[Illustration:
FRONTISPIECE.—Half-stereogram of Mount Ellsworth, drawn to illustrate
the form of the displacement and the progress of the erosion.
The base of the figure represents the sea-level. The remote half shows
the result of uplift alone; the near half, the result of uplift and
erosion,
or the actual condition. (See page 95.)
]
DEPARTMENT OF THE INTERIOR.
U. S. GEOGRAPHICAL AND GEOLOGICAL SURVEY OF THE ROCKY MOUNTAIN REGION.
J. W. POWELL, IN CHARGE.
REPORT
ON THE
GEOLOGY OF THE HENRY MOUNTAINS.
By G. K. GILBERT.
[Illustration: [Logo]]
WASHINGTON:
GOVERNMENT PRINTING OFFICE.
1877.
DEPARTMENT OF THE INTERIOR,
U. S. GEOGRAPHICAL AND GEOLOGICAL SURVEY
OF THE ROCKY MOUNTAIN REGION,
_Washington, D. C., March 5, 1877_.
SIR: I have the honor to transmit herewith a report on the Geology of
the Henry Mountains, by Mr. G. K. Gilbert.
I am, with great respect, your obedient servant,
J. W. POWELL,
_In charge_.
The Hon. SECRETARY OF THE INTERIOR,
_Washington, D. C._
DEPARTMENT OF THE INTERIOR,
U. S. GEOGRAPHICAL AND GEOLOGICAL SURVEY
OF THE ROCKY MOUNTAIN REGION,
_Washington, D. C., March 1, 1877_.
DEAR SIR: I submit herewith my report on the Geology of the Henry
Mountains, prepared from material gathered under your direction in the
years 1875 and 1876.
I am, with great respect, your obedient servant,
G. K. GILBERT.
Prof. J. W. POWELL,
_In charge_.
PREFACE.
If these pages fail to give a correct account of the structure of the
Henry Mountains the fault is mine and I have no excuse. In all the
earlier exploration of the Rocky Mountain Region, as well as in much of
the more recent survey, the geologist has merely accompanied the
geographer and has had no voice in the determination of either the route
or the rate of travel. When the structure of a mountain was in doubt he
was rarely able to visit the points which should resolve the doubt, but
was compelled to turn regretfully away. Not so in the survey of the
Henry Mountains. Geological exploration had shown that they were well
disposed for examination, and that they promised to give the key to a
type of structure which was at best obscurely known; and I was sent by
Professor Powell to make a study of them, without restriction as to my
order or method. I was limited only in time, the snow stopping my work
two months after it was begun. Two months would be far too short a
period in which to survey a thousand square miles in Pennsylvania or
Illinois, but among the Colorado Plateaus it proved sufficient. A few
comprehensive views from mountain tops gave the general distribution of
the formations, and the remainder of the time was spent in the
examination of the localities which best displayed the peculiar features
of the structure. So thorough was the display and so satisfactory the
examination, that in preparing my report I have felt less than ever
before the desire to revisit the field and prove my conclusions by more
extended observation.
In the description of the details of the structure a demand arose for a
greater number of geographic titles than were readily suggested by
natural forms or other accidents, and recourse was had to the names of
geologists. Except that the present members of my own corps are not
included, the names chosen are of those whose cognate studies have given
me most aid. Mr. Steward and Mr. Howell saw the Henry Mountains before I
did, and gleaned something of their structure from a distance; Dr.
Newberry, Mr. Marvine, Dr. Peale, and Mr. Holmes have described allied
phenomena in Colorado and New Mexico; and the works of Messrs. Jukes,
Geikie, Scrope, and Dana have been among my chief sources of information
in regard to igneous mountains in general. If any of these gentlemen
feel offended that their names have been attached to natural features so
insignificant, I can assure them that the affront will never be repeated
by the future denizens of the region. The herders who build their hut at
the base of the Newberry Arch are sure to call it “the Cedar Knoll”; the
Jukes Butte will be dubbed “Pilot Knob”, and the Scrope, “Rocky Point”.
During the preparation of my report every part of the discussion has
been submitted to Professor Powell for criticism, and many of his
suggestions are embodied in the text. Similar and valuable aid was
received from Capt. C. E. Dutton and Mr. William B. Taylor in the study
of the physical problems to which the discussion of the intrusive
phenomena gave rise. Captain Dutton rendered an important service also
by the study of the collection of igneous rocks, and his report,
included in the fourth chapter, testifies to the thoroughness of his
work. The supervision of the publication has fallen in large share upon
Mr. J. C. Pilling, and the text has had the advantage of his literary
criticism, as well as of his watchful care.
G. K. G.
CONTENTS.
Page.
CHAPTER I. INTRODUCTORY 1
The rock series 3
Unconformities 8
The Great Folds 11
Cliffs and plateaus 13
How to reach the Henry Mountains 14
CHAPTER II. STRUCTURE OF THE HENRY MOUNTAINS 18
CHAPTER III. DETAILED DESCRIPTION OF THE MOUNTAINS 22
Mount Ellsworth 22
Mount Holmes 27
Mount Hillers 30
Mount Pennell 35
Mount Ellen 38
Stereogram of the Henry Mountains 49
CHAPTER IV. THE LACCOLITE 51
The Henry Mountains intrusives, by _Captain C. E. Dutton_ 61
The question of cause 72
The stretching of strata 80
The conditions of rock flexure 83
The question of cover and the question of age 84
The history of the laccolite 95
Laccolites of other regions 97
Possible analogues of the laccolite 98
CHAPTER V. LAND SCULPTURE 99
I. Erosion 99
A. Processes of erosion 99
Weathering 100
Transportation 101
Corrasion 101
B. Conditions controlling erosion 102
Rate of erosion and declivity 102
Rate of erosion and rock texture 103
Rate of erosion and climate 103
Transportation and comminution 106
Transportation and declivity 108
Transportation and quantity of water 109
Corrasion and transportation 111
Corrasion and declivity 112
Declivity and quantity of water 113
II. Sculpture 115
Sculpture and declivity 115
The law of structure 115
The law of divides 116
Sculpture and climate 117
Bad-lands 120
Equal action and interdependence 123
III. Systems of drainage 124
The stability of drainage lines 124
The instability of drainage lines 125
The stability of divides 138
The instability of divides 139
Consequent and inconsequent drainage 143
The drainage of the Henry Mountains 144
CHAPTER VI. ECONOMIC 151
CHAPTER I.
INTRODUCTORY.
The Henry Mountains have been visited only by the explorer. Previous to
1869 they were not placed upon any map, nor was mention made of them in
any of the published accounts of exploration or survey in the Rocky
Mountain region. In that year Professor Powell while descending the
Colorado River in boats passed near their foot, and gave to them the
name which they bear in honor of Prof. Joseph Henry, the distinguished
physicist. In 1872 Prof A. H. Thompson, engaged in the continuance of
the survey of the river, led a party across the mountains by the
Penellen Pass, and climbed some of the highest peaks. Frontiersmen in
search of farming and grazing lands or of the precious metals have since
that time paid several visits to the mountains; but no survey was made
of them until the years 1875 and 1876, when Mr. Walter H. Graves and the
writer visited them for that purpose.
They are situated in Southern Utah, and are crossed by the meridian of
110° 45′ and the thirty-eighth parallel. They stand upon the right bank
of the Colorado River of the West, and between its tributaries, the
Dirty Devil and the Escalante.
At the time of their discovery by Professor Powell the mountains were in
the center of the largest unexplored district in the territory of the
United States—a district which by its peculiar ruggedness had turned
aside all previous travelers. Up to that time the greater part of the
knowledge that had been gained of the interior of the continent had been
acquired in the search for routes for transcontinental railways; and the
cañons of the Colorado Basin, opposing more serious obstacles to travel
than the mountain ranges which were met in other latitudes, were by
common consent avoided by the engineers.
The same general causes which have rendered the region so difficult of
access and passage have made it a desert, almost without economic value.
The physical conditions of elevation and aridity which have caused it to
be so deeply carved in cañons, have prevented the streams with which it
is scantily watered from being bordered by tracts of land which can be
irrigated; and agriculture without irrigation being in that climate an
impossibility, there is nothing to attract the farmer. As will be
explained in the sequel, the mountains offer no inducements to the miner
of the precious metals. There is timber upon their flanks and there is
coal near at hand, but both are too far removed from other economic
interests to find the market which would give them value. It is only for
purposes of grazing that they can be said to have a money value, and so
distant are they at present from any market that even that value is
small.
But while the Henry Mountains contribute almost nothing to our direct
material interests, they offer in common with the plateaus which
surround them a field of surpassing interest to the student of
structural geology. The deep carving of the land which renders it so
inhospitable to the traveler and the settler, is to the geologist a
dissection which lays bare the very anatomy of the rocks, and the dry
climate which makes the region a naked desert, soilless and almost
plantless, perfects the preparation for his examination.
The study of the mountains is further facilitated by their isolation.
They mark a limited system of disturbances, which interrupt a region of
geological calm, and structurally, as well as topographically, stand by
themselves.
The Henry Mountains are not a range, and have no trend; they are simply
a group of five individual mountains, separated by low passes and
arranged without discernible system. The highest rise about 5,000 feet
above the plateau at their base and 11,000 feet above the level of the
ocean. Projecting so far above the surface of the desert, they act as
local condensers of moisture, and receive a comparatively generous
supply of rain. Springs abound upon their flanks, and their upper slopes
are clothed with a luxuriant herbage and with groves of timber. The
smaller mountains and the foot-hills of the larger are less generously
watered and but scantily clothed with vegetation. Their extent is small.
From Ellen Peak to Mount Ellsworth, the two summits which are the most
widely separated, the distance is but twenty-eight miles, and a circle
of eighteen miles radius will include the whole group.
Mount Ellen which is the most northerly of the group, has an extreme
altitude of 11,250 feet, and surpasses all its companions in horizontal
extent as well as altitude. Its crest-line is continuous for two miles,
with an elevation varying little from 11,000 feet. From it there radiate
spurs in all directions, descending to a series of foot-hills as
conspicuous in their topography as they are interesting in their
structure. In some places the base of the mountain is guarded by a
continuous, steep ridge, through which a passage must be sought by the
approaching traveler, but within which movement in any direction is
comparatively unimpeded.
Mount Pennell is a single peak rising to an altitude of 11,150 feet. On
one side its slopes join those of Mount Ellen in Pennellen Pass (7,550
feet), and on the other those of Mount Hillers in the Dinah Creek Pass
(7,300 feet). Its profiles are simple, and it lacks the salient spurs
that characterize Mount Ellen. From the west it is difficult of
approach, being guarded by a barrier ridge continuous with that of Mount
Ellen.
Mount Hillers is more rugged in its character, and although compact in
its general form, is carved in deep gorges and massive spurs. Its
rugosity is contrasted by the smoothness of its pedestal, which to the
south and west and north is a sloping plain merging with the surrounding
plateau.
Mount Ellsworth (8,000 feet) and Mount Holmes (7,750 feet) stand close
together, but at a little distance from the others. The pass which
separates them from Mount Hillers has an altitude of 5,250 feet. They
are single peaks, peculiarly rugged in their forms, and unwatered by
springs. They stand almost upon the brink of the Colorado, which here
flows through a cañon 1,500 feet in depth.
THE ROCK SERIES.
The sedimentary rocks which occur in the Henry Mountains and their
immediate vicinity, range from the summit of the Cretaceous to the
summit of the Carboniferous. It is probable that they were covered at
one time by some thousands of feet of Tertiary strata, but from the
immediate banks of the Colorado these have been entirely eroded, and
their nearest vestiges lie thirty miles to the westward, where they have
been protected by the lava-beds of the Aquarius Plateau.
_Cretaceous._—The Cretaceous strata do not reach to the Colorado River,
but they extend to the Henry Mountains, and are well displayed upon the
flanks. They include four principal sandstones, with intervening shales,
in the following (descending) order:
1. The Ma-suk′ Sandstone, yellow, heavy-bedded 500 feet.
2. The Ma-suk′ Shale, gray, argillaceous, and toward the
top slightly arenaceous 500 feet.
3. The Blue Gate Sandstone, yellow and heavy-bedded 500 feet.
4. The Blue Gate Shale, blue-black and argillaceous,
weathering to a fine gray clay (_Inoceramus deformis_
and _I. problematicus_) 1,000 feet.
5. The Tu-nunk′ Sandstone, yellow and heavy-bedded 100 feet.
6. The Tu-nunk′ Shale, blue-black and argillaceous,
weathering to a fine gray clay (_Inoceramus
problematicus_ and _Baculites anceps_) 400 feet.
7. The Henry’s Fork Group, consisting of—
_a._ Friable yellow sandstone with numerous fossils
(_Ostrea prudentia_, _Gryphea Pitcheri_, _Exogyra
læviuscula_, _Exogyra ponderosa_, _Plicatula
hydrotheca_, _Camptonectes platessa_, and _Callista
Deweyi_) 10 feet.
_b._ Arenaceous shales, purple, green, and white, with
local beds of conglomerate 190 feet.
_c._ Coarse sandstone and conglomerate, with many white
grains and pebbles, interleaved with local beds of
purple and red shale, and containing immense
silicified tree-trunks 300 feet.
—————
Total Cretaceous 3,500 feet.
The three upper sandstones, the Masuk, the Blue Gate, and the Tununk,
are so nearly identical in their lithologic characters that I was unable
to discriminate them in localities where their sequence was unknown.
This was especially the case upon the summits of Mounts Ellen and
Pennell where they occur in a somewhat metamorphosed condition. All of
them contain thin beds of coal, none of which are continuous over large
areas, and only one of which was observed of workable thickness. At the
western foot of Mount Ellen, a bed four feet thick lies at the base of
the Blue Gate Sandstone.
There is almost equal difficulty in discriminating the Masuk, the Blue
Gate, and the Tununk shales. The first is usually of a paler color and
is more apt to include arenaceous bands. It has not been found to
contain fossils, while the lower shales rarely fail to afford them when
search is made. The Blue Gate and Tununk shales are typical examples of
fine argillaceous sediments. They are beautifully laminated and are
remarkably homogeneous. It is only in fresh escarpments that the
lamination is seen, the weathered surface presenting a structureless
clay. The fossils of these shales are so numerous, when they have been
sought out and studied, that they will probably serve not merely to
discriminate the two, but also to correlate them with some of the beds
which have been examined elsewhere in the Colorado Basin. For the
present I am unable to refer any of the Cretaceous rocks above the
Henry’s Fork Group to the divisions which have been recognized
elsewhere, and it is for this reason that I have given local, and
perhaps temporary names to such beds as I have need to mention in the
discussion of the structure of the mountains.
The fossils of the Henry’s Fork Group have been more fully collected,
and they have been referred without question by Dr. White to the group
of that name, as recognized in the Green River Basin (Geology of the
Uinta Mountains, pp. 82 and 94). The white grit which lies at the base
of the group is a conspicuous bed of unusual persistence, and is
recognized wherever Cretaceous rocks are found in the upper basin of the
Colorado.
_Jura-Trias._—The rocks which intervene between the base of the
Cretaceous and the summit of the Carboniferous are of doubtful age,
having been referred to the Trias by one geologist and to the Jura and
Trias by others, while the fossils recently discovered by Mr. Howell
(Geology of Uinta Mountains, page 80) lead to the suspicion at least
that they are all Jurassic. It is probable that the uncertainty will
soon be dispelled by the more thorough working of Mr. Howell’s new
localities; but while it remains, it seems best to recognize its
existence in our nomenclature, and I shall include the whole of the
doubtful series under the title of _Jura-Trias_. At the Henry Mountains
it is easily divided into four groups, as follows:
1. Flaming Gorge Group; arenaceous shales or bad-land
sandstones, purple and white at top and red below 1,200 feet.
2. Gray Cliff Group; massive cross-laminated sandstone,
buff to red in color 500 feet.
3. Vermilion Cliff Group; massive cross-laminated
sandstone, red, with a purple band at the top 500 feet.
4. Shin-ar´-ump Group; consisting of—
_a._ Variegated clay shale; purple and white above and
chocolate below, with silicified wood 300 feet.
_b._ Gray conglomerate, with silicified wood; the
“Shinarump Conglomerate” 30 feet.
_c._ Chocolate-colored shale, in part sandy 400 feet.
—————
Total Jura-Trias 2,930 feet.
The rock of the Flaming Gorge Group is of a peculiar character. It is
ordinarily so soft that in its manner of weathering it appears to be a
shale. It is eroded so much more rapidly than the Henry’s Fork
conglomerate above it, that the latter is undermined, and always appears
in the topography as the cap of a cliff. Nevertheless, it is not
strictly speaking a shale. The chief product of its weathering is sand,
and wherever it can be examined in an unweathered condition it is found
to be a fine-grained sandstone, massive and cross-laminated like those
of the Gray and Vermilion Cliffs, but devoid of a firm cement. In a
number of localities it has acquired, locally and accidentally, a
cement, and it is there hardly distinguishable from the firmer
sandstones which underlie it. In the immediate vicinity of the Henry
Mountains it varies little except in color from summit to base, but in
other localities not far distant it is interrupted near the base by
thick beds of gypsum and gypsiferous clays, and by a sectile,
fossiliferous limestone.
The Gray Cliff and Vermilion Cliff sandstones are often difficult to
distinguish, but the latter is usually the firmer, standing in bold
relief in the topography, with level top, and at its edge a precipitous
face. The former is apt to weather into a wilderness of dome-like
pinnacles, so steepsided that they cannot often be scaled by the
experienced mountaineer, and separated by narrow clefts which are
equally impassable.
The colors of the two sandstones are not invariable. The lower, which
although not reddened throughout its mass is usually stained upon its
surface with a uniform deep color, appears in Mount Ellsworth and at
other points of elevation with as pale a tint as that of the Gray Cliff.
The latter sandstone, on the other hand, where it lies low, is often as
deep in color as the Vermilion. Standing upon one of the summits of the
Henry Mountains and looking eastward, I found myself unable to
distinguish the Gray Cliff Sandstone by color either from the lower part
of the Flaming Gorge Group or from the Vermilion Sandstone. The
bleaching of the redder sandstone in Mount Ellsworth is probably a
result of metamorphism; the reddening of the gray sandstone may depend
on the hydration of the iron which it contains.
The thickness of individual strata in these great sandstones is
remarkable, and is one of the elements which must be taken into account
in the discussion of the problem—which to my mind is yet unsolved—of the
manner in which such immense quantities of homogeneous sand were
accumulated. Ordinarily the depth of strata is indefinable, on account
of the impossibility of distinguishing stratification from lamination;
but where, as in this case, the lamination is oblique to the
stratification, the upper and lower limits of each stratum are
definitely marked. I have at several points measured single strata with
thicknesses of about fifty feet, and near Waterpocket Cañon a stratum of
Vermilion Cliff sandstone was found to be 105 feet thick.
One other measurement is worthy of record; the inclination which oblique
lamination bears to the plane of the stratum in which it occurs appears
to have a definite limit. The maximum of a series of measurements made
at points where to the eye the dip seemed to be unusually great, is 24°.
The sandy layers at the base of the Shinarump Group are characterized by
profuse ripple-marks.
_Carboniferous._—Beneath the Jura-Trias is the Carboniferous. A few
hundred feet of its upper member, the Aubrey Sandstone, are exposed near
the summit of Mount Ellsworth. At that point the sandstone is altered to
the condition of a quartzite, but where it is cut by the upper and lower
cañons of the Dirty Devil River it is massive and cross-laminated,
differing from the Gray Cliff sandstone chiefly in the abundance of its
calcareous cement.
UNCONFORMITIES.
From the Masuk Sandstone to the Aubrey Sandstone, inclusive, there is
perfect conformity of dip. The fold system of the region, of which a
description will be found in succeeding pages, was established after the
deposition of all these strata, and the whole series were flexed
together. Nevertheless, the strata do not represent continuous
deposition. There were intervals in which the sea receded and exposed to
erosion the sediments which it had accumulated. Shallow valleys and
waterways were excavated, and when the sea returned it deposited new
sediments upon the somewhat uneven surface of the old.
The first occurrence of this sort was at the close of the Aubrey epoch.
Its evidence was not found in the Henry Mountains; but at the confluence
of the Paria with the Colorado, eighty miles to the southeast, the
surface of the upper member of the Aubrey Group, which is there a cherty
limestone, is unevenly worn, and in its depressions are beds of
conglomerate, the pebbles of which are derived from the chert of the
limestone itself. The shaly, rippled sandstones which succeed this
conglomerate indicate that the water remained shallow for a time, and in
the middle of the Shinarump epoch the region was once more abandoned by
the sea. The chocolate shales and shaly sands were unevenly worn, and
the first deposit that the returning sea spread over them was a
conglomerate. The evidence of this break is found at many points. The
Shinarump conglomerate although remarkably persistent for a conglomerate
thins out and disappears at a number of points, and at the margins of
its areas it is evident to the eye that it occupies depressions of the
surface on which it rests.
The next break is at the base of the Vermilion Cliff Group. In the
region of the Virgin River and Kanab Creek the change from the
variegated shales of the Upper Shinarump to the homogeneous sandstone of
the Vermilion Cliff is gradual, the interval being filled by a series of
alternating shales and sandstones; but further to the east, in the
region of the Henry Mountains and Waterpocket Cañon, the change is
abrupt, and the firm sandstone rests directly upon the soft shale. The
abruptness of the change would suggest that the currents which brought
the sand had swept away all evidence of the intermediate conditions
which are likely to have connected the epochs represented by the two
sediments; but in one locality, at least, there is direct evidence that
the surface of the clay was exposed to the air before it was covered by
the sand. On the northern flank of Mount Ellsworth are the vestiges of a
system of mud-cracks, such as form where wet clays are dried in the sun.
Where the under surface of the Vermilion sandstone is exposed to view,
it is seen to be marked by a network of ridges which once occupied the
sun-cracks of the Shinarump clay; and where the clay is seen in
juxtaposition, tapering fillets of sand can be traced from the ridges
downward ten feet into the clay.
[Illustration:
FIG. 1.—Fossil Suncracks in the Shinarump Shale.
]
From the base of the Vermilion to the summit of the Cretaceous no
evidence of land erosion has been found; but the association of coal
seams with all of the Cretaceous sandstones except the lowest, shows
that the sea-bottom was frequently brought to the surface of the water
if it was not carried above.
Thus it is evident that the strata of the Henry Mountain region do not
represent continuous sedimentation. At the close of the Aubrey epoch, in
the middle of the Shinarump, and again at the close of the Shinarump,
not merely was the accumulation of sediments interrupted, but the
process was reversed, and a portion of the deposits which had already
been formed were excavated by the agency of rains and rivers, and swept
away to some other region. Each break is indefinite, alike as regards
the interval during which the record of the sea was interrupted and as
regards the extent of the record which was at the same time obliterated.
And yet the evidence of these breaks is of such nature that it would
probably elude observation if a single section only were examined, and
in a region masked by the soil and vegetation of a humid climate it
would hardly be discovered except by accident. The parallelism of
contiguous strata is not alone sufficient evidence that they were
consecutive in time.
At the close of the Cretaceous period there came an epoch of
disturbance. The system of strata which has been described was bent into
great waves, and the crests of the waves were lifted so high above the
sea that they lost thousands of feet by erosion.
In the troughs between the waves lakes remained, in which the material
removed from the crests was redeposited, and by a later change the lake
waters rose so as to cover the truncated crests, and deposit upon the
worn edges of the upbent strata a series of unconforming, fresh-water,
Tertiary sediments.
Thus was produced the only _unconformity of dip_ which involves the
Henry Mountain rocks, and even this is not to be observed in the
immediate vicinity of the mountains, for a later erosion has thence
removed all of the Tertiary strata, and has resumed the degradation of
the older beds.
THE GREAT FOLDS.
The disturbances at the close of the Cretaceous period were of the
Kaibab type[1]. It seems as though the crust of the earth had been
divided into great blocks, each many miles in extent, which were moved
from their original positions in various ways. Some were carried up and
others down, and the majority were left higher at one margin than at the
other. But although they moved independently, they were not cleft
asunder. The strata remained continuous, and were flexed instead of
faulted at the margins of the blocks. Subsequent erosion has obliterated
in great part the inequality of the surface, and the higher-lying blocks
do not stand as mountains, but are outlined by zones of tilted strata
which mark the flexures by which the blocks are separated. Along the
zones of flexure it frequently happens that a hard stratum outcropping
between two that are softer will be preserved from erosion and form a
long, continuous ridge. Such ridges, and other forms produced by the
erosion of the flexures, are conspicuous features of the topography, and
the tracing out of the limits of the blocks is a simple matter. Indeed
the flexures are the first elements of the structure to attract
attention, and it is easy in studying them to overlook the fact that
they merely mark the limits between displaced masses of great extent. If
the reader will examine Plate I at the end of the volume, he will
observe that the system of parallel ridges and valleys which follow the
line of the Waterpocket flexure are very conspicuous features; but it is
only by some such generalization as that given in the stereogram of the
same region (Plate II) that the full structural significance of the
flexures can be realized. Each map was obtained by photography from a
model in relief, in which the proportionate heights of the several
features were not exaggerated. The stereogram was produced by the
restoration of the top of the Cretaceous, the Masuk sandstone, in the
form and position it would have, had there been no erosion of the
region, but displacement only.
Footnote 1:
For a definition of the Kaibab structure, see “Geology of the Uinta
Mountains,” pp. 14 and 17, and American Journal of Science for July
and August, 1876, pp. 21 and 85.
I must caution the reader against an implication of rigidity which might
attach to the meaning of the word “block”, as I have used it in speaking
of the great displaced rock masses. To what extent they may be regarded
as rigid is uncertain, but the presence upon their surfaces of numerous
minor flexures, such as appear in the stereogram, would seem to imply
that their rigidity is not of a high order.
In the northwest corner of the area represented by the stereogram are a
few faults belonging to a system which occupies a large area in that
direction. The system of faults and the system of flexures are
independent, the latter having originated at the close of the Cretaceous
period, and the former after the formation of the Tertiary rocks of the
region, which are referred by Professor Powell to the Bitter Creek
epoch. Over a large district the Tertiary strata were covered by a deep
mantle of lava, which has protected them from erosion to such an extent
that the structure of the district is portrayed in its topography. The
district is its own stereogram, each uplifted block constituting a
mountain and each depressed block flooring a valley.
Not all the displacements of the later system are by faulting, but by
far the greater number. Of the earlier system of displacements none are
simple faults, but a few are combinations of fault and flexure.
[Illustration:
FIG. 2.—Cross-section of the Waterpocket Flexure, opposite the Masuk
Plateau. Scale, one inch = 3,500 feet. 1, Masuk Sandstone. 2, Masuk
Shale. 3, Blue Gate Sandstone. 4, Blue Gate Shale. 5, Tununk
Sandstone. 6, Tununk Shale. a, Gryphea Sandstone. 7, Henry’s Fork
Conglomerate. 8, Flaming Gorge Shale. 9, Fossiliferous Limestone.
10, Gray Cliff Sandstone. 11, Vermilion Cliff Sandstone. 12, Upper
Shinarump Shale. 13, Shinarump Conglomerate. 14, Lower Shinarump
Shale. 15, Aubrey Sandstone.
]
[Illustration:
FIG. 3.—View of the Waterpocket Cañon and the Waterpocket Flexure. The
cliff at the left is capped by the Henry’s Fork Conglomerate. The
arched rocks at the right are the Gray and Vermilion Cliff
Sandstones.
]
[Illustration:
FIG. 4.—Waterpocket Flexure, as seen from the south end of Mount
Ellen.
]
The Waterpocket flexure, represented in the stereogram (Plate II), is
better known in detail than any other of the great flexures of Southern
Utah. It is far from following a straight line, but like most lines of
orographic disturbance swerves to the right and left, while maintaining
a general trend. The amount of its “throw”, or the difference in level
between adjacent parts of the two blocks which it divides, is
inconstant, its maximum being 7,000 feet. At some points the flexed
strata are inclined at an angle of 50° while at others their greatest
dip is but 15°. Toward the north the flexure twice divides. One of its
branches, the Blue Gate flexure, has a throw in the same direction, and
by its separation diminishes the throw of the main flexure. The other,
the Red Gate flexure, has a throw in the opposite direction, and by its
separation increases the throw of the main flexure. Or in other words,
the blocks at the west of the main flexure stand higher than those at
the east; and of two blocks which lie at the west, that at the north of
the Red Gate flexure stands higher than that at the south; while of two
blocks at the east, that which lies northwest of the Blue Gate flexure
is higher than the one at the southeast.
CLIFFS AND PLATEAUS.
Let us now return to the topographic map (Plate I) and examine the forms
into which erosion has wrought the disturbed strata. Thanks to the
aridity of the climate, the erosion has been greatly influenced by the
varying texture of the rocks. Every hard stratum, if inclined, stands
forth in a ridge, or if level, caps a plateau. The Masuk Sandstone,
undermined by the weathering of the Masuk Shale, breaks off everywhere
in a cliff which completely encircles the Masuk Plateau. The plateau
stands upon the Blue Gate Sandstone, and this breaking off in a cliff
upon all sides constitutes another plateau. The Blue Gate Plateau, in
turn, rests upon the Tununk, and that again upon the Henry’s Fork.
Passing either to the north or to the south from the Masuk Plateau, one
descends a great geological stairway, of which each step is a hard
sandstone and each riser a soft shale. Toward the Waterpocket flexure
the edge of each plateau is upturned, and if one goes westward from the
Masuk Plateau, he will cross in the first mile the upturned rocks of all
the lower tables.
The preservation of the Masuk Plateau is due in part to the fact that it
lies in a slight synclinal, but chiefly to the arrangement of the
drainage lines. No streams cross the Henry Mountains, but all go around,
and the plateau occupies the divide between those which flow southward
to the Colorado and those which flow northward to the Dirty Devil.
The antithesis of the Masuk Plateau is to be seen in the Circle Cliffs,
on the summit of the Waterpocket fold. At the lowest point of the Masuk
synclinal a circling cliff has been formed, which facing outward
surrounds a plateau. At the highest point of the Waterpocket fold, which
is in a certain sense anticlinal, a circling cliff has been formed
which, facing inward, surrounds a valley. The two phenomena are alike
illustrations of the law that in regions of inclined strata cliffs face
toward districts of elevation and away from districts of depression.
HOW TO REACH THE HENRY MOUNTAINS.
[Illustration:
FIG. 5.—Ways and Means.
]
No one but a geologist will ever profitably seek out the Henry
Mountains, and I will therefore, in marking out a route by which they
may be reached, select whenever there is option those paths which will
give him the best introduction to this wonderful land. There is no
wagon-road to the mountains, and although a wagon might carry his
baggage the greater part of the way, he must provide himself with other
means of transportation. At Salt Lake City he can procure pack-mules and
pack-saddles, or _apparajos_, and everything necessary for a mountain
“outfit”. His route southward follows the line of the Utah Southern
Railway to Juab Valley, and then touches the Mormon towns of Nephi,
Gunnison, and Salina. At Salina he halts his train for a day while he
rides a few miles up the creek to see the unconformity between the
Tertiary above, and the Jura-Trias and Cretaceous below. This is at
present the last settlement on the route, but there are “ranches” as far
as Rabbit Valley, and if he delays a few years he will find a town
there. By way of the “Twist” road and King’s Meadows he goes to Grass
Valley, and thence to Fish Lake. The lake lies between two upheaved
blocks of trachyte, and covers one which is relatively depressed, and
tilted to the north. At the south end of the lake he stands on the
higher end of the depressed block, and if he follows the shore to the
outlet at the north he will find that the water is contained by a
moraine, which has been thrown across the valley by an ancient glacier,
descending from the mountain at the west. From Fish Lake he goes to
Rabbit Valley, and there delays a day or two to climb Thousand Lake
Mountain. Looking west from the summit he sees the lava-capped plateaus
of the faulted district among which he has journeyed since he left the
“Twist”—huge tables of trachyte bounded by cliffs of displacement, of
which only the sharpest edges have been worn away; and when his eye has
become accustomed to the _facies_ of the faults, he perceives that there
is an identity in structure between the great and the small features.
Just as the whole district is divided into blocks, of which the
dimensions are measured by miles, and the displacements by thousands of
feet, so the greater blocks are sometimes divided into smaller, of which
the dimensions are measured by rods and furlongs and the displacements
by tens and hundreds of feet. Looking eastward he sees the region of the
great flexures spread out before him like a map. The Waterpocket flexure
starts from the very mountain beneath him, and curving to the right,
runs far to the south and is lost in the distance. Beyond it are the
Henry Mountains springing abruptly from the desert; and against the
horizon are outlined other island mountains, gray in the distance. To
the left is the San Rafael Fold, the rival of the Waterpocket in
grandeur; and all about are tables and cliffs. The vivid hues of the
naked rocks are obscured only by the desert haze, and the whole
structure is pictured forth by form and color.
[Illustration:
FIG. 6.—The Unconformity of the lower cañon of Salina Creek. The
horizontal strata are Tertiary; the inclined, Cretaceous.
]
To reach the Henry Mountains from Rabbit Valley, he must cross the
Waterpocket flexure; and so continuous and steep are the monoclinal
ridges which follow the line of flexure, that there are but four points
known where he can effect a passage. Except at these points, the barrier
is impassable from Thousand Lake Mountain to the Colorado River, a
distance of eighty miles. The most difficult and circuitous route I will
not describe. The remaining three diverge but slightly from each other.
Starting from Rabbit Valley he follows for a few miles the valley of the
Dirty Devil, which here, through the “Red Gate”, passes from the
trachyte plateaus and enters the land of cañons. He does not follow it
far, but where the river enters a cañon in the Aubrey Sandstone, bears
to the left, and by the aid of a trail which Indians have made finds a
sinuous but easy pathway along a monoclinal valley, following the
outcrop of the lower Shinarump. At his right the Aubrey Sandstone rises
to form the plateau through which the river defiles. At his left the
Vermilion Sandstone stands in a vertical wall. Beneath his feet are the
shaly sandstones of the Shinarump Group, bare of vegetation and
displaying a profusion of ripple-marks, such as is rarely if ever
equaled. A ride of twelve miles brings him once more to the Dirty Devil
River, which emerging from its Carboniferous cañon dives at once into a
still deeper cañon through the Vermilion and Gray Cliff Sandstones. He
can follow the river if he tries, and emerge with it beyond the flexure;
but the way is difficult and the Indian trail he has followed thus far
leads on to another cañon. The monoclinal valley which has opened so
easy a way continues for fifteen miles farther, and in that distance is
crossed by four waterways, each of which leads by a narrow cañon through
the great sandstones. The first and fourth are impassable. The second
carries no permanent stream, and is called the “Capitol Cañon”. The
third affords passage to Temple Creek. The smoothest road lies through
Capitol Cañon, but the Temple Creek Cañon has an advantage in the
presence of water, and is furthermore attractive by reason of the
picture-writings on the walls.
He has now to cross the Blue Gate flexure, and to do this he leaves
Temple Creek a little below the mouth of its cañon. Seeking once more
the guidance of a trail, he journeys southeastward over the gypsum and
sand of the Flaming Gorge Group to a pass which from a distance he has
detected in the monoclinal ridge marking the Henry’s Fork conglomerate.
Through this pass Tantalus Creek sometimes runs on its way to join the
Dirty Devil, and he may find a stream of muddy water; but the bottom is
more likely to be dry with the exception of a few pools. Passing through
the gap he finds before him a similar opening in the Tununk Ridge, and
beyond that a break in the Blue Gate Cliff. From Tantalus Creek he
ascends to the pass in the Blue Gate Cliff, and climbing to the summit
of a sharp divide in the shales descends again to Lewis Creek, which
there follows a cañon through the Blue Gate Plateau. Here he finds
bowlders of the Henry Mountain trachyte—for Lewis Creek rises in the
Henry Mountains—and a few hours’ ride toward the sources of the stream
brings him to the base of Mount Ellen.
_Distances._
From Salt Lake City to Salina 155 miles.
From Salina to Fish Lake 38 miles.
From Fish Lake to Rabbit Valley 27 miles.
From Rabbit Valley to Temple Creek Cañon 27 miles.
From Temple Creek Cañon to Lewis Creek 18 miles.
Thence to the base of Mount Ellen 10 miles.
———
Total from Salt Lake City to the Henry Mountains 275 miles.