.
]
The effect of this process on the course of a stream which runs
obliquely across inclined beds is shown in Figure 67. The outcrops of a
series of hard and soft strata, _H_, _H_, _H_ and _S_, _S_, are
represented in ground plan, and the direction of their dip is indicated
by the arrow. Supposing that a stream is thrown across them in the
direction of the dotted lines and that the land is then degraded, the
following changes will take place. The portion of the stream from _c_ to
_d_ will sink through the soft rock down to the surface of the hard, and
then follow down the slope of the hard, until at last its whole course
will be transferred to the line of separation between the two, and its
position (with reference to the outcrops which will then have succeeded
the original) will be represented by the line _g c_. The portion from
_e_ to _d_ sinking first through the hard bed and then through the soft,
will be deflected in the same manner to the position _e h g_. The points
_e_ and _c_ will retain their original relations to the strata. The same
changes will affect the portion from _e_ to _f_; and the original
oblique course will be converted into two sets of courses, of which one
will follow the strike of the strata and the other will cross the strike
at right angles.
The character of these changes is independent of the direction of the
current. They are not individually of great amount, and they do not
often divert streams from one drainage system to another nor change
their general directions. Their chief effects are seen in the details of
drainage systems and in the production of topographic forms. The
tendency of hard strata to rid themselves of waterways and of soft
strata to accumulate them, is a prime element of the process which
carves hills from the hard and valleys from the soft. Where hard rocks
are crossed by waterways they cannot stand higher than the adjacent
parts of the waterways; but where they are not so crossed they become
divides, and the “law of divides” conspires with the “law of structure”
to carve eminences from them.
The tendency of waterways to escape from hard strata and to abide in
soft, and their tendency to follow the strike of soft strata and to
cross hard at right angles, are tendencies only and do not always
prevail. They are opposed by the tendency of drainage lines to
stability. If the dip of the strata is small, or if the differences of
hardness are slight, or if the changes of texture are gradual instead of
abrupt, monoclinal shifting is greatly reduced.
Waterpocket Cañon is one of the most remarkable of monoclinal valleys;
and it serves to illustrate both the rule of monoclinal shifting and its
exception. The principal bed of soft rock which outcrops along the line
of the Waterpocket flexure is the Flaming Gorge shale, having a
thickness of more than one thousand feet. Through nearly the whole
extent of the outcrop a valley is carved from it, but the valley is not
a unit in drainage. At the north it is crossed by the Dirty Devil River
and by Temple and Tantalus Creeks, and the adjacent portions slope
toward those streams. At the south it is occupied for thirty miles by a
single waterway—the longest monoclinal drainage line with which I am
acquainted. The valley here bears the name of Waterpocket Cañon, and
descends all the way from the Masuk Plateau to the Colorado River. The
upper part of the cañon is dry except in time of rain, but the lower
carries a perpetual stream known as Hoxie Creek. Whatever may have been
the original meanderings of the latter they are now restrained, and it
is limited to the narrow belt in which the shale outcrops. As the cañon
is worn deeper the channel steadily shifts its position down the slope
of the underlying Gray Cliff sandstone, and carves away the shale. But
there is one exceptional point where it has not done this. When the
bottom of the cañon was a thousand feet higher the creek failed, at a
place where the dip of the strata was comparatively small, to shift its
channel as it deepened it, and began to cut its way into the massive
sandstone. Having once entered the hard rock it could not retreat but
sank deeper and deeper, carving a narrow gorge through which it still
runs making a detour from the main valley. The traveler who follows down
Waterpocket Cañon now comes to a place where the creek turns from the
open cañon of the shale and enters a dark cleft in the sandstone. He can
follow the course of the water (on foot), and will be repaid for the
wetting of his feet by the strange beauty of the defile. For nearly
three miles he will thread his way through a gorge walled in by the
smooth, curved faces of the massive sandstone, and so narrow and devious
that it is gloomy for lack of sunlight; and then he will emerge once
more into the open cañon. Or if he prefer he can keep to his saddle, to
the open daylight, and to the outcrop of the shale, and riding over a
low divide can reach the mouth of the gorge in half the distance.
[Illustration:
FIG. 68.—Waterpocket Cañon and the Horseshoe Bend of Hoxie Creek.
]
THE STABILITY OF DIVIDES.
The rain drops which fall upon the two sides of a divide flow in
opposite directions. However near to the dividing line they reach the
earth the work of each is apportioned to its own slope. It disintegrates
and transports the material of its own drainage slope only. The divide
is the line across which no water flows—across which there is no
transportation. It receives the minimum of water, for it has only that
which falls directly upon it, and every other point receives in addition
that which flows from higher points. It is higher than the surfaces
which adjoin it, and since less water is applied to its degradation it
tends to remain higher. It tends to maintain its position.
Opposed to this tendency there are others which lead to
THE INSTABILITY OF DIVIDES,
and which will now be considered.
_Ponding, Planation, and Alluviation._
Whenever by ponding, a stream or a system of streams which have belonged
to one drainage system are diverted so as to join another there is
coincidently a change of divides. The general divide between the two
systems is shifted from one side to the other of the area which changes
its allegiance. The line which was formerly the main divide becomes
instead a subordinate divide separating portions of the drainage system
which has increased its area; and on the other hand a line which had
been a subordinate divide is promoted to the rank of a main divide. In
like manner the shifting of streams from one system of drainage to
another by the extension of flood-plains, or by the building of alluvial
cones or deltas, involves a simultaneous shifting of the divides which
bound the drainage systems.
The changes which are produced by these methods are per saltum. When a
pond or lake opens a new outlet and abandons its old one there is a
short interregnum during which the drainage is divided between the two
outlets, and the watershed separating the drainage systems is double.
But in no other sense is the change gradual. The divide occupies no
intermediate positions between its original and its final. And the same
may be said of the changes by planation and alluviation. In each case a
tract of country is transferred bodily from one river system to another,
and in each case the watershed makes a leap.
But there are other methods of change, by which dividing lines move
_slowly_ across the land; and to these we will proceed.
_Monoclinal Shifting._
In regions of inclined strata, the same process which gathers the
waterways into the outcrops of the softer beds converts the outcrops of
the harder into divides. As the degradation progresses the waterways and
divides descend obliquely and retain the same relations to the beds. The
waterways continuously select the soft because they resist erosion
feebly, and the watersheds as continuously select the hard because they
resist erosion strongly. If the inclination of the strata is gentle,
each hard bed becomes the cap of a sloping table bounded by a cliff, and
the erosion of the cliff is by sapping. The divide is at the brow of the
cliff, and as successive fragments of the hard rock break away and roll
down the slope the divide is shifted. The process is illustrated in the
Pink Cliffs of Southern Utah. They face to the south, and their
escarpment is drained by streams flowing to the Colorado. The table
which they limit inclines to the north and bears the headwaters of the
Sevier. As the erosion of the cliffs steadily carries them back and
restricts the table, the drainage area of the Colorado is increased and
that of the “Great Basin”, to which the Sevier River is tributary, is
diminished.
[Illustration:
FIG. 69.—Ideal cross-section of inclined strata, to show the Shifting
of Divides in Cliff Erosion. Successive positions of a divide are
indicated at _a_, _b_, and _c_.
]
_Unequal and Equal Declivities._
[Illustration:
FIG. 70.—Cross-profile of a bad-land divide separating slopes of
Unequal Declivity. Two stages of erosion are indicated, to
illustrate the horizontal shifting of the divide.
]
In homogeneous material, and with equal quantities of water, the rate of
erosion of two slopes depends upon their declivities. The steeper is
degraded the faster. It is evident that when the two slopes are upon
opposite sides of a divide the more rapid wearing of the steeper carries
the divide toward the side of the gentler. The action ceases and the
divide becomes stationary only when the profile of the divide has been
rendered symmetric.
It is to this law that bad-lands owe much of their beauty. They acquire
their smooth curves under what I have called the “law of divides”, but
the symmetry of each ridge and each spur is due to the law of equal
declivities. By the law of divides all the slopes upon one side of a
ridge are made interdependent. By the law of equal declivities a
relation is established between the slopes which adjoin the crest on
opposite sides, and by this means the slopes of the whole ridge, from
base to base, are rendered interdependent.
One result of the interdependence of slopes is that a bad-land ridge
separating two waterways which have the same level, stands midway
between them; while a ridge separating two waterways which have
different levels, stands nearer to the one which is higher.
It results also that if one of the waterways is corraded more rapidly
than the other the divide moves steadily toward the latter, and
eventually, if the process continues, reaches it. When this occurs, the
stream with the higher valley abandons the lower part of its course and
joins its water to that of the lower stream. Thus from the shifting of
divides there arises yet another method of the shifting of waterways, a
method which it will be convenient to characterize as that of
_abstraction_. A stream which for any reason is able to corrade its
bottom more rapidly than do its neighbors, expands its valley at their
expense, and eventually “abstracts” them. And conversely, a stream which
for any reason is able to corrade its bottom less rapidly than its
neighbors, has its valley contracted by their encroachments and is
eventually “abstracted” by one or the other.
The diverse circumstances which may lead to these results need not be
enumerated, but there is one case which is specially noteworthy on
account of its relation to the principles of sculpture. Suppose that two
streams which run parallel and near to each other corrade the same
material and degrade their channels at the same rate. Their divide will
run midway. But if in the course of time one of the streams encounters a
peculiarly hard mass of rock while the other does not, its rate of
corrasion above the obstruction will be checked. The unobstructed stream
will outstrip it, will encroach upon its valley, and will at last
abstract it; and the incipient corrasion of the hard mass will be
stopped. Thus by abstraction as well as by monoclinal shifting, streams
are eliminated from hard rocks.
_Résumé._—There is a tendency to permanence on the part of drainage
lines and divides, and they are not displaced without adequate cause.
Hence every change which is known to occur demands and admits of an
explanation.
(_a_) There are four ways in which abrupt changes are made. Streams are
diverted from one drainage system to another, and the watersheds which
separate the systems are rearranged,
(1) by _ponding_, due to the elevation or depression of portions of
the land;
(2) by _planation_, or the extension of flood-plains by lateral
corrasion;
(3) by _alluviation_, or in the process of building alluvial cones and
deltas; and
(4) by _abstraction_.
(_b_) There are two ways in which gradual changes are effected:
(1) When the rock texture is variable, it modifies and controls by
_monoclinal shifting_ the distribution in detail of divides and
waterways.
(2) When the rock texture is uniform, the positions of divides
are adjusted in accordance with the principle of _equal
declivities_.
The abrupt changes are of geographic import; the gradual, of
topographic.
The methods which have been enumerated are not the only ones by which
drainage systems are modified, but they are the chief. Very rarely
streams are “ponded” and diverted to new courses through the damming of
their valleys by glaciers or by volcanic _ejecta_ or by land-slips. More
frequently they are obstructed by the growing alluvial cones of stronger
streams, but only the smallest streams will yield their “right of way”
for such cause, and the results are insignificant.
The rotation of the earth, just as it gives direction to the trade-winds
and to ocean currents, tends to deflect rivers. In the southern
hemisphere streams are crowded against their left banks and in northern
against the right. But this influence is exceedingly small. Mr. Ferrel’s
investigations show that in latitude 45° and for a current velocity of
ten miles an hour, it is measured by less than one twenty-thousandth
part of the weight of the water (American Journal of Science, January,
1861). If its effects are ever appreciable it must be where lateral
corrasion is rapid; and even there it is probable that the chief result
is an inclination of the flood-plain toward one bank or the other,
amounting at most to two or three minutes.
CONSEQUENT AND INCONSEQUENT DRAINAGE.
If a series of sediments accumulated in an ocean or lake be subjected to
a system of displacements while still under water, and then be converted
to dry land by elevation _en masse_ or by the retirement of the water,
the rains which fall on them will inaugurate a drainage system perfectly
conformable with the system of displacements. Streams will rise along
the crest of each anticlinal, will flow from it in the direction of the
steepest dip, will unite in the synclinals, and will follow them
lengthwise. The axis of each synclinal will be marked by a watercourse;
the axis of each anticlinal by a watershed. Such a system is said to be
_consequent_ on the structure.
If however a rock series is affected by a system of displacements after
the series has become continental, it will have already acquired a
system of waterways, and _provided the displacements are produced
slowly_ the waters will not be diverted from their accustomed ways. The
effect of local elevation will be to stimulate local corrasion, and each
river that crosses a line of uplift will inch by inch as the land rises
deepen its channel and valorously maintain its original course. It will
result that the directions of the drainage lines will be independent of
the displacements. Such a drainage system is said to be _antecedent_ to
the structure.
But if in the latter case the displacements are produced rapidly the
drainage system will be rearranged and will become consequent to the
structure. It has frequently happened that displacements formed with
moderate rapidity have given rise to a drainage system of mixed
character in which the courses of the larger streams are antecedent and
those of the smaller are consequent.
There is a fourth case. Suppose a rock series that has been folded and
eroded to be again submerged, and to receive a new accumulation of
unconforming sediments. Suppose further that it once more emerges and
that the new sediments are eroded from its surface. Then the drainage
system will have been given by the form of the upper surface of the
superior strata, but will be independent of the structure of the
inferior series, into which it will descend vertically as the
degradation progresses. Such a drainage system is said to be
_superimposed by sedimentation_ upon the structure of the older series
of strata.
Fifth. The drainage of an alluvial cone or of a delta is independent of
the structure of the bed-rock beneath; and if in the course of time
erosion takes the place of deposition and the alluvial formation is cut
through, the drainage system which is acquired by the rocks beneath is
not consequent upon their structure but is _superimposed by
alluviation_.
Sixth. The drainage of a district of planation is independent of the
structure of the rock from which it is carved; and when in the progress
of degradation the beds favorable to lateral corrasion are destroyed and
the waterways become permanent, their system may be said to be
_superimposed by planation_.
In brief, systems of drainage, in their relation to structure, are
(A) _consequent_
(a) by emergence, when the displacements are subaqueous, and
(b) by sudden displacement;
(B) _antecedent_; and
(C) _superimposed_
(a) by sedimentation, or subaqueous deposition,
(b) by alluviation, or subaërial deposition, and
(c) by planation.
THE DRAINAGE OF THE HENRY MOUNTAINS
is consequent on the laccolitic displacements. The uplifting of a
laccolite, like the upbuilding of a volcanic cone, is an event of so
rapid progress that the corrasion of a stream bed cannot keep pace with
it. We do not know that the site of the mountains was dry land at the
time of their elevation; but if it was, then whatever streams crossed it
were obstructed and turned from their courses. If it was not, there were
no preëxistent waterways, and the new ones, formed by the first rain
which fell upon the domes of strata, radiated from the crests in all
directions. The result in either case would be the same, and we cannot
determine from the present drainage system whether the domes were lifted
from the bed of the Tertiary lake or arose after its subsidence.
But while the drainage of the Henry Mountains is consequent as a whole,
it is not consequent in all its details, and the character of its
partial inconsequence is worthy of examination.
Let us begin with the simplest case. The drainage system of Mount
Ellsworth is more purely consequent than any other with which I am
acquainted. In the accompanying chart the point _c_ marks the crest of
the Ellsworth dome; the inner circle represents the line of maximum dip
of the arching strata and the outer circle the limit of the disturbance.
It will be seen that all the waterways radiate from the crest and follow
closely the directions in which the strata incline. At _a_ the Ellsworth
arch touches that of Mount Holmes and at _b_ that of Mount Hillers; and
the effect of the compound inclination is to modify the directions of a
few of the waterways.
[Illustration:
FIG. 71.—Drainage system of the Ellsworth Arch.
]
Turning now to Mount Holmes, we find that its two domes are not equally
respected by the drainage lines. The crest of the Greater arch (see
Figure 72) is the center of a radiating system, but the crest of the
Lesser arch is not; and waterways arising on the Greater traverse the
Lesser from side to side. More than this, a waterway after following the
margin of the Lesser arch turns toward it and penetrates the flank of
the arch for some distance. In a word, the drainage of the Greater arch
is consequent on the structure, while the drainage of the Lesser arch is
inconsequent.
[Illustration:
FIG. 72.—Drainage system of the Holmes Arches.
]
There are at least two ways in which this state of affairs may have
arisen.
First, the Greater arch may have been lifted so long before the Lesser
that its waterways were carved too deeply to be diverted by the gentle
flexure of the latter. The drainage of the Lesser would in that case be
classed as antecedent. If the Lesser arch were first formed and carved,
the lifting of the Greater might throw a stream across its summit; but
it could not initiate the waterways which skirt the slopes of the
Lesser, especially if those slopes were already furrowed by streams
which descended them. If the establishment of the drainage system
depended on the order of uplift, the Greater arch is surely the older.
Second, the drainage of the Lesser arch may have been imposed upon it by
planation at a very late stage of the degradation. Whatever was the
origin of the arches, and whatever was the depth of cover which they
sustained, the Greater is certain to have been a center of drainage from
the time of its formation. When it was first lifted it became a drainage
center because it was an eminence; and afterward it remained an eminence
because it was a drainage center. When in the progress of the denudation
its dikes were exposed, their hardness checked the wear of the summit
and its eminence became more pronounced. It was perhaps at about this
time that the last of the Cretaceous rocks were removed from the summits
and slopes of the two arches and the Flaming Gorge shale was laid bare,
and so soon as this occurred the conditions for lateral corrasion were
complete. With trachyte in the peaks and shale upon the slopes planation
would naturally result, and a drainage system would be arranged about
the dikes as a center without regard to the curves of the strata. The
subsequent removal of the shale would impart its drainage to the
underlying sandstones.
Either hypothesis is competent to explain the facts, but the data do not
warrant the adoption of one to the exclusion of the other. The waterways
of the Lesser arch may be either antecedent, or superimposed by
planation. The Greater arch may have been the first to rise or the last.
The drainage of Mount Hillers is consequent to the main uplift and to
the majority of the minor, but to the Pulpit arch it is inconsequent. In
this case there is no question that the arch has been truncated by
planation. (Figure 73.) The Hillers dome, rising five times as high as
the Pulpit, became the center of drainage for the cluster, and the
trachyte-laden streams which it sent forth were able to pare away
completely the lower arch while it was still unprotected by the hardness
of its nucleus. The foot-plain of Mount Hillers, which extends unbroken
to the outcrop of the Henry’s Fork conglomerate, is continued on several
lines across the Pulpit arch, although in the intervals the central area
is deeply excavated. The planation stage is just completed, and an epoch
of fixed waterways is inaugurated.
[Illustration:
FIG. 73.—Cross-section of the Pulpit Arch, showing its truncation.
]
The drainage of Mount Pennell is consequent in regard to the main
uplift, but inconsequent to some of the minor. A stream which rises on
the north flank not merely runs across one of the upper series of
laccolites,—a companion to the Sentinel,—but has cut into it and divided
it nearly to the base. It is probable that the position of the waterway
was fixed by planation, but no remnant of the plain was seen.
Too little is known of the structure of the central area of Mount Ellen
to assert its relation to the drainage. About its base there are five
laccolites which have lost all or nearly all their cover, and each of
these is a local center of drainage, avoided by the streams which head
in the mountain crest. Four others have been laid bare at a few points
only, and these are each crossed by one or two streams from higher
levels. The remainder are not exposed at all, and their arches are
crossed by numerous parallel streams. The Crescent arch is freshly
truncated by planation, and the Dana and Maze bear proof that they have
at some time been truncated. The laccolites which stand highest with
reference to the general surface are exempt from cross-drainage, and the
arches which lie low are completely overrun.
If we go back in imagination to a time when the erosion of the mountain
was so little advanced that the stream beds were three thousand feet
higher than they now are, we may suppose that very little trachyte was
laid bare. As the surface was degraded and a few laccolites were
exposed, it would probably happen that some of the then existing streams
would be so placed as to run across the trachyte. But being unarmed as
yet by the _débris_ of similar material they would corrade it very
slowly; and the adjoining streams having only shale to encounter, would
so far outstrip them as eventually to divert them by the process of
“abstraction”. In this way the first-bared laccolites might be freed
from cross-drainage and permitted to acquire such radiating systems of
waterways as we find them to possess. At a later stage when trachyte was
exposed at many points and all streams were loaded with its waste, the
power to corrade was increased, and the lower-lying laccolites could not
turn aside the streams which overran them.
The work of planation is so frequently seen about the flanks of the
Henry Mountains that there seems no violence in referring all the
cross-drainage of lateral arches to its action; and if that is done the
history of the erosion of the mountains takes the following form:
When the laccolites were intruded, the mounds which they uplifted either
rose from the bed of a lake or else turned back all streams which
crossed their sites; and in either case they established upon their
flanks a new and “consequent” set of waterways. The highest mounds
became centers of drainage, and sent their streams either across or
between the lower. All the streams of the disturbed region rose within
it and flowed outward. The degradation of the mounds probably began
before the uplift was complete, but of this there is no evidence. As it
proceeded the convex forms of the mounds were quickly obliterated and
concave profiles were substituted. The rocks which were first excavated
were not uniform in texture, but they were all sedimentary and were soft
as compared to the trachyte. The Tertiary and probably the Upper
Cretaceous were removed from the summits before any of the igneous rocks
were brought to light, and during their removal the tendency of divides
to permanence kept the drainage centers or maxima of surface at
substantially the same points. When at length the trachyte was reached
its hardness introduced a new factor. The eminences which contained it
were established more firmly as maxima, and their rate of degradation
was checked. With the checking of summit degradation and the addition of
trachyte to the transported material, planation began upon the flanks,
and by its action the whole drainage has been reformed. One by one the
lower laccolites are unearthed, and each one adds to the complexity and
to the permanence of the drainage.
If the displacements were completed before the erosion began, the
mountains were then of greater magnitude than at any later date. Before
the igneous nuclei were laid bare and while sedimentary rocks only were
subject to erosion, the rate of degradation was more rapid than it has
been since the hardness and toughness of the trachyte have opposed it.
If the surrounding plain has been worn away at a uniform rate, the
height of the mountains (above the plain) must have first diminished to
a minimum and afterward increased. The minimum occurred at the beginning
of the erosion of the trachyte, and at that time the mountains may even
have been reduced to the rank of hills. They owe their present
magnitude, not to the uplifting of the land in Middle Tertiary time, but
to the contrast between the incoherence of the sandstones and shales of
the Mesozoic series and the extreme durability of the laccolites which
their destruction has laid bare. And if the waste of the plain shall
continue at a like uniform rate in the future, it is safe to prophesy
that the mountains will for a while continue to increase in relative
altitude. The phase which will give the maximum resistance to
degradation has been reached in none of the mountains, except perhaps
Mount Hillers. In Mount Ellen the laccolites of the upper zone only have
been denuded; the greater masses which underlie them will hold their
place more stubbornly. The main bodies of Mounts Ellsworth, Holmes, and
Pennell are unassailed, and the present prominence of their forms has
been accomplished simply by the valor of their skirmish lines of dikes
and spurs. In attaching to the least of the peaks the name of my friend
Mr. Holmes, I am confident that I commemorate his attainments by a
monument which will be more conspicuous to future generations and races
than it is to the present.