GENERAL OUTLINE OF RAIN AND RIVER EROSION.
_Elements of erosion._--The general process of subaerial erosion is
divisible into the several sub-processes of weathering, transportation,
and corrasion.[5]
[5] There is an admirable exposition of this subject in
Gilbert's "Henry Mountains."
_Weathering_ is the term applied to all those processes which
disintegrate and disrupt exposed surfaces of rock. It is accomplished
chiefly by solution, changes in temperature, the wedge-work of ice and
roots, the borings of animals, and such chemical changes as surface
water and air effect. The products of weathering are transported by the
direct action of gravity, by glaciers, by winds, and by running water.
Of these the last is the most important.
_Corrasion_ is accomplished chiefly by the mechanical wear of streams,
aided by the hard fragments such as sand, gravel and bowlders, which
they carry. The solution effected by the waters of a stream may also be
regarded as a part of corrasion. Under ordinary circumstances solution
by streams is relatively unimportant, but where the rock is relatively
soluble, and where conditions are not favorable for abrasion, solution
may be more important than mechanical wear.
So soon as sea bottom is raised to the estate of land, it is attacked by
the several processes of degradation. The processes of weathering at
once begin to loosen the material of the surface if it be solid; winds
shift the finer particles about, and with the first shower
transportation by running water begins. Weathering prepares the material
for transportation and transportation leads to corrasion. Since the goal
of all material transported by running water is the sea, subaerial
erosion means degradation of the surface.
_Erosion without valleys._--In the work of degradation the valley
becomes the site of greatest activity, and in the following pages
especial attention is given to the development of valleys and to the
phases of topography to which their development leads.
If a new land surface were to come into existence, composed of materials
which were perfectly homogeneous, with slopes of absolute uniformity in
all directions, and if the rain, the winds and all other surface
agencies acted uniformly over the entire area, valleys would not be
developed. That portion of the rainfall which was not evaporated and did
not sink beneath the surface, would flow off the land in a sheet. The
wear which it would effect would be equal in all directions from the
center. If the angle of the slope were constant from center to shore, or
if it increased shoreward, the wear effected by this sheet of water
would be greatest at the shore, because here the sheet of flowing water
would be deepest and swiftest, and therefore most effective in
corrasion.
_The beginning of a valley._--But land masses as we know them do not
have equal and uniform slopes to the sea in all directions, nor is the
material over any considerable area perfectly homogeneous. Departure
from these conditions, even in the smallest degree, would lead to very
different results.
That the surface of newly emerged land masses would, as a rule, not be
rough, is evident from the fact that the bottom of the sea is usually
rather smooth. Much of it indeed is so nearly plane that if the water
were withdrawn, the eye would scarcely detect any departure from
planeness. The topography of a land mass newly exposed either by its own
elevation or by the withdrawal of the sea, would ordinarily be similar
to that which would exist in the vicinity of Necedah and east of Camp
Douglas, if the few lone hills were removed, and the very shallow
valleys filled. Though such a surface would seem to be moderately
uniform as to its slopes, and homogeneous as to its material, neither
the uniformity nor the homogeneity are perfect, and the rain water would
not run off in sheets, and the wear would not be equal at all points.
Let it be supposed that an area of shallow sea bottom is raised above
the sea, and that the elevation proceeds until the land has an altitude
of several hundred feet. So soon as it appears above the sea, the rain
falling upon it begins to modify its surface. Some of the water
evaporates at once, and has little effect on the surface; some of it
sinks beneath the surface and finds its way underground to the sea; and
some of it runs off over the surface and performs the work
characteristic of streams. So far as concerns modifications of the
surface, the run-off is the most important part.
The run-off of the surface would tend to gather in the depressions of
the surface, however slight they may be. This tendency is shown on
almost every hillside during and after a considerable shower. The water
concentrated in the depressions is in excess of that flowing over other
parts of the surface, and therefore flows faster. Flowing faster, it
erodes the surface over which it flows more rapidly, and as a result the
initial depressions are deepened, and _washes_ or _gullies_ are started.
Should the run-off not find irregularities of slope, it would, at the
outset, fail of concentration; but should it find the material more
easily eroded along certain lines than along others, the lines of easier
wear would become the sites of greater erosion. This would lead to the
development of gullies, that is, to irregularities of slope. Either
inequality of slope or material may therefore determine the location of
a gully, and one of these conditions is indispensable.
Once started, each wash or gully becomes the cause of its own growth,
for the gully developed by the water of one shower, determines greater
concentration of water during the next. Greater concentration means
faster flow, faster flow means more rapid wear, and this means
corresponding enlargement of the depression through which the flow takes
place. The enlargement effected by successive showers affects a gully in
all dimensions. The water coming in at its head carries the head back
into the land (head erosion), thus lengthening the gully; the water
coming in at its sides wears back the lateral slopes, thus widening it;
and the water flowing along its bottom deepens it. Thus gullies grow to
be ravines, and farther enlargement by the same processes converts
ravines into valleys. A river valley therefore is often but a gully
grown big.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XIII.
FIG. 1.
A very young valley.
Illustration: FIG. 2.
A valley in a later stage of development.
Illustration: FIG. 3.
Young valleys.]
_The course of a valley._--In the lengthening of a gully or valley
headward, the growth will be in the direction of greatest wear. Thus in
Plate XIII, Fig. 1, if the water coming in at the head of the gully
effects most wear in the direction a, the head of the gully will
advance in that direction; if there be most wear in the direction b or
c, the head will advance toward one of these points. The direction of
greatest wear will be determined either by the slope of the surface, or
by the nature of the surface material. The slope may lead to the
concentration of the entering waters along one line, and the surface
material may be less resistant in one direction than in another. If
these factors favor the same direction of head-growth, the lengthening
will be more rapid than if but one is favorable. If there be more rapid
growth along two lines, as b and c, Plate XIII, Fig. 1, than between
them, two gullies may develop (Plate XIII, Fig. 2). The frequent and
tortuous windings common to ravines and valleys are therefore to be
explained by the inequalities of slope or material which affected the
surface while the valley was developing.
_Tributary valleys._--Following out this simple conception of valley
growth, we have to inquire how a valley system (a main valley and its
tributaries) is developed. The conditions which determine the location
and development of gullies in a new land surface, determine the location
and development of tributary gullies. In flowing over the lateral slopes
of a gully or ravine, the water finds either slope or surface material
failing of uniformity. Both conditions lead to the concentration of the
water along certain lines, and concentration of flow on the slope of an
erosion depression, be it valley or gully, leads to the development of
a tributary depression. In its growth, the tributary repeats, in all
essential respects, the history of its main. It is lengthened headward
by water coming in at its upper end, is widened by side wash, and
deepened by the downward cutting of the water which flows along its
axis. The factors controlling its development are the same as those
which controlled the valley to which it is tributary.
There is one peculiarity of the courses of tributaries which deserves
mention. Tributaries, as a rule, join their mains with an acute angle up
stream. In general, new land surfaces, such as are now under
consideration, slope toward the sea. If a tributary gully were to start
back from its main at right angles, more water would come in on the side
away from the shore, on account of the seaward slope of the land. This
would be true of the head of the gully as well as of other portions, and
the effect would be to turn the head more and more toward parallelism
with the main valley. Local irregularities of surface may, and
frequently do, interfere with these normal relations, so that the
general course of a tributary is occasionally at right angles to its
main. Still more rarely does the general course of a tributary make an
acute angle with its main on the down stream side. Local irregularities
of surface determine the windings of a tributary, so that their courses
for longer or shorter distances may be in violation of the general rule
(c, Fig. 43); but on the whole, the valleys of a system whose
history has not been interrupted in a region where the surface material
is not notably heterogeneous, follow the course indicated above. This is
shown by nearly every drainage system on the Atlantic Coastal plain
which represents more nearly than any other portion of our continent,
the conditions here under consideration. Fig. 12 represents the drainage
system of the Mullica river in southern New Jersey and is a type of the
Coastal plain river system.
_How a valley gets a stream._--Valleys may become somewhat deep and long
and wide without possessing permanent streams, though from their
inception they have _temporary_ streams, the water for which is
furnished by showers or melting snow. Yet sooner or later, valleys come
to have permanent streams. How are they acquired? Does the valley find
the stream or the stream the valley? For the answer to these questions,
a brief digression will be helpful.
[Illustration: Fig. 12.--A typical river system of the Coastal plain
type.]
In cultivated regions, wells are of frequent occurrence. In a flat
region of uniform structure, the depth at which well water may be
obtained is essentially constant at all points. If holes (wells 1 and 2,
Fig. 13) be excavated below this level, water seeps into them, and in a
series of wells the water stands at a nearly common level. This means
that the sub-structure is full of water up to that level. These
relations are illustrated by Fig. 13. The diagram represents a vertical
section through a flat region from the surface (s s) down below the
bottom of wells. The water stands at the same level in the two cells (1
and 2), and the plane through them, at the surface of the water, is the
_ground water level_. If in such a surface a valley were to be cut until
its bottom was below the ground water level, the water would seep into
it, as it does into the wells; and if the amount were sufficient, a
permanent stream would be established. This is illustrated in Fig. 13.
The line A A represents the ground water level, and the level at which
the water stands in the wells, under ordinary circumstances. The bottom
of the valley is below the level of the ground water, and the water
seeps into it from either side. Its tendency is to fill the valley to
the level A A. But instead of accumulating in the open valley as it does
in the enclosed wells, it flows away, and the ground water level on
either hand is drawn down.
[Illustration: Fig. 13.--Diagram illustrating the relations of ground
water to streams.]
The level of the ground water fluctuates. It is depressed when the
season is dry (A' A'), and raised when precipitation is abundant (A''
A''). When it is raised, the water in the wells rises, and the stream in
the valley is swollen. When it falls, the ground water surface is
depressed, and the water in the wells becomes lower. If the water
surface sinks below the bottom of the wells, the wells "go dry;" if
below the bottom of the valley, the valley becomes for the time being, a
"dry run." When a well is below the lowest ground-water level its supply
of water never fails, and when the valley is sufficiently below the same
level, its stream does not cease to flow, even in periods of drought. On
account of the free evaporation in the open valley, the valley
depression must be somewhat below the level necessary for a well, in
order that the flow may be constant.
It will be seen that _intermittent_ streams, that is, streams which flow
in wet seasons and fail in dry, are intermediate between streams which
flow after showers only, and those which flow without interruption. In
the figure the stream would become dry if the ground water level sank to
A' A'.
It is to be noted that a permanent stream does not normally precede its
valley, but that the valley, developed through gully-hood and
ravine-hood to valley-hood by means of the temporary streams supplied by
the run-off of occasional showers, _finds a stream_, just as diggers of
wells find water. The case is not altered if the stream be fed by
springs, for the valley finds the spring, as truly as the well-digger
finds a "vein" of water.
_Limits of a valley._--So soon as a valley acquires a permanent stream,
its development goes on without the interruption to which it was subject
while the stream was intermittent. The permanent stream, like the
temporary one which preceded it, tends to deepen and widen its valley,
and, under certain conditions, to lengthen it as well. The means by
which these enlargements are affected are the same as before. There are
limits, however, in length, depth, and width, beyond which a valley may
not go. No stream can cut below the level of the water into which it
flows, and it can cut to that level only at its outlet. Up stream from
that point, a gentle gradient will be established over which the water
will flow without cutting. In this condition the stream is _at grade_.
Its channel has reached _baselevel_, that is, the level to which the
stream can wear its bed. This grade is, however, not necessarily
permanent, for what was baselevel for a small stream in an early stage
of its development, is not necessarily baselevel for the larger stream
which succeeds it at a later time.
Weathering, wash, and lateral corrasion of the stream continue to widen
the valley after it has reached baselevel. The bluffs of valleys are
thus forced to recede, and the valley is widened at the expense of the
upland. Two valleys widening on opposite sides of a divide, narrow the
divide between them, and may ultimately wear it out. When this is
accomplished, the two valleys become one. The limit to which a valley
may widen on either side is therefore its neighboring valley, and since,
after two valleys have become one by the elimination of the ridge
between them, there are still valleys on either hand, the final result
of the widening of all valleys must be to reduce all the area which
they drain to baselevel. As this process goes forward, the upper flat
into which the valleys were cut is being restricted in area, while the
lower flats developed by the streams in the valley bottoms are being
enlarged. Thus the lower flats grow at the expense of the higher.
There are also limits in length which a valley may not exceed. The head
of any valley may recede until some other valley is reached. The
recession may not stop even there, for if, on opposite sides of a
divide, erosion is unequal, as between 1A and 1B, Fig. 14, the divide
will be moved toward the side of less rapid erosion, and it will cease
to recede only when erosion on the two sides becomes equal (4A and
4B). In homogeneous material this will be when the slopes on the two
sides are equal.
[Illustration: Fig. 14.--Diagram showing the shifting of a divide. The
slopes 1A and 1B are unequal. The steeper slope is worn more rapidly and
the divide is shifted from 1 to 4, where the two slopes become equal and
the migration of the divide ceases.]
It should be noted that the lengthening of a valley headward is not
normally the work of the permanent stream, for the permanent stream
begins some distance below the head of the valley. At the head,
therefore, erosion goes on as at the beginning, even after a permanent
stream is acquired.
Under certain circumstances, the valley may be lengthened at its
debouchure. If the detritus carried by it is deposited at its mouth, or
if the sea bottom beyond that point rise, the land may be extended
seaward, and over this extension the stream will find its way. Thus at
their lower, as well as at their upper ends, both the stream and its
valley may be lengthened.
_A cycle of erosion._--If, along the borders of a new-born land mass, a
series of valleys were developed, essentially parallel to one another,
they would constitute depressions separated by elevations, representing
the original surface not yet notably affected by erosion (see Plate XIV,
Fig. 1). These inter-valley areas might at first be wide or narrow, but
in process of time they would necessarily become narrow, for, once, a
valley is started, all the water which enters it from either side helps
to wear back its slopes, and the wearing back of the slopes means the
widening of the valleys on the one hand and the narrowing of the
inter-valley ridges on the other. Not only would the water running over
the slopes of a valley wear back its walls, but many other processes
conspire to the same end. The wetting and drying, the freezing and the
thawing, the roots of plants and the borings of animals, all tend to
loosen the material on the slopes or walls of the valleys, and gravity
helps the loosened material to descend. Once in the valley bottom, the
running water is likely to carry it off, landing it finally in the sea.
Thus the growth of the valley is not the result of running water alone,
though this is the most important single factor in the process.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XIV.
FIG. 1.
The same valleys as shown in Plate XIII, Fig. 3, in a later stage of
development.
Illustration: FIG. 2.
Same valleys as shown in Fig. 1, in a still later stage of
development.]
Even if valleys developed no tributaries, they would, in the course of
time, widen to such an extent as to nearly obliterate the intervening
ridges. The surface, however, would not easily be reduced to perfect
flatness. For a long time at least there would remain something of slope
from the central axis of the former inter-stream ridge, toward the
streams on either hand; but if the process of erosion went on for a
sufficiently long period of time, the inter-stream ridge would be
brought very low, and the result would be an essentially flat surface
between the streams, much below the level of the old one.
The first valleys which started on the land surface (see Plate XIII,
Fig. 3) would be almost sure to develop numerous tributaries. Into
tributary valleys water would flow from their sides and from their
heads, and as a result they would widen and deepen and lengthen just as
their mains had done before them. By lengthening headward they would
work back from their mains some part, or even all of the way across the
divides separating the main valleys. By this process, the tributaries
cut the divides between the main streams into shorter cross-ridges. With
the development of tributary valleys there would be many lines of
drainage instead of two, working at the area between two main streams.
The result would be that the surface would be brought low much more
rapidly, for it is clear that many valleys within the area between the
main streams, widening at the same time, would diminish the aggregate
area of the upland much more rapidly than two alone could do.
The same thing is made clear in another way. It will be seen (Plate XIV,
Figs. 1 and 2) that the tributaries would presently dissect an area of
uniform surface, tending to cut it into a series of short ridges or
hills. In this way the amount of sloping surface is greatly increased,
and as a result, every shower would have much more effect in washing
loose materials down to lower levels, whence the streams could carry
them to the sea.
[Illustration: Fig. 15.--Cross-sections showing various stages of
erosion in one cycle.]
The successive stages in the process of lowering a surface are suggested
by Fig. 15, which represents a series of cross-sections of a land mass
in process of degradation. The uppermost section represents a level
surface crossed by young valleys. The next lower represents the same
surface at a later stage, when the valleys have grown larger, while the
third and succeeding sections represent still later stages in the
process of degradation. Plate XIII, Fig. 3, and Plate XIV, Figs. 1 and
2, represent in another way the successive stages of stream work in the
general process of degradation.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XV.
Diagram illustrating how a hard inclined layer of rock becomes a ridge
in the process of degradation.]
In this manner a series of rivers, operating for a sufficiently long
period of time, might reduce even a high land mass to a low level,
scarcely above the sea. The new level would be developed soonest near
the sea, and the areas farthest from it would be the last--other things
being equal--to be brought low. The time necessary for the development
of such a surface is known as a _cycle of erosion_, and the resulting
surface is a _base-level plain_, that is, a plain as near sea level as
river erosion can bring it. At a stage shortly preceding the base-level
stage the surface would be a _peneplain_. A peneplain, therefore, is a
surface which has been brought toward, but not to base-level. Land
surfaces are often spoken of as young or old in their erosion history
according to the stage of advancement which has been made toward
base-leveling. Thus the Colorado canyon, deep and impressive as it is,
is, in terms of erosion, a young valley, for the river has done but a
small part of the work which must be done in order to bring its basin to
baselevel.
_Effects of unequal hardness._--The process of erosion thus sketched
would ultimately bring the surface of the land down to base-level, and
in case the material of the land were homogeneous, the last points to be
reduced would be those most remote from the axes of the streams doing
the work of leveling. But if the material of the land were of unequal
hardness, those parts which were hardest would resist the action of
erosion most effectively. The areas of softer rock would be brought low,
and the outcrops of hard rock (Plate XV) would constitute ridges during
the later stages of an erosion cycle. If there were bodies of hard rock,
such as the Baraboo quartzite, surrounded by sandstone, such as the
Potsdam, the sandstone on either hand would be worn down much more
readily than the quartzite, and in the course of degradation the latter
would come to stand out prominently. The region in the vicinity of Devil's
lake is in that stage of erosion in which the quartzite ridges are
conspicuous (Plate XXXVII). The less resistant sandstone has been
removed from about them, and erosion has not advanced so far since the
isolation of the quartzite ridges as to greatly lower their crests. The
harder strata are at a level where surface water can still work
effectively, even though slowly, upon them, and in spite of their great
resistance they will ultimately be brought down to the common level. It
will be seen that, from the point of view of subaerial erosion, a
base-level plain is the only land surface which is in a condition of
approximate stability.
_Falls and rapids._--If in lowering its channel a stream crosses one
layer of rock much harder than the next underlying, the deepening will
go on more rapidly on the less resistant bed. Where the stream crosses
from the harder to the less hard, the gradient is likely to become
steep, and a rapids is formed. These conditions are suggested in Fig. 16
which represents the successive profiles (a b, a c, d e, f e, g e,
and h e) of a stream crossing from a harder to a softer formation. Below
the point a the stream is flowing over rock which is easily eroded, while
above that point its course is over a harder formation. Just below a
(profile a b) the gradient has become so steep that there are rapids.
Under these conditions, erosion is rapid just beyond the crossing of the
hard layer, and the gradient becomes higher and higher. When the steep
slope of the rapids approaches verticality, the rapids become a _fall_
(profile a c).
[Illustration: Fig. 16.--Diagram to illustrate the development of a
rapid and fall. The upper layer is harder than the strata below. The
successive profiles of the stream below the hard layer are represented
by the lines a b, a c, d e, f e, g e, and h e.]
As the water falls over the precipitous face and strikes upon the softer
rock below, part of it rebounds against the base of the vertical face
(Fig. 16). The result of wear at this point is the undermining of the
hard layer above, and sooner or later, portions of it will fall. This
will occasion the recession of the fall (profile d e and f e). As the
fall recedes, it grows less and less high. When the recession has
reached the point i, or, in other words, when the gradient of the stream
below the fall crosses the junction of the beds of unequal hardness, as
it ultimately must, effective undermining ceases, and the end of the
fall is at hand.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XVI.
Skillett Falls, in the Potsdam formation, three miles southwest of
Baraboo. The several small falls are occasioned by slight inequalities
in the hardness of the layers.]
When the effective undercutting ceases because the softer bed is no
longer accessible, the point of maximum wear is transferred to the top
of the hard bed just where the water begins to fall (g, Fig. 16). The
wear here is no greater than before, though it is greater relatively.
The relatively greater wear at this point destroys the verticality of
the face, converting it into a steep slope. When this happens, the fall
is a thing of the past, and rapids succeed. With continued flow the bed
of the rapids becomes less and less steep, until it is finally reduced
to the normal gradient of the stream (h e), when the rapids disappear.
When thin layers of rock in a stream's course vary in hardness, softer
beds alternating with harder ones, a series of falls such as shown in
Plate XVI, may result. As they work up stream, these falls will be
obliterated one by one. Thus it is seen that falls and rapids are not
permanent features of the landscape. They belong to the younger period
of a valley's history, rather than to the older. They are marks of
topographic youth.
_Narrows._--Where a stream crosses a hard layer or ridge of rock lying
between softer ones, the valley will not widen so rapidly in the hard
rock as above and below. If the hard beds be vertical, so that their
outcrop is not shifted as the degradation of the surface proceeds, a
notable constriction of the valley results. Such a constriction is a
_narrows_. The Upper and Lower narrows of the Baraboo (Plate IV)
are good examples of the effect of hard rock on the widening of a
valley.
_Erosion of folded strata._--The processes of river erosion would not be
essentially different in case the land mass upon which erosion operated
were made of tilted and folded strata. The folds would, at the outset,
determine the position of the drainage lines, for the main streams would
flow in the troughs (synclines) between the folds (anticlines). Once
developed, the streams would lower their beds, widen their valleys, and
lengthen their courses, and in the long process of time they would bring
the area drained nearly to sea-level, just as in the preceding case. It
was under such conditions that the general processes of subaerial
erosion operated in south central Wisconsin, after the uplift of the
quartzite and before the deposition of the Potsdam sandstone. It was
then that the principal features of the topography of the quartzite were
developed.
In regions of folded strata, certain beds are likely to be more
resistant than others. Where harder beds alternate with softer, the
former finally come to stand out as ridges, while the outcrops of the
latter mark the sites of the valleys. Such alternations of beds of
unequal resistance give rise to various peculiarities of drainage,
particularly in the courses of tributaries. These peculiarities find no
illustration in this region and are not here discussed.
_Base-level plains and peneplains._--It is important to notice that a
plane surface (base-level) developed by streams could only be developed
at elevations but slightly above the sea, that is, at levels at which
running water ceases to be an effective agent of erosion; for so long as
a stream is actively deepening its valley, its tendency is to roughen
the area which it drains, not to make it smooth. The Colorado river,
flowing through high land, makes a deep gorge. All the streams of the
western plateaus have deep valleys, and the manifest result of their
action is to roughen the surface; but given time enough, and the streams
will have cut their beds to low gradients. Then, though deepening of the
valleys will cease, widening will not, and inch by inch and shower by
shower the elevated lands between the valleys will be reduced in area,
and ultimately the whole will be brought down nearly to the level of the
stream beds. This is illustrated by Fig. 15.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XVII.
A group of mounds on the plain southwest from Camp Douglas. The
base-level surface is well shown, and above it rise the remnants of the
higher plain from which the lower was reduced.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XVIII.
Castle Rock near Camp Douglas. In this view the relation of the erosion
remnant to the extensive base-leveled surface is well shown.]
It is important to notice further that if the original surface on which
erosion began is level, there is no stage intermediate between the
beginning and the end of an erosion cycle, when the surface is again
level, or nearly so, though in the stage of a cycle next preceding the
last--the peneplain stage (fourth profile, Fig. 15)--the surface
approaches flatness. It is also important to notice that when streams
have cut a land surface down to the level at which they cease to erode,
that surface will still possess some slight slope, and that to seaward.
No definite degree of slope can be fixed upon as marking a base-level.
The angle of slope which would practically stop erosion in a region of
slight rainfall would be great enough to allow of erosion if the
precipitation were greater. All that can be said, therefore, is that the
angle of slope must be low. The Mississippi has a fall of less than a
foot per mile for some hundreds of miles above the gulf. A small stream
in a similar situation would have ceased to lower its channel before so
low a gradient was reached.
The nearest approach to a base-leveled region within the area here under
consideration is in the vicinity of Camp Douglas and Necedah (see Plate
I). This is indeed one of the best examples of a base-leveled plain
known. Here the broad plain, extending in some directions as far as the
eye can reach, is as low as it could be reduced by the streams which
developed it. The erosion cycle which produced the plain was, however,
not completed, for above the plain rise a few conspicuous hills (Plates
XVII and XVIII, and Fig. 17), and to the west of it lie the highlands
marking the level from which the low plain was reduced.
Where a region has been clearly base-leveled, isolated masses or ridges
of resistant rock may still stand out conspicuously above it. The
quartzite hill at Necedah is an example. Such hills are known as
_monadnocks_. This name was taken from Mount Monadnock which owes its
origin to the removal of the surrounding less resistant beds. The name
has now become generic. Many of the isolated hills on the peneplain east
of Camp Douglas are perhaps due to superior resistance, though the rock
of which they are composed belongs to the same formation as that which
has been removed.
[Illustration: Fig. 17.--Sketch, looking northwest from Camp Douglas.]
CHARACTERISTICS OF VALLEYS AT VARIOUS STAGES OF DEVELOPMENT.
In the early stages of its development a depression made by erosion has
steep lateral slopes, the exact character of which is determined by many
considerations. Its normal cross-section is usually described as
V-shaped (Fig. 18). In the early stages of its development, especially
if in unconsolidated material, the slopes are normally convex inward. If
cut in solid rock, the cross section may be the same, though many
variations are likely to appear, due especially to the structure of the
rock and to inequalities of hardness. If a stream be swift enough to
carry off not only all the detritus descending from its slopes, but to
abrade its bed effectively besides, a steep-sided gorge develops. If it
becomes deep, it is a canyon. For the development of a canyon, the
material of the walls must be such as is capable of standing at a high
angle. A canyon always indicates that the down-cutting of a stream keeps
well ahead of the widening.
[Illustration: Fig. 18.--Diagrammatic cross-section of a young valley.]
Of young valleys in loose material (drift) there are many examples in
the eastern portion of the area here described. Shallow canyons or
gorges in rock are also found. The gorge of Skillett creek at and above
the Pewit's nest about three miles southwest from Baraboo, the gorge of
Dell creek two miles south of Kilbourn City, and the Dalles of the
Wisconsin at Kilbourn City may serve as illustrations of this type of
valley.
[Illustration: Fig. 19.--Diagrammatic profile of a young valley.]
The profile of a valley at the stage of its development corresponding to
the above section is represented diagrammatically by the curve A B in
Fig. 19. The sketch (Pl. XIX, Fig. 1) represents a bird's-eye view of a
valley in the same stage of development.
[Illustration: Fig. 20.--Diagrammatic cross-section of a valley at a
stage corresponding with that shown in Plate XIX, Fig. 2.]
At a stage of development later than that represented by the V-shaped
cross-section, the corresponding section is U-shaped, as shown in Fig.
20. The same form is sketched in Plate XIX, Fig. 2. This represents a
stage of development where detritus descending the slopes is not all
carried away by the stream, and where the valley is being widened faster
than it is deepened. Its slopes are therefore becoming gentler. The
profile of the valley at this stage would be much the same as that in
the preceding, except that the gradient in the lower portion would be
lower.
Still later the cross section of the valley assumes the shape shown in
Fig. 21, and in perspective the form sketched in Plate XX, Fig. 1. This
transformation is effected partly by erosion, and partly by deposition
in the valley. When a stream has cut its valley as low as conditions
allow, it becomes sluggish. A sluggish stream is easily turned from side
to side, and, directed against its banks, it may undercut them, causing
them to recede at the point of undercutting. In its meanderings, it
undercuts at various points at various times, and the aggregate result
is the widening of the valley. By this process alone the stream would
develop a flat grade. At the same time all the drainage which comes in
at the sides tends to carry the walls of the valley farther from its
axis.
[Illustration: Fig. 21.--Diagrammatic cross-section of a valley at a
stage later than that shown in Fig. 20.]
A sluggish stream is also generally a depositing stream. Its deposits
tend to aggrade (build up) the flat which its meanderings develop. When
a valley bottom is built up, it becomes wider at the same time, for the
valley is, as a rule, wider at any given level than at any lower one.
Thus the U-shaped valley is finally converted into a valley with a flat
bottom, the flat being due in large part to erosion, and in smaller part
to deposition. Under exceptional circumstances the relative importance
of these two factors may be reversed.
It will be seen that the cross-section of a valley affords a clue to its
age. A valley without a flat is young, and increasing age is indicated
by increasing width. Valleys illustrating all stages of development are
to be found in the Devil's lake region. The valley of Honey creek
southwest of Devil's lake may be taken as an illustration of a valley at
an intermediate stage of development, while examples of old valleys are
found in the flat country about Camp Douglas and Necedah.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XIX.
FIG. 1.
Sketch of a valley at the stage of development corresponding to the
cross section shown in Fig. 18.
Illustration: FIG. 2.
Sketch of a valley at the stage of development corresponding to the
cross section shown in Fig. 20.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XX.
FIG. 1.
Sketch of a part of a valley at the stage of development corresponding
to the cross section shown in Fig 21.
Illustration: FIG. 2.
Sketch of a section of the Baraboo valley.]
_Transportation and Deposition._
Sediment is carried by streams in two ways: (1) by being rolled along
the bottom, and (2) by being held in suspension. Dissolved mineral
matter (which is not sediment) is also carried in the water. By means of
that rolled along the bottom and carried in suspension, especially the
former, the stream as already stated abrades its bed.
The transporting power of a stream of given size varies with its
velocity. Increase in the declivity or the volume of a stream increases
its velocity and therefore its transportive power. The transportation
effected by a stream is influenced (1) by its transporting power, and (2)
by the size and amount of material available for carriage. Fine material
is carried with a less expenditure of energy than an equal amount of
coarse. With the same expenditure of energy therefore a stream can carry
a greater amount of the former than of the latter.
Since the transportation effected by a stream is dependent on its
gradient, its size, and the size and amount of material available, it
follows that when these conditions change so as to decrease the carrying
power of the river, deposition will follow, if the stream was previously
fully loaded. In other words, a stream will deposit when it becomes
overloaded.
Overloading may come about in the following ways: (1) By decrease in
gradient, checking velocity and therefore carrying power; (2) by
decrease in amount of water, which may result from evaporation,
absorption, etc.; (3) by change in the shape of the channel, so that the
friction of flow is increased, and therefore the force available for
transportation lessened; (4) by lateral drainage bringing in more
sediment than the main stream can carry; (5) by change in the character
of the material to which the stream has access; for if it becomes finer,
the coarse material previously carried will be dropped, and the fine
taken; and (6) by the checking of velocity when a stream flows into a
body of standing water.
_Topographic forms resulting from stream deposition._--The topographic
forms resulting from stream deposition are various. At the bottoms of
steep slopes, temporary streams build _alluvial cones_ or _fans_. Along
its flood-plain portion, a stream deposits more or less sediment on its
flats. The part played by deposition in building a river flat has
already been alluded to. A depositing stream often wanders about in an
apparently aimless way across its flood plain. At the bends in its
course, cutting is often taking place on the outside of a curve while
deposition is going on in the inside. The valley of the Baraboo
illustrates this process of cutting and building. Fig. 2, Plate XX, is
based upon the features of the valley within the city of Baraboo.
Besides depositing on its flood-plain, a stream often deposits in its
channel. Any obstruction of a channel which checks the current of a
loaded stream occasions deposition. In this way "bars" are formed. Once
started, the bar increases in size, for it becomes an obstacle to flow,
and so the cause of its own growth. It may be built up nearly to the
surface of the stream, and in low water, it may become an island by the
depression of the surface water. In some parts of its course, as about
Merrimac, the Wisconsin river is marked by such islands at low water,
and by a much larger number of bars.
At their debouchures, streams give up their loads of sediment. Under
favorable conditions deltas are built, but delta-building has not
entered into the physical history of this region to any notable extent.
_Rejuvenation of Streams._
After the development of a base-level plain, its surface would suffer
little change (except that effected by underground water) so long as it
maintained its position. But if, after its development, a base-level
plain were elevated, the old surface in a new position would be subject
to a new series of changes identical in kind with those which had gone
before. The elevation would give the established streams greater fall,
and they would reassume the characteristics of youth. The greater fall
would accelerate their velocities; the increased velocities would entail
increased erosion; increased erosion would result in the deepening of
the valleys, and the deepening of the valleys would lead to the
roughening of the surface. But in the course of time, the _rejuvenated_
streams would have cut their valleys as low as the new altitude of the
land permitted, that is, to a new base-level. The process of deepening
would then stop, and the limit of vertical relief which the streams were
capable of developing, would be attained. But the valleys would not stop
widening when they stopped deepening, and as they widened, the
intervening divides would become narrower, and ultimately lower. In the
course of time they would be destroyed, giving rise to a new level
surface much below the old one, but developed in the same position which
the old one occupied when it originated; that is, a position but little
above sea level.
If at some intermediate stage in the development of a second base-level
plain, say at a time when the streams, rejuvenated by uplift, had
brought half the elevated surface down to a new base-level, another
uplift were to occur, the half completed cycle would be brought to an
end, and a new one begun. The streams would again be quickened, and as a
result they would promptly cut new and deeper channels in the bottoms of
the great valleys which had already been developed. The topography which
would result is suggested by the following diagram (Fig. 22) which
illustrates the cross-section which would be found after the following
sequence of events: (1) The development of a base-level, A A; (2)
uplift, rejuvenation of the streams, and a new cycle of erosion half
completed, the new base-level being at B B; (3) a second uplift,
bringing the second (incomplete) cycle of erosion to a close, and by
rejuvenating the streams, inaugurating the third cycle. As represented
in the diagram, the third cycle has not progressed far, being
represented only by the narrow valley C. The base-level is now 2-2, and
the valley represented in the diagram has not yet reached it.
[Illustration: Fig. 22.--Diagram (cross-section), illustrating the
topographic effect of rejuvenation by uplift.]
[Illustration: Fig. 23.--Normal profile of a valley bottom in a
non-mountainous region.]
The rejuvenation of a stream shows itself in another way. The normal
profile of a valley bottom in a non-mountainous region is a gentle
curve, concave upward with gradient increasing from debouchure to
source. Such a profile is shown in Fig. 23. Fig. 24, on the other hand,
is the profile of a rejuvenated stream. The valley once had a profile
similar to that shown in Fig. 23. Below B its former continuation is
marked by the dotted line B C. Since rejuvenation the stream has
deepened the lower part of its valley, and established there a profile
in harmony with the new conditions. The upper end of the new curve has
not yet reached beyond B.
[Illustration: Fig. 24.--Profile of a stream rejuvenated by uplift.]
_Underground Water._
In what has preceded, reference has been made only to the results
accomplished by the water which runs off over the surface. The water
which sinks beneath it is, however, of no small importance in reducing a
land surface. The enormous amount of mineral matter in solution in
spring water bears witness to the efficiency of the ground water in
dissolving rock, for since the water did not contain the mineral matter
when it entered the soil, it must have acquired it below the surface. By
this means alone, areas of more soluble rock are lowered below those of
less solubility. Furthermore, the water is still active as a solvent
agent after a surface has been reduced to so low a gradient that the
run-off ceases to erode mechanically.