Popular Science Monthly/Volume 7/October 1875/Physical Features of the Colorado Valley III
|PHYSICAL FEATURES OF THE COLORADO VALLEY.|
By Major J. W. POWELL.
THE more important topographic features in the valley of the Colorado are mountains, hills, hog-backs, bad-lands, alcove-lands, cliffs, buttes, and canons. The primary agency in the production of these features is upheaval, i. e., upheaval in relation to the level of the sea, though it may possibly be down-throw in relation to the centre of the earth. This movement in portions of the crust of the earth may be by great folds, with anticlinal or synclinal axes, and by monoclinal folds and faults.
The second great agency is erosion, and the action of this agency is conditioned on the character of the displacements above mentioned, the texture and constitution of the rocks, and the amount and relative distribution of the rains.
In a district of country, the different portions of which lie at different altitudes above the sea, the higher the region the greater the amount of rainfall, and hence the eroding agency increases in some well-observed but not accurately-defined ratio, from the low to the high lands. The power of running water, in corrading channels and transporting the products of erosion, increases with the velocity of the stream in geometric ratio, and hence the degradation of the rocks increases with the inclination of the slopes. Thus altitude and inclination both are important elements in the problem.
Let me state this in another way. We may consider the level of the sea to be a grand base-level, below which the dry lands cannot be eroded; but we may also have, for local and temporary purposes, other base levels of erosion, which are the levels of the beds of the principal streams which carry away the products of erosion. (I take some liberty in using the term level in this connection, as the action of a running stream in wearing its channel ceases, for all practical purposes, before its bed has quite reached the level of the lower end of the stream. What I have called the base-level would, in fact, be an imaginary surface, inclining slightly in all its parts toward the lower end of the principal stream draining the area through which the level is supposed to extend, or having the inclination of its parts varied in direction as determined by tributary streams.) Where such a stream crosses a series of rocks in its course, some of which are hard, and others soft, the harder beds form a series of temporary dams, above which the corrasion of the channel through the softer beds is checked, and thus we may have a series of base-levels of erosion, below which the rocks on either side of the river, though exceedingly friable, cannot be degraded. In these districts of country, the first work of rains and rivers is to cut channels, and divide the country into hills, and perhaps mountains, by many meandering grooves or water-courses, and when these have reached their local base-levels, under the existing conditions, the hills are washed down, but not carried entirely away.
With this explanation I may combine the statements concerning elevation and inclination into this single expression, that the more elevated any district of country is, above its base-level of denudation, the more rapidly it is degraded by rains and rivers.
The second condition in the progress of erosion is the character of the beds to be eroded. Softer beds are acted upon more rapidly than the harder. The districts which are composed of softer rocks are rapidly excavated, so as to become valleys or plains, while the districts composed of harder rocks remain longer as hills and mountains.
Where the beds are of stratified material, so that the change from harder to softer materials is from bed to bed, rather than from district to district, and in a vertical or inclined direction, rather than an horizontal, the topographic features, which I have described as hogbacks and cliffs of erosion, are produced. The difference between hogbacks and cliffs of erosion is chiefly due to the amount of dip or inclination of the beds.
But there is another condition necessary to the production of cliffs and hog-backs in their typical forms. The country must be arid, for, where there is a great amount of rainfall, the water penetrates and permeates the rocks, and breaks them up, or rots them, to use an expression which has been employed with this meaning; and the difference between the durability of the harder beds and that of the softer is, to some extent, compensated for by this agency, though doubtless ridges and cliffs may be produced in less arid climates, as we find them in the Appalachian System, but not so well marked. In a region of country where there is a greater amount of rainfall, the tendency is to produce hills and mountains, rather than plateaus and ridges, with escarpments.
Now let us examine the character of the channels which running streams carve. Where the rocks to be carved are approximately horizontal, and composed of stratified beds of varying thickness, the tendency is to cut channels with escarpments or cliffs ', but if the beds are greatly inclined, or composed of unstratified material, the tendency is to cut channels with more flaring and irregular walls. These tendencies are more clearly defined when the meteorologic conditions are favorable—that is, if a stream cuts through stratified rocks, in an arid region, and carries the waters from a district more plentifully supplied, the cliff character of the walls is increased; and where a stream runs through unstratified rocks, in a district well supplied with rains, the walls or banks of the stream are cut down in more gentle slopes.
For purposes of discussion, it will be convenient to call the deep
Section of Wall in the Grand Cañon.
channels of streams through table-lands, in arid climates, cañons; and the deep channels of streams through heterogeneous beds, in a moist climate, water-gaps, or narrows, and ravines.
Having in view the forms which are produced by erosion, it will be convenient to classify the methods of erosion as follows: First, corrasion by running streams, and, second, erosion by rains; the first producing channels along well-defined lines, the second producing tie general surface features of the landscape.
Of the first class we have two varieties:
A. The corrasion of water-gaps.
B. The corrasion of cañons.
Of the second class we have three varieties:
A. Cliff erosion, where the beds are slightly inclined, and are of heterogeneous structure, some soft and others hard; and, for the production of the best-marked forms, the climate should be arid. Here the progress of erosion is chiefly by undermining.
B. Hog-back erosion, where the beds have a greater inclination, but are still of heterogeneous structure. Here the progress of erosion is by undermining and surface-washing, and the typical forms would require an arid climate.
C. Hill and mountain erosion, where the beds may lie in any shape, and be composed of any material not included in the other classes, and the progress of erosion is chiefly by surface-washing. The typical forms are found in a moist climate.
There is still another agency in the production of topographic features, viz., the eruption of molten matter from below the general surface. The beds formed are soon modified by erosion, and then the forms produced are due to that agency, and fall under the general series. But there is a time, immediately after the eruption, when these beds lie in forms due to igneous dynamics, and the most important features produced are cones. These cones are very conspicuous features of the landscape over much of the region drained by the Colorado River.
The district of country drained by the Colorado and its tributaries is divided into two parts, by a well-marked line of displacements. The lower third of the valley, which lies southward from this line, is but little above the level of the sea, except that here and there ranges of mountains are found. From this region, there is usually a bold step to a higher.
The upper two-thirds of the area drained by the Colorado is from 4,000 to 8,000 feet above the level of the sea, with mountain-ranges on the east, north, and west, of greater altitude. The bold step from the lower country to the table-lands is usually an escarpment in rocks of the Carboniferous Age, marked, here and there, by beds of lava, and along its margin stand many volcanic cones. San Francisco Mountain is made up of a group of these beds of eruptive matter, covering stratified rocks. This higher region is the one to which we have given especial attention in the previous discussion.
The principal condensation of moisture occurs on and about the mountains standing on the rim of the basin, the region within being arid.
Bad-lands, alcove-lands, plains of naked rock, plains of drifting sands, mesas, plateaus, buttes, hog-backs, cliffs, volcanic cones, volcanic mountains, cañons, cañon valleys, and valleys, are all found in this region, and make up its topographic features. Mountains, hills, and small elevated valleys, are the features of the irregular boundary belt.
No valley is found along the course of the Colorado, from the Grand Wash toward the sources of the river, until we reach the head of Labyrinth Cañon. For this entire distance the base-level of erosion is below the general surface-level of the country adjacent to the river, but at Gunnison's Valley we have a local base-level of erosion which has resulted in the production of low plains and hills for a number of miles back from the stream. North of the Cañon of Desolation and south of the Uinta Mountains, another local base-level of erosion is found, so near to the general surface of the country that we find a district of valleys and low hills stretching back from Green River, up the Uinta to the west, and White River to the east, for many miles. North of the Uinta Mountains a third local base-level of erosion is seen, but its influence on the topographic features is confined to a small area of 200 or 300 square miles. Going up the chief lateral streams of the Colorado, we find one or more of these local base-levels of erosion, where the streams course through valleys.
Where these local base-levels of erosion exist, forming valley and hill regions, the streams no longer cut their channels deeper, and the waters of the streams, running at a low angle, course slowly along, and are not able to carry away the products of surface-wash, and these are deposited along the flood-plains, in part, and in the valleys, among hills, and on the gentler slopes. This results in a redistribution of the material in irregular beds and aggregations.
In this region, there are occasional local storms of great violence. Such storms may occur in any particular district only at intervals of many years, possibly centuries. When such a one does occur, it reopens great numbers of channels that have been filled by the ordinary wash of rains, and often cuts a new channel through beds which have accumulated in the manner above described. The structure of these beds is well exposed, and we find beds of clay, beds of sand, and beds of gravel occurring in a very irregular way, due to the vicissitudes of local wash, and, where the progress of erosion has been more or less by undermining, larger fragments or bowlders are found, and these bowlders are sometimes mixed with clay, and sometimes with sand and gravel, and where thin sheets of eruptive rocks have been torn to pieces, more or less by undermining (for such is the usual way in this country), the beds appear to contain erratics, and in fact some of the rocks are erratics, for in the various changes in the levels produced they have often been transported many miles, not by sudden and rapid excursions, but moved a little from time to time.
Again, the beds from which they were derived, doubtless, in many cases have been broken up or lost, and these fragments only remain to attest the existence of such beds in some former time, and all stages may be observed, from the beds the edges only of which have been broken up, to those that have only fragments remaining or have entirely disappeared. Another interesting fact has been observed, that these erratics or bowlders are often found distributed somewhat in lines due to the undermining of lines of cliffs. Often where we have cliffs capped with a bed of lava, former and more advanced positions of these lines of cliffs can be recognized by the position of lines of lava-fragments which are seen in the valley or plains in front of the cliffs. It will be seen that these local accumulations of material, due to the excess of erosion over that of transportation, greatly resemble the accumulations of "the Drift." Especially is this true where I have studied the latter in the valley of the Mississippi, and I have been led to query whether it may not be possible to refer the origin of the Drift of the valley of the Mississippi, in part at least, to some such action as this; not that I question the evidence of extended glacial action in that region, but may it not be that this glacial action has only resulted in somewhat modifying a vast accumulation of irregularly-bedded material, originally due to the fact that the grand base-level of erosion had been reached by the running streams of that region, and hills and mountains had been degraded by having the material of which they were composed scattered over lower lands, without being carried away by streams to the sea?
All the mountain-forms of this region are due to erosion; all the cañons, channels of living rivers and intermittent streams, were carved by the running waters, and they represent an amount of corrasion difficult to comprehend. But the carving of the canons and mountains is insignificant, when compared with the denudation of the whole area, as evidenced in the cliffs of erosion. Beds hundreds of feet in thickness and hundreds of thousands of square miles in extent, beds of granite and beds of schist, beds of marble and beds of sandstone, crumbling shales and adamantine lavas, have slowly yielded to the silent and unseen powers of the air, and crumbled into dust and been washed away by the rains and carried into the sea by the rivers.
The story we have told is a history of the war of the elements to beat back the march of the lands from ocean-depths.
And yet the conditions necessary to great erosion in the valley of the Colorado are not found to exceed those of many other regions. In fact, the aridity of the climate is such that this may be considered a region of lesser, rather than greater, erosion. We may suppose that, had this country been favored with an amount of rainfall similar to that of the Appalachian country, and many other districts on the surface of the earth, the base-level of erosion of the entire area would have been the level of the sea; and, under such circumstances, though the erosion would have been much greater than we now find, the evidences of erosion would have been more or less obliterated. As it is, we are able to study erosion in this country, and find evidences of its progress and its great magnitude, from the very fact that the conditions of erosion have been imperfect.
It is proper to remark here that erosion does not increase in ratio to the increase of the precipitation of moisture, as might be supposed; for, with the increase of rains there will be an increase of vegetation, which serves as a protection to the rocks, and distributes erosion more evenly, and it may be that a great increase of rains in this region would only produce a different series of topographic outlines, without greatly increasing the general degradation of the valley of the Colorado.
To a more thorough discussion of this subject I hope to return at some future time.
From the considerations heretofore presented, it is not thought necessary to refer the exhibition of erosion shown in the canons and cliffs to a more vigorous action of aqueous dynamics than now exists, for, as I have stated, a greater precipitation of moisture would have resulted in a very different class of topographic features. Instead of canons, we should have had water-gaps and ravines; instead of valleys with cliff-like walls, we should have had valleys bounded by hills and slopes; and if the conclusions to which we have arrived are true, the arid conditions now existing must have extended back for a period of time of sufficient length to produce the present canons and cliffs. But there are facts which seem to warrant the conclusion that this condition has existed for a much longer period than that necessary for the production of the present features; that is, the characteristics of the present topography have existed for a long time. There are evidences that the lines of cliffs themselves have been carried back for great distances as cliffs by undermining, which is a process carried on only in an arid region.
The evidence is of this character: I have stated that the drainage of the inclined plateaus is usually from the brink of the cliffs backward; i. e., the water falling on the plateau does not find its way immediately over the cliffs, but runs from the very brink or edge of the plateau back toward the middle or farther side, which is usually found against the foot of another line of cliffs, and here the waters are turned toward some greater channel, which runs against the dip and cuts through the cliffs. Now, the water-ways at the heads of these streams that have their sources near the brink of the cliffs would always be small, shallow, and ramifying into many minute branches if the line of cliffs were a fixed or immovable line, but we often find that the cliffs have been carried back by the undermining process until all these minute ramifications have been cut off; and we find cañons opening on the faces of the cliffs, the waters of which run backward as above described.
Let us suppose that we have a line of cliffs with an escarpment facing the south. The rain, falling on the escarpment and in the region south of the cliffs, would run toward the south or along the foot of the cliffs until it reached some more important water-channel; the rain falling on the plateau, from the brink of the cliffs backward, would run toward the north, and the waters falling on this upper region would excavate channels for themselves, and, under proper conditions, canons would be cut. As the cliffs are undermined and this line carried back into the plateau, the area with a southern drainage would be increased, the area with a northern drainage correspondingly diminished, and, when the process had continued for a sufficient length of time, we would find the southern edge of the plateau carried away by this undermining process, until all the heads of the streams were cut off and until the line had reached the cañons.
Gradually, during the progress of erosion, the excavation of the bottom of the cañons would cease, as the supply of water running through them would be cut off, and such cañons would have to be considered as comparatively ancient. Such facts are frequently observed in this cañon and cliff country.
From such considerations, it seems that we may safely conclude that the cliff topography has prevailed in that region for a long time. There are evidences also that there were cañons here before the present cañons were carved. The facts in relation to this matter can be better stated when we come to discuss the geology of the region,
Mr. G. K. Gilbert, a geologist of Lieutenant Wheeler's corps, in a paper communicated to the Philosophical Society of Washington, in 1873, deduced a similar conclusion from an independent series of facts observed in Western Utah. The basin of Great Salt Lake, a portion of what Fremont designated the "Great Basin," has now so dry a climate that its waters gather in its lowest parts and evaporate, and have no outlet to the sea. In a former period, however, there was more rain, the valley was filled with water to its brim, and in place of the Salt Lake Desert, there was a broad and deep fresh lake, discharging its surplus into the Columbia River. The epoch of this lake Mr. Gilbert finds reason to consider identical with the Glacial Epoch, and it was of limited duration. Among its vestiges are deposits of fossiliferous marl, which are conspicuously contrasted with the gravels and sand that now slowly accumulate in the same region, borne by the intermittent streams that descend from the mountains. Where the beds are superposed, the marls testify to a moist climate and the gravels to a climate so dry that the basin was never filled with water. But above the marls are found only scattered and thin deposits of gravel, while below them the gravel-beds are omnipresent and of great depth, and hence it was reasoned that the arid period that preceded the Glacial Epoch was many times longer than that which has followed it.
Even during the Glacial Epoch, Mr. Gilbert considers that "the Atlantic slope, and the region of the Great Basin, were contrasted in climate, just as now. The general causes that covered the humid East with a mantle of ice, sufficed, in the arid West, only to flood the valleys with fresh water, and send a few ice-streams down the highest mountain-gorges."
Records of More Ancient Lands.—The summit of the Kaibab Plateau is more than 6,000 feet above the river, and I have already mentioned that the summit of the plateau is also the summit of rocks of the Carboniferous Age. These beds are about 3,500 feet in thickness, and beneath them we have 1,000 feet of conformable rocks of undetermined age. This gives us 4,500 feet, from the summit of the plateau down to the non-conformable beds. Still beneath these we have 1,500 feet, so that we have more than 1,500 feet of other rocks exposed in the depths of the Grand Cañon. Standing on some rock, which has fallen from the wall into the river—a rock so large that its top lies above the water—and looking overhead, we see a thousand feet of crystalline schists, with dikes of greenstone, and dikes and beds of granite. Heretofore we have given the general name granite to this group of rocks; still, above them we can see beds of hard, vitreous sandstone of many colors, but chiefly dark red. This group of rocks adds but little more than 500 feet to the height of the walls, and yet the beds are 10,000 feet in thickness. How can this be? The beds themselves are non-conformable with the overlying Carboniferous rocks; that is, the Carboniferous rocks are spread over their upturned edges.
In the figure (p. 672) we have a section of the rocks of the Grand Cañon. A, A, represents the granite; a, a, dikes and eruptive beds; B, B, these non-conformable rocks. It will be seen that the beds incline to the right. The horizontal beds above, C, C, are rocks of Carboniferous Age, with underlying conformable beds. The distance along the wall marked by the line x, y, is the only part of its height represented by these rocks, but the beds are inclined, and their thickness must be measured by determining the thickness of each bed. This is done by measuring the several beds along lines normal to the planes of stratification; and, in this manner, we find them to be 10,000 feet in thickness.
Doubtless, at some time before the Carboniferous rocks C, C, were formed, the beds B, B, extended off to the left, but between the periods of deposition of the two series, B, B, and C, C, there was a period of erosion. The beds themselves are records of the invasion of the sea; the line of separation, the record of a long time when the region was dry land. The events in the history of this intervening time, the period of dry land, one might suppose were all lost. What plants lived here, we cannot learn; what animals roamed over the hills, we know not; and yet there is a history which is not lost, for we find that after these beds were formed as sediments beneath the sea, and still after they had been folded, and the sea had left them, and the rains had fallen on the country long enough to carry out 10,000 feet of rocks, the extension of these beds to the south, which were cut away, and yet before the overlying Carboniferous rocks were formed as sediments of sand and triturated coral-reefs, and ground shells and pulverized bones, some interesting events occurred, the records of which are well preserved. This region of country was fissured, and the rocks displaced so as to form faults, and through the fissures floods of lava were poured, which, on cooling, formed beds of trap or greenstone. This greenstone was doubtless poured out on the dry land, for it bears evidence of being eroded by rains and streams prior to the deposition of the overlying rocks.
Let us go down again, and examine the junction between these red rocks, with their intrusive dikes and overlying beds of greenstone, and the crystalline schists below.
We find these lower rocks to be composed chiefly of metamorphosed sandstones and shales, which have been folded so many times, squeezed, and heated, that their original structure, as sandstones and shales, is greatly obscured, or entirely destroyed, so that they are called metamorphic crystalline schists.
Dame Nature kneaded this batch of dough very thoroughly. After these beds were deposited, after they were folded, and still after they were deeply eroded, they were fractured, and through the fissures came floods of molten granite, which now stands in dikes, or lies in beds, and the metamorphosed sandstones and shales, and the beds of granite, present evidences of erosion subsequent to the periods just mentioned, yet antecedent to the deposition of the non-conformable sandstones.
Here, then, we have evidences of another and more ancient period of erosion, or dry land. Three times has this great region been left high and dry by the ever-shifting sea; three times have the rocks been fractured and faulted; three times have floods of lava been poured up through the crevices; and three times have the clouds gathered over the rocks, and carved out valleys with their storms. The first time was after the deposition of the schists; the second was after the deposition of the red sandstones; the third time is the present time. The plateaus and mountains of the first and second periods have been destroyed or buried; their eventful history is lost; the rivers that ran into the sea are dead, and their waters are now rolling as tides, or coursing in other channels. Were there cañons then? I think not. The conditions necessary to the formation of canons are exceptional in the world's history.
We have looked back unnumbered centuries into the past, and seen the time when the schists in the depths of the Grand Canon were first formed as sedimentary beds beneath the sea; we have seen this long period followed by another of dry land—so long that even hundreds, or perhaps thousands, of feet of beds were washed away by the rains; and, in turn, followed by another period of ocean triumph, so long, that at least 10,000 feet of sandstones were accumulated as sediments, when the sea yielded dominion to the powers of the air, and the region was again dry land. But aerial forces carried away the 10,000 feet of rocks, by a process slow yet unrelenting, until the sea again rolled over the land, and more than 10,000 feet of rocky beds were built over the bottom of the sea; and then again the restless sea retired, and the golden, purple, and black hosts of heaven made missiles of their own misty bodies—balls of hail, flakes of snow, and drops of rain—and when the storm of war came, the new rocks fled to the sea. Now we have cañon-gorges and deeply-eroded valleys, and still the hills are disappearing, the mountains themselves are wasting away, the plateaus are dissolving, and the geologist, in the light of the past history of the earth, makes prophecy of a time when this desolate land of Titanic rocks shall become a valley of many valleys, and yet again the sea will invade the land, and the coral animals build their reefs in the infinitesimal laboratories of life, and lowly beings shall weave nacre-lined shrouds for themselves, and the shrouds shall remain entombed in the bottom of the sea, when the people shall be changed, by the chemistry of life, into new forms; monsters of the deep shall live and die, and their bones be buried in the coral sands. Then other mountains and other hills shall be washed into the Colorado Sea, and coral-reefs, and shales, and bones, and disintegrated mountains, shall be made into beds of rock, for a new land, where new rivers shall flow.
Thus ever the land and sea are changing; old lands are buried, and new lands are born, and with advancing periods new complexities of rock are found; new complexities of life evolved.