Popular Science Monthly/Volume 7/August 1875/Physical Features of the Colorado Valley I
|←Notes|| Popular Science Monthly Volume 7 August 1875 (1875)
Physical Features of the Colorado Valley I
By J. W. Powell
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THE topographical features of the valley of the Colorado, or the area drained by the Colorado River and its tributaries, are, in many respects, unique, as some of these features, perhaps, are not re-produced, except to a very limited extent, on any other portion of the surface of the globe. Mountains, hills, plateaus, plains, and valleys, are here found, as elsewhere throughout the earth; but, in addition to these topographic elements in the scenic features of the region, we find buttes, outlying masses of stratified rocks, often of great altitude, not as dome-shaped or conical mounds, but usually having angular outlines; their sides are vertical walls, terraced or buttressed, and broken by deep, reentering angles, and often naked of soil and vegetation.
Then we find lines of cliffs, abrupt escarpments of rock, of great length and great height, revealing the cut edges of strata swept away from the lower side. Thirdly, we find cañons, narrow gorges, scores or hundreds of miles in length, and hundreds or thousands of feet in depth, with walls of precipitous rocks.
In the arid region of the Western portion of the United States, there are certain tracts of country which have received the name of Mauvaises Terres, or Bad Lands. These are dreary wastes—naked hills, with rounded or conical forms, composed of sand, sandy clays, or fine fragments of shaly rocks, with steep slopes, and, yielding to the pressure of the foot, they are climbed only by the greatest toil, and it is a labor of no inconsiderable magnitude to penetrate or cross such a district of country. The steep hills are crowded together, and the water-ways separating them are deep arroyas. Where the rocks, or sandy clays and shales, of which the hills are composed, are interstratified with occasional harder beds, the slopes are terraced; and when these thinly-bedded, though harder, rocks prevail, the ontlines
of the topography are changed, and present angular surfaces, and give rise to another type of topographic features, which I have denominated Alcove Lands,
The agencies and conditions under which all of these features have been formed deserve mention, and in this and following chapters I shall briefly discuss this subject, in a manner as free from technical terms as will be consistent with accurate description.
The discussion will by no means be exhaustive, and I hope hereafter to treat this subject in a more thorough manner. In view of these facts, I shall not attempt any logical classification of the elements of the topography, nor of the agencies and conditions under which they were produced; but, commencing at the north, at the initial point of the exploration, I shall take them up in geographic order, as we proceed down the river.
Bad Lands and Alcove Lands north of the Uinta Mountains.—The area north of the Uinta Mountains embraced in the survey is but small Through the middle of it runs Green River, in a deep, narrow valley, the sides or walls of which sometimes approach so near to each other, and are so precipitous, as to form a canon.
The general surface of the country, on the north of this district, is about 1,000 feet above the river, with peaks, here and there, rising a few hundred feet higher; but south, toward the Uinta Mountains, this general surface, within a few miles of the river, gradually descends, and at the foot of the mountains we find a valley on either side, with a direction transverse to that of the course of Green River, and parallel to the mountain-range.
To the north, the water-ways are all deeply eroded; the permanent streams have flood-plains of greater or lesser extent, but the channels of the wet-weather streams, i. e., those which are dry during the greater part of the year, are narrow, and much broken by abrupt falls.
The rocks arc the sediments of a dead lake, and are quite variable in lithologic characteristics. We find thinly-laminated shales, hard limestones, breaking with an angular fracture, crumbling Bad-Land rocks, and homogeneous, heavily-bedded sandstones.
The scenic features of the country are alike variable. On the cliffs about Green River City, towers and buttes are seen as you look from below, always regarded by the passing traveler as strange freaks of Nature. The limestones, interstratified with shales, give terraced and buttressed characteristics to the escarpments of the canons and narrow valleys.
Immediately south of Bitter Creek, on the east side of Green River, there is a small district of country which we have called the Alcove Land. On the east it is drained by Little Bitter Creek, a dry gulch much of the year. This runs north into Bitter Creek, a permanent stream, which empties into the Green. The crest of this water-shed is an irregular line, only two to four miles back from the river, but usually more than 1,000 feet above it, so that the waters have a rapid descent, and every shower-born rill has excavated a deep, narrow channel, and these narrow canons are so close to each other as to be separated by walls of rock so steep, in most places, that they cannot be scaled, and many of these little cañons are so broken by falls as to be impassable in either direction.
The whole country is cut, in this way, into irregular, angular blocks, standing as buttresses, benches, and towers, about deep water-ways and gloomy alcoves.
The conditions under which the cañons have been carved will be more elaborately discussed hereafter.
To the west of Green River, and back some miles, between Black's Fork and Henry's Fork, we have a region of buff, chocolate, and lead-colored Bad Lands. This Bad-Land country differs from the Alcove Land, above mentioned, in that its outlines are everywhere beautifully rounded, as the rocks of which it is composed crumble quickly under atmospheric agencies, so that an exposure of solid rock is rarely seen; but we have the same abrupt descent of the streams, and the same elaborate system of water-channels. Here we have loose, incoherent sandstones, shales, and clays, carved, by a net-work of running waters, into domes and cones, with flowing outlines. But still there is no vegetation, and the loose earth is naked. Occasionally, a thin stratum of harder rock will be found. Such strata will here and there form shelves or steps upon the sides of the mountains.
Traces of iron, and rarer minerals, are found in these beds, and, on exposure to the air, the chemical agencies give a greater variety of colors, so that the mountains and cones, and the strange forms of the Bad Lands, are elaborately and beautifully painted; not with the delicate tints of verdure, but with brilliant colors, that, are gorgeous when first seen, but which soon pall on the senses.
The Uinta Mountains.—To the west of Green River stand the Wasatch Mountains, a system of peaks, tables, and elevated valleys, having a northerly and southerly direction, nearly parallel to the river. The range known as the Uinta Mountains stands at right angles to the Wasatch, extending toward the east, and no definite line of division can be noticed. The Wasatch is a great trunk, with a branch called the Uinta. Near the junction, the two ranges have about the same altitude, and the gulches of their summits are filled with perpetual snow; but, toward the east, the Uinta peaks are lower, gradually diminishing in altitude, until they are lost in low ridges and hills. Through this range Green River runs, and a series of cañons forms its channel.
To a person studying the physical geography of this country, without a knowledge of its geology, it would seem very strange that the river should cut through the mountains, when apparently it might have passed around them to the east, through valleys, for there are such along the north side of the Uintas, extending to the east, where the mountains are degraded to hills; and, passing around these, there are other valleys, extending to the Green, on the south side of the range. Then, why did the river run through the mountains?
The first explanation suggested is, that it followed a previously-formed fissure through the range; but very little examination will
show that this explanation is unsatisfactory. The proof is abundant that the river cut its own channel; that the cañons are gorges of corrasion. Again, the question returns to us, Why did not the stream turn around this great obstruction, rather than pass through it? The answer is, that the river had the right of way; in other words, it was running ore the mountains were formed: not before the rocks, of which the mountains are composed, were deposited, but before the formations were folded, so as to make a mountain-range.
The contracting or shriveling of the earth causes the rocks near the surface to wrinkle or fold, and such a fold was started athwart the course of the river. Had it been suddenly formed, it would have been an obstruction sufficient to turn the water in a new course to the east, beyond the extension of the wrinkle; but the emergence of the fold above the general surface of the country was little or no faster than the progress of the corrasion of the channel. We may say, then, that the river did not cut its way down through the mountains, from a height of many thousand feet above its present site; but, having an elevation differing but little, perhaps, from what it now has, as the fold was lifted, it cleared away the obstruction by cutting a canon and the walls were thus elevated on either side. The river preserved its level, but mountains were lifted up; as the saw revolves on a fixed pivot, while the log through which it cuts is moved along. The river was the saw which cut the mountains in two.
Recurring to the time before this wrinkle was formed, there were beds of sandstone, shale, and limestone, more than 24,000 feet in thickness, spread horizontally over a broad stretch of this country. Then the summit of the fold slowly emerged, until the lower beds of sandstone were lifted to the altitude at first occupied by the upper beds, and if these upper beds had not been carried away, they would now be found more than 24,000 feet above the river, and we should have a billow of sandstone, with its axis lying in an easterly and westerly direction, more than 100 miles in length, 50 miles in breadth, and over 24,000 feet higher than the present altitude of the river, gently rounded from its central line above to the foot of the slope on either side. But as the rocks were lifted, rains fell upon them and gathered into streams, and the wash of the rains and the corrasion of the rivers cut the billow down almost as fast as it rose, so that the present altitude of these mountains marks only the difference between the elevation and the denudation.
It has been said that the elevation of the wrinkle was 24,000 feet, but it is probable that this is not the entire amount, for the present altitude of the river, above the sea, is nearly 6,000 feet, and when this folding began we have reason to believe that the general surface of this country was but slightly above that general standard of comparison.
Then there were down-turned as well as up-turned wrinkles, or, as the geologist would say, there were synclinal as well as anticlinal folds. Had there been no degradation of the fold, there would have been a bed of rock turned over its summit 24,000 feet above the present level of the river. Now that bed is gone from the mountains, yet it can be seen turned up on edge against the flanks of the mountains, dipping under the beds of rocks found still farther out from the range. Follow it down, and doubtless we could trace it to a depth much below the level of the sea. While the folds were forming, the up-turned flexures were cut down, and the troughs in the down-turned flexures were filled up, and we have more than 8,000 feet of these later sediments to the north of the Uinta Mountains.
It will thus be seen that the upheaval was not marked by a great convulsion, for the lifting of the rocks was so slow that the rains removed the sandstones almost as fast as they came up. The mountains were not thrust up as peaks, but a great block was slowly lifted, and from this the mountains were carved by the clouds—patient artists, who take what time may be necessary for their work.
We speak of mountains forming clouds about their tops: the clouds have formed the mountains. Lilt a district of granite, or marble, into their region, and they gather about it, and hurl their storms against it, beating the rocks into sands, and then they carry them out into the sea, carving out cañons, gulches, and valleys, and leaving plateaus and mountains embossed on the surface.
Instead of having a rounded billow, we have an irregular table, with beds dipping to the north, on the north side of the axis, and to the south, on the south side, and in passing over the truncated fold we pass over their upturned edges.
Go out on the flank of the fold, and find the bed of rock which would form the summit of the great wrinkle, had there been no erosion, and there sink a shaft 24,000 feet, and you will be able to study a certain succession of beds of sandstones, shales, and limestones. Go two or three miles farther from the mountains, and sink a shaft; the first 8,000 feet or more will be through sandstones and shales, unlike those seen in the first section; then you will strike the summit of the first section. Continuing down for 24,000 feet, the first will be reproduced, stratum for stratum. Now start on either side of the fold, and cross to its centre; and you will pass over the same series of strata in the same order as you would in descending the first-mentioned shaft, and in the second also, below the upper 8,000 feet. Now pass again from the centre to the flank of the fold, in either direction, and you can study the same rocks in the same order as you would in ascending these shafts. It will thus be seen that in these truncated wrinkles we are enabled to study geological formations without descending into the depths of the earth.
Fig. 1 has been constructed for the purpose of graphically expressing some of the important facts observed in the great Uinta Fold. In this, the beds are seen to turn up in a great flexure, and to be cut away above, the higher beds more than the lower; thus 4, 4-4, 4, has been cut away much more than 5, 5-5, 5; and 10, 10-10, 10 has suffered much less erosion than the beds above it. The only place where the water has carried it away is at Y, the bottom of the cañon.
In this diagram, the line A-B represents the lowest line of observation, as exhibited in the bed of the river. All below this line is theoretical. The line C-D represents the level of the sea. The stratum, E, E-E, E was the last deposited antecedent to the commencement of the emergence of the summit of the fold. Had there been no erosion of the fold, the beds intervening between the broken line I, I, I (which is a continuation of the lines E, E-E, E), and the irregular line which represents the surface of the country, cutting the edges of the eroded beds, and passing through the lowest. No. 10, at Y, would still be found, but they have been carried away.
The diagram does not properly represent the entire amount of erosion, from the fact that the vertical scale is exaggerated, and the beds have been extended beyond their proper limits, for the purpose of representing more clearly other facts of interest.
It will be seen that in passing along the line A-B (the bottom of the river-channel), from the shaft F to the bottom of the cañon Y, we are able to observe the beds 4, 5, 6, 7, 8, 9, 10, in the same order that we would in descending the shaft F. The beds 1-1, 2-2 have been deposited since the emergence of the summit of the fold, and hence never extended quite across it; yet the lower members of these beds, doubtless, at one time extended much farther up on the flanks of the fold. They have been cut away, however, as represented in the diagram. Let the lines H-H-H, H, represent the limit of the continuation of these beds. In the shaft G these beds also are exposed above those seen in shaft F.
The altitude of the rocks above the line of observation (A, B) is exaggerated about five times. If they were reduced to one-fifth, the proportion between the rocks seen in the various escarpments of these mountains, and those carried away below the broken lines, would be properly represented.
By sinking a shaft, only a little surface along the edge of the strata could be seen; but on the sides of the fold they are exposed for many miles, and often the top or bottom is cleared off for a great space, revealing even the ripple-marks of the ancient sea, or rounded impressions of rain-drops which fell in that elder time; or the sands have buried shells and bones of ancient animals, and they are still encased in the rock; and even impressions of leaves that were buried in the mud can yet be seen in such a fine state of preservation that you can trace their delicate veins.
In speaking of the great upheaval of rocks from which the Uinta ^Mountains are carved, I have spoken of wrinkling and folding, as if the rocks were always flexed; but these displacements are sometimes attended with fractures, on one side of which the rocks are upheaved, or thrown down on the other. Such displacements are called faults. Faults like these are seen in many places in the Uinta Mountains; one great one, on the north side, the throw of which is nearly 20,000 feet, and many others are found of lesser magnitude.
In speaking of elevation and depression by faulting or folding, it must be understood that reference is made to a change of altitude in relation to the surface of the sea, so that upheaval or throw is only relative to this general standard of comparison. But during the geological ages represented in the folding and carving of the Uinta Mountains, it is possible the level of the sea itself has been changed by the shrinking of the earth, and a part, at least, of the apparent upheaval above mentioned may be accounted for by a depression of the formations in synclinal folds, and the letting down of broad areas of the earth's surface by lateral contraction exhibited, in corrugation.
It has already been said that the cutting off of the fold has left the upturned edges of the formations exposed to view. Some of these beds are quite hard, others are composed of very soft material; so that there are alternating beds of harder and softer rocks running in an easterly and westerly direction, both on the north and south side of the range. The soft rocks, yielding much more readily to atmospheric degradation, have been washed out in irregular valleys, between intervening ridges of harder rock, so that we have a series of nearly parallel valleys, and also a series of intervening parallel ridges, and both valleys and ridges are approximately parallel to the range. But, as the great fold of the Uinta Mountains is greatly complicated by minor oblique and transverse flexures, while the general direction of these ridges is as described, they are turned back and forth from these lines in gentle or abrupt curves. These ridges are sometimes low mountain-ranges.
So, if we approach these mountains from either direction, north or south, we first meet with ridges, or, as they are usually called in the Western country, hog-backs. In many places these are so steep as to form a complete barrier to progress.
Usually the slope away from the side of the mountain corresponds above with the dip of the rock, and is gentle or steep, as the dip is lesser or greater. The side of the hog-back, next to the mountain, is composed of the cut edges of the strata, and varies greatly with the texture of the rocks; but usually it is steep or broken, sometimes but tressed, sometimes terraced, sometimes columned and fluted.
On the south side of the Yampa Plateau, near the head of Cliff-Creek Valley, there is an abrupt, oblique flexure, on the side of the great fold, by which the rocks are turned up, so as to stand vertically. In the rocks at this place there are two very hard conglomerates; the intervening strata are soft sandstones and marls, and have been carried away, and the conglomerates stand as vertical walls, 30 or 40 feet in thickness, 50 to 300 feet in height, and several miles in length, and between these is a broad avenue, or narrow valley, beset with ragged bowlders of conglomerate.
The drainage of these narrow valleys between the hog-backs is not always along their lengths, but the water is sometimes carried by channels crossing them and cuttings through intervening; ridges; hence there are numbers of transverse streams and wet-weather channels running across valleys and through ridges.
Now, if the great axis of the Uinta Fold was everywhere the summit of a water-shed, we should find the streams heading along that irregular line running off to the flank of the fold on either side; hut, as the fold is bisected by Green River, some of the minor water-courses, especially those near the river, and those near the centre of the fold, follow the strike of the rocks directly into that stream. On the north side, some head back near the summit of the fold, and run to the north, crossing the hog-backs in a direction with the dip, and then turn, at the foot of the mountains, and run into the Green, where the waters take a general southerly direction. Others, again, head back on the hog-backs, or even beyond them, on the plains and the Bad Lands to the north, and cut quite through the hog-backs and mountains in a direction against the dip of the rocks, and empty into the Green. This is especially true where the river has its easterly and westerly direction through Brown's Park. On the other side of the range, streams head high up in the mountains, and cut directly or obliquely against the upturned edges of the strata, and run in a general direction with the dip of the strata until they reach the long valleys between hog-backs; then down these valleys they turn, sometimes cutting through intervening ridges, until they find their way into the Green, where they are turned to the south, away from the mountain.
It will thus be seen that the relation of the direction of the streams to the dip of the rocks is very complex, and, for convenience of description, I have elsewhere classified these valleys, on the basis of these relations, in the following order:
Order 1.—Transverse valleys, having a direction at right angles to the strike.
Order 2.—Longitudinal valleys, having a direction the same as the strike.
Of the first order, three varieties are noticed:
a, diaclinal, those which pass through a fold. (Fig. 3.)
b, cataclinal, valleys that run in the direction of the dip. (Fig. 4.)
c, anaclinal, valleys that run against the dip of the beds. (Fig. 7.)
Of the second order, we have, also, three varieties:
A, anticlinal valleys, which follow anticlinal axes. (Fig. 5.)
B, synclinal valleys, which follow synclinal axes. (Fig. 6.)
C, monoclinal valleys, which run in the direction of the strike between the axes of the fold—one side of the valley formed of the summits of the beds, the other composed of the cut edges of the formation. (Fig. 8.)
Many of the valleys are thus simple in their relations to the folds; but, as we may have two systems of displacements, a valley may belong to one class, in relation to one fold, and to another in its relation to a second. Such we designate as complex valleys.
Again, a valley may belong to one class in one part of its course and to another elsewhere in its course. Such we designate as compound valleys. It will be further noticed that valleys may have many branches, but, in relegating a valley to its class, we consider only the stem of the valley proper, and not its branches.
A great diversity in the features of all these valleys is observed. Most of these modifications are due to three principal causes: First, a greater or lesser inclination of the rocks. Second, the texture of the beds—that is, their greater or lesser degree of heterogeneity. The third class of modifying influences is found in the eruptive beds.
The last-mentioned agencies are not found in the region under immediate discussion.
The explanation of the cañons of Green River will assist us in understanding the origin of the lateral valleys and cañons. The streams were there before the mountains were made; that is, the streams carved out the valleys, and left the mountains. The direction of the streams is indubitable evidence that the elevation of the fold was so slow as not to divert the streams, although the total amount of elevation was many thousands of feet. Had the fold been lifted more rapidly than the principal streams could have cut their channels, Green River would have been turned about it, and all the smaller streams and water-ways would have been cataclinal.
Thus it is that the study of the structural characteristics of the valleys and cañons teaches us, in no obscure way, the relation between the progress of upheaval and that of erosion and corrasion, showing that these latter were pari passu with the former, and that the agencies of Nature produce great results—results no less than the carving of a mountain-range out of a much larger block lifted from beneath the sea; not by an extravagant and violent use of power, but by the slow agencies which may be observed generally throughout the world, still acting in the same slow, patient manner.
There are yet some interesting facts to be observed concerning these inter-hog-back valleys. Their floors are usually lower than the general surface farther away from the mountains. There seem to be two causes for this: The great fold having been lifted and truncated prior to the exposure of the rocks farther away from the mountains, its strata present their edges, instead of their upper surfaces, to the down-falling rain, and the softer beds are not so well shielded by the harder. Erosion hence progresses more rapidly than where the beds are approximately horizontal.
Again, the mountains, with peaks among the clouds, condense their moisture, and a greater quantity of rain falls on them, or in their vicinity. The region of country adjacent to the mountains receives
a portion of this extra rainfall, so that this dynamic agency increases from the plains to the summits of the mountains, probably in some direct ratio. This increase of the eroding agency, and the greater exposure of the soft beds, probably account for the fact that the lowest country is at the foot of the mountains.
There is a limit to the effect of these conditions, for it should be observed that no valley can be eroded below the level of the principal stream, which carries away the products of its surface degradation; and where the floor of such a valley has been cut down nearly to the level of such a stream, it receives the débris of the adjacent cliffs and mountains, and in this way the rocks composing the floor are usually masked, to a greater or lesser extent. The same topographic facts under like conditions, are found on the eastern slope of the Rocky Mountains, in Colorado Territory, and the valleys which run into the South Platte from the south, between the hog-backs, are lower than the mesas and plateaus farther away from the mountains, but not lower than the flood-plain of the river.
I have endeavored above to explain the relation of the valleys of the Uinta Mountains to the stratigraphy, or structural geology, of the region, and, further, to state the conclusion reached, that the drainage was established antecedent to the corrugation or displacement of the beds by faulting and folding. I propose to call such valleys, including the orders and varieties before mentioned, antecedent valleys.
In other parts of the mountain-region of the West, valleys are found having directions dependent on corrugation. I propose to call these consequent valleys. Such valleys have been observed only in limited areas, and have not been thoroughly studied, and I omit further discussion of them.
In many cases, there can be no doubt that the present courses of the streams were determined by conditions not found in the rocks through which the channels are now carved, but that the beds in which the streams had their origin, when the district last appeared above the level of the sea, have been swept away. I propose to call such superimposed valleys. Thus the valleys under consideration, if classified on the basis of their relation to the rocks in which they originated, would be called consequent valleys; but, if classified on the basis of their relation to the rocks in which they are now found, would be called superimposed valleys.
- From "Report on United States Geological and Geographical Survey of the Territories. Second Division." Major J. W. Powell in charge.