« PreviousContinue »
we remarked that there were no indications in the geological formations of that portion of the West over which we have traversed of long-continued deep-water deposits until we pass up into the Cretaceous epoch. The lower portion of No. 1, or the Dakota Group, which ushered in the Cretaceous epoch in this portion of the West, is composed of coarse sand, pebbles, &c., with ripple marks, oblique laminæ, and with other indications of shallow water and change of currents. The same characters are seen throughout the formation wherever it is exhibited. We also know from the mumerous impressions of leaves, and some beds of impure lignite, that dry land could not have been far distant. But as we pass up through Nos. 2, 3, and 4, whatever changes of land may have occurred in the mean time, we think there were periods at least when the sea was of considerable depth and suffered a quiet deposition to go on. W infer this from the fine and homogeneous character of the sediments. Throughout No. 4 we have a fine plastic clay which continues up into No.5, when a gradual change takes place from the introduction of yellowish ferruginous matter, and a slow increase of sandy sediments. Toward the middle of No. 5 the sand begins to predominate until the upper part becomes a coarse ferruginous sandstone, with all the indications of shallow-water deposits. We know, also, from fragments of wood and impressions of leaves which have been found quite widely distributed in the upper part of No. 5, that dry land could not have been far away. We also infer from the character of the molluscan remains that the great Cretaceous sea which had so long spread its vast waters over this region was becoming shallow, and that a new epoch was approaching. As we arise in No. 4, and pass up into No. 5, there is an evident increase in the number of gasteropoda, indicating shoal waters. We have already remarked their peculiar Tertiary aspect, which seemed to point directly to that epoch, showing that it was not far distant. We may now ask the cause of this apparent approach to land, as foreshadowed by the lithological as well as the paleontological characters of the Upper Cretaceous formation No. 5. We think that the facts indicate that during the deposition of this formation the western portion of the continent was slowly rising above the ocean level, the waters on the one side reccding toward the Pacific, and on the other toward the Atlantic, introducing the great Tertiary epoch which had already been foretold in the Cretaceous. At the commencement of the Tertiary period, throughout the central portions of the continent, lakes, estuaries, &c., more or less salt, at length becoming brackish, and finally fresh water, existed, and a new flora and fauna were introduced. The subterranean expansive power which was quietly lifting up the country still continued, although no bursting of the earth's crust had commenced. These brackish water-deposits, which appear to mark the dawn of the Tertiary period in the West, are distributed quite widely over the central portions of the Rocky Mountain district, and then, by a general subsidence or a vast increase of fresh water, the true lignite deposits spread themselves over large areas and probably covered much of the country now occupied by the mountain ranges, and were doubtless more or less intimately connected with the Tertiary beds on the Pacific coast. What barriers separated them from the Tertiary formations along the Pacific it is impossible from our present limited knowledge of the geology of the intermediate region to determine.
We have remarked that the probable period of the bursting of the earth's crust which resulted in the formation of those abrupt mountain crests or ridges, occurred somewhere near the close of the accumulation of the true lignite deposits. We believe this for the following reasons: Whenever we observe the lignite beds in the vicinity of the mountain ranges we find them more or less inclined in the same direction with the older fossiliferous rocks, though, as a general rule, dipping at a smaller angle, because more remote from the axis of the disturbing power. Of course, as the land was slowly elevated toward the surface of the waters, the newer Tertiary beds would be suljected to the erosive action of water first, and thus continuing downward, as the mass was slowly rising, until the granitic nucleus was exposed. The Tertiary rocks, being composed for the most part of loose, yielding material, sands, clays and lignites, would be worn away from the surface for some distance from the axis of elevation. Although the lignite Tertiary beds are developed in full force all along the base of the larger ranges of mountains, it is not unlikely that some of these ridges formed barriers or lofty shores to these great Tertiary lakes. It would seem as if this country during the Tertiary period was not unlike the Undine region of the north, so called by the geographer Nicollet on account of the great number of fresh-water lakes distributed over that district.
Near the Black Hills these beds are worn away from the immediate base of the mountains, and it is doubtful from any proofs that we can now obtain whether the Tertiary lake extended over the country at that time occupied by the Black Hills. West of this range, the lignite Tertiary beds incline from the western slope 5 to 10 degrees. All along the Bighorn Mountains, the same features, only more strongly marked, are seen. These beds often lie quite high upon the slopes of the mountains, conforming to the Cretaceous rocks and sometimes inclining at a high angle. Between the western extremity of the Bighorn Range and the Sweet Water Mountains on the North Platte
they are more disturbed than at any other locality. The lignite Tertiary strata are nearly vertical and the hard layers of sandstone or limestone extend in long projecting lines across the country, while the intermediate yielding beds of clay, sand, and lignite are smoothed and leveled by atmospheric agencies and clothed with a thick turf of grass. All along the Laramie Range, from the Red Buttes to Deer Creek, until the lignite beds are concealed by the White River Group, the same features are seen, though the strata incline less, being more remote from the anticlinal crest. On both sides of the Wind River Mountains the same phenomena occur, and other examples might be cited pointing to the same conclusions, but enough has been said to show that it is probable that the lignite Tertiary beds partook of the same movements that have elevated the older fossiliferous rocks. We therefore infer that the fracture of the earth's crust in this portion of the West, by which the nucleus of the mountains was revealed, occurred near the time of the accumulation of the lignite deposits or at the close of that epoch.
Again, although there is not a strict unconformability between the true lignite beds and the Wind River Group, the latter incline in the same direction, only at a much smaller angle. Near the source of Wind River the Wind River Group rests directly upon Cretaceous formation No. 2. · At this point the Cretaceous rocks incline from 100 to 25°, while the Wind River beds dip from 1° to 5o. As we ascend the valley of Wind River towards its source, we pass, for a long distance, the steeply inclined Cretaceous and Jurassic rocks, along the margins of the mountains on our left hand, while on our right, but a few hundred yards distant, the naked, almost vertical walls of the lower portion of the Wind River Group* are seen, the strata, however, seldom inclining more than one degree.
The same examples may be observed on the west side of the Wind River Mountains, where the Wind River beds lie high upon the sides of the western slope in a very slightly inclined position and in some localities covering the very summit, showing clearly that even the dividing crest of the mountains was beneath the waters during the deposition of this group. Along the margins of both the Wind River and the Bighorn Mountains these beds seem to have risen in an undisturbed or in a nearly horizontal condition. We have already expressed the opinion in a previous paper,t that the Wind River Group was intermediate in age between the lignite Tertiary and the White River beds, and in point of time filled up a chronological chasm. We have inferred this from the fact that these beds seem to possess paleontological and lithological characters intermediate between the two. They contain casts of a species of Viripara which is undistinguishable from V. trochiformis, and fragments of a Trionyx apparently the same with that occurring in the liguite beds, also fragments of a Testudo which, so far as we can determine, is identical with the T. Nebrascensis of the White River beds. If we look also‘at the composition of the Wind River beds, we find that their light color, indurated arenaceous and argillaceous character, and their general appearance after erosion, favor the correctness of the inference in regard to their intermediate position. From the facts before us in regard to this group, we conclude that even after the crust broke, the country continued slowly rising while the Wind River deposits were accumulating, and that the upper portions when not eroded away were elevated high upon the sides of the mountains in a nearly horizontal position.
Again, the White River beds hold a similar position with reference to the lignite formations as the Wind River Group. They are seldom disturbed, and only in a few instances do they incline as much as 5o. They, however, occur high up on the mountain slopes along both sides of the Laramie Range, showing that they partook of the gradual elevation of the country, after the crust was broken and the mountain district began to approach its present configuration. On the west side of the Black Hills, where the White River beds probably began their origin, we find only the lower strata of this group, usually reposing directly upon Cretaceous rocks, though in a few localities upon lignite formations. But as we descend south and southwestward, these lower beds disappear and more recent ones take their place, until they pass into the Pliocene sands of the Loup River Group, and then, in turn, still farther southward, are lost in the Loess or yellow marl deposits. We can only account for these phenomena on the supposition that this great Tertiary fresh-water lake had its commencement in the White River Valley, and as the Black Hills, and of course the whole Rocky Mountain district, arose slowly toward its present elevation, the waters gradually receded southward and southwestward, and then more recent beds continued to be accumulated, until this formation spread itself over the vast area which it now occupies. We thus think that, by means of these Cretaceous and Tertiary deposits of the West, we can yet trace step by step the progress of that grand development which has given the present geographical conformation to the West, and originated the. fountains from which flow those mighty rivers which may well be called the commercial arteries of the American continent.
Another illustration of the gradual and long-continued rise of the country may be found in the immense chasms or cañons which have been formed by the streams along
* Same as the Wahsatch Group, 1878.
the mountain sides. We can only account for them on the supposition that as the anticlinal crest was slowly emerging from the sea, the myriad sources of our great rivers were seeking their natural channels, and that these branches or tributaries began this erosive action long before the great thoroughfares, the valleys of the Mississippi and the Missouri, were marked out. The erosion would go on as the mountains continued slowly rising at an almost imperceptible rate, and in process of time the stupendous channels which everywhere meet us along the immediate sides of the mountains would be formed. If we examine the barometrical profiles, already referred to, we see at a glance that in traversing the country from the Mississippi to the foot of the mountains the ascent is very gradual, but increases as we approach the upheaved ridges. In an equal proportion will the rapidity and consequently the erosive power of the streams be increased so that we may readily account for those grand displays of the erosive action of water which occur so frequently along the mountain sides. Eastward from the mountains, beyond this immediate influence, the descent is so gradual that the Missouri flows quietly along over its yielding alluvial bed, transporting its sediments to the Gulf of Mexico.
That the progressive elevation of the country continued up to our present period, or at least until near the time of the deposition of the most recent superficial deposits, we think we have evidence derived from the terraces, which are seen all along the streams. The elevation of these terraces increases as we approach the sources of the rivers, averaging from a few feet to 150 or 200 feet in height. This subject will be discussed more fully in a future article.
We conclude, therefore, that the barometrical profiles, constructed from explorations across our continent, and geological data, indicate a long-continued quiet expansion of the earth's crust, commencing toward the close of the Cretaceous epoch and extending even to our present period; that near the close of the accumulation of the Tertiary lignite deposits, the crust of the earth had reached its utmost tension, the long lines of fractures had commenced, and the anticlinal crests of the mountain ranges were marked out. In a previous paper in this Journal, we remarked that there is no unconformability in any of the fossiliferous sedimentary strata in the Northwest, from the Potsdam sandstone to the summits of the true lignite Tertiary. We believe, therefore, that the elevated ridges which form the nuclei of the mountain ranges began to emerge above the surface of the surrounding country near the close of the Eocene period. We think also that the evidence is clear that there were periods of subsidence and repose; but the thought which we wish to illustrate is, that there was a slow, long-continued, quiet, upward tendency which began near the close of the Cretaceous epoch and culminated in the present configuration of the western portion of our continent near the commencement of our present period.
WASHINGTON, D. C., January 1, 1862.
ART. XXXIV.- REMARKS ON THE GEOLOGICAL FORMATIONS ALONG THE EASTERN MARGINS OF THE ROCKY MOUNTAINS.*
By F. V. HAYDEN.
(From the American Journal of Science, May, 1868. ] On several former occasions I have described the different geological periods represented by the rocks uplifted along the margins of the Rocky Mountains and especially along the eastern slope. Examinations over a great extent of country in considerable detail, from latitude 490 south nearly to the Arkansas River, have shown me that quite marked lithological and paleontological changes occur in them all as we proceed from the north southward. It is the purpose of this article to note this fact somewhat more in detail than hitherto. Beginning with the nucleus of the Rocky Mountains at any point along the eastern range, we find it composed of massive granite rocks, mostly red feldspathic, but not unfrequently gray or other shades of color; then a series of metamorphic rocks (as they are usually called, though no doubt all the granites should be included with them), syenites, diorites, clay, mica, and hornblende slates, and igneous rocks of various kinds here and there.
Proceeding outward, we find the Silurian period represented by the Potsdam sandstone, Devonian wanting, then Carboniferous, Red Beds (Triassic ?), Jurassic, Creta
* This article refers only to the eastern ranges of the Rocky Mountains, extending south to the Arkansas. The same remarks may or may not apply to other portions.
ceous, and Tertiary, all connected together in the regular order of sequence, and all but the most recent Tertiary in strict conformity. The Tertiary deposits do not exhibit any marked change either in their mineral or fossil contents from the northern portion of our domain to the Arkansas, but the Cretaceous beds present several quite marked changes. Nos.* 5 and 4 maintain their peculiar characters as shown on the Upper Missouri, wherever they are exposed all along the eastern slope, except that they contain comparatively few fossils, yet a few characteristic species are found wherever these beds are seen, which identify them. On the Missouri River, No. 3 attains a great thickness, 400 to 600 feet, presenting massive escarpments of yellow chalk, and it can be traced all the way across the prairie country lying between 98° and 100° longitude. At Forts Hayes and Wallace on the Union Pacific Railway, Eastern Division, there are massive beds of this chalk which is sawed into building blocks with a common saw, and in many instances it is nearly as white as our chalk of commerce and might be used for the same purposes.
The two characteristic species of fossils of this division are found everywhere, Ostrea congesta and Inoceramus problematicus. All along the slope of the mountains No. 3 still retains its chalky nature, but becomes quite shaly, none of the layers ever becoming more than one or two inches in thickness. This is the case at the sources of the Missouri along the Bighorn and Wind River Mountains also, from the South Pass to Pike's Peak, and on the western slope wherever this bed is exposed. Near Denver, at Marshall's coal-mine, No. 3 has been changed by heat into a grayish compact limestone, quite hard and brittle in its fracture, which makes an excellent flux in smelting ores. But this change is local, for 16 miles north of this point it presents the same laminated character. It seems that No. 3 loses its massive chalky character, by which it first attracted attention on the Missouri River, in its westward extension, so that along the margins of the mountains, except in one locality, it cannot prove of any economical value, while between 980 and 1000 longitude it becomes very useful not only for lime, but also for building purposes. No. 2, like Nos. 4 and 5, retains its dark plastic clay character everywhere that it has been observed, but, like the others, it is not nearly as well developed in Colorado as on the Upper Missouri. Near Fort Benton it attains a thickness of 200 to 400 feet, while in Colorado it is not more than 50 to 150 feet. Between longitude 96° and 990 No. 1 retains its deep rustred sandy characters with dicotyledonous leaves from the Missouri River to the Arkansas, but nowhere along the margins of the mountains from latitude 490 to Pike's Peak have I ever seen any well-defined palæontological proof of its existence. Near Fort Benton are a series of Cretaceous beds containing some seams of impure lignite and numerous species of fossils, not one of which is identical with those so abundant in Nos. 4 and 5 lower down on the Missouri. These beds have been placed provisionally in the general section as a portion of No. 1, but the region about Fort Benton needs a more careful examination before any positive conclusions can be arrived at. Around the Black Hills is a bed of massive siliceous rocks, some layers forming a pudding stone, which in some localities takes the name of fortification rocks. These hold a position between No. 2 Cretaceous and the Jurassic marls. The same are seen along the margin of the Bighorn Mountains, in which I observed a bed of impure lignite, an abundance of silicified wood, and some uncharacteristic Saurian bones. From the Wind River Mountains to Pike's Peak these same siliceous and pebble cemented rocks occur holding the same geological position, forming, as it were, beds of transition between the Cretaceous and the Jurassic periods. I have carefully examined these rocks for hundreds of miles and have never yet detected any organic remains, animal or vegetable, in them.
The Jurassic beds, as revealed along the mountains, possess peculiar and marked lithological characters, so that having identified them by the fossils in one locality we can trace them over great areas. They were first shown to exist in the West in the form of a zone engirdling the Black Hills. They here attain a thickness from 200 to 300 feet at least, and from the beds in this locality alone have fossils enough been collected of such unmistakable Jurassic types as to prove their existence beyond a doubt. But these beds have also been shown, since they were first made known in the Black Hills, to be exposed along the margins of the Bighorn and Wind River Mountains near Red Buttes, on North Platte, and in numerous localities in the Laramie Plains, and westward to Fort Bridger. So numerous are the species now known from the West and so close are the affinities of most of them to well-known Jurassic types that it is not necessary for me in this place to detail the evidence in support of that statement.
It is sufficient to remark that the Jurassic system is quite plainly represented along the margins of the different ranges of mountains north of latitude 42, but proceeding southward from Deer Creek on the North Platte, the Jurassic beds diminish in force until near Cache la Poudre it becomes doubtful whether they are represented at all'
* The different divisions of the Cretaceous period, as shown on the Missouri River, have received geographical names, as Fort Benton Group, &c., but I use the old divisious by figures for brevity.
At this point there is a thin bed, perhaps 20 to 50 feet in thickness, of greenish-gray arenaceous marl overlying the Red Beds, which seem to occupy the place of the Jurassic. This seems to thin out more and more as we proceed southward toward the Arkansas. From Deer Creek 100 miles north of Fort Laramie to Denver, a distance of 400 miles, I have searched in vain for any organic remains in the rocks which appear to represent the Jurassic period of the Black Hills, Bighorn and Wind River Mountains. In the Red Beds or supposed Triassic no organic remains have been found north of the Arkansas, and they do not differ much lithologically in their southward extension, except that they seem to be much thicker and more gypsiferous northward. In the far north the Carboniferous rocks are in many localities 500 to 1,500 feet in thickness, and even as far south as the Red Buttes the massive beds of limestone, with true Carboniferous fossils, are exposed 500 to 1,000 feet thick, and are quite distinct from the red or variegated beds. But as we proceed southward from this point the Carboniferous limestones seem to lose their usual lithological characters and the Red Beds prevail. At the head of Pole Creek on the eastern margin and in the Laramie Plains west, the Carboniferous rocks are mostly of a red arenaceous character, with a few layers 2 to 10 feet in thickness of whitish or yellowish limestone. From these limestones I collected Productus Prattenianus, Athyris subtilita, and other well-known Carboniferous forms.
Above these Red Beds, which contain intercalated layers of limestone, is a considerable thickness of purely red arenaceous beds, but in studying all these rocks with some care from Pole Creek nearly to Pike's Peak, I could not separate the Red Beds from the Carboniferous by any break in continuity, and I was rather inclined to the opinion that inasmuch as a large portion of the gypsiferous or variegated beds could be shown to be Carboniferous, they might possibly all be included in that period. The Potsdam sandstone, the only portion of the Silurian era ever detected along the eastern slope of the Rocky Mountains north of the Arkansas, seems to fade out entirely south of the Red Buttes on the North Platte. It is well defined around the Black Hills, Bighorn, and Wind River Mountains. Near the Red Buttes there is a bed of siliceous pudding stone resting on the metamorphic rocks which may be the Potsdam in its southern extension, but south of Fort Laramie to Pike's Peak it is somewhat doubtful whether any trace of it exists. If it occurs at all it a very thin layer, for the most part concealed. So far as I could determine, the Carboniferous rocks rest directly (though not conforming) upon the metamorphic rocks. There is also some change in the nuclei of the mountain ranges southward. At the north the feldspathic and the gray granites prevail, but southward the syenites and igneous rocks form the central portions of the mountains almost entirely. It is rare to see true granite,
The above remarks, founded on observations that have been made over a very great extent of country through a period of many years, lead me to the following conclusions:
1st. That all the formations of the West undergo more or less change both in their mineral and fossil contents in their extension toward the west and south. They all seem to reach their culmination not far from the central portion of the great area drained by the Missouri, and lose to a great extent their distinctive characters beyond its limits.
2d. The Potsdam sandstone and the Jurassic beds present more remarkable changes than any of the others. While north both these formations are well marked, both lithologically and palæontologically in their southward extension they gradually fade out, so that south of Fort Laramie to Pike's Peak it becomes a matter of doubt whether they exist at all. The inference therefore is that these groups of rocks are not well defined, if they occur at all south of the Arkansas. In support of this statement is the fact that although this southern region has been traversed in every direction by multitudes of explorers for thirty years past, among whom have been geologists of high reputation, yet south of latitude 40° not a single animal fossil has ever been detected with Jurassic affinities, and it is quite doubtful whether any have been found with Triassic or Permian relations;* even the few plants that have been found are doubtful in their affinities and are regarded as probably Cretaceous or Permian. I have made these remarks from the fact that all the observations that have been made by explorers in the West during the past will, ere many years, be put to the rigid test of a most careful scrutiny, and an error by whomsoever made, though sustained by the highest authority in the land, will fall to the ground before the light of true science as the dead bark from a tree. The case with which the Rocky Mountain region can soon be reached, in a few years, when our great national highways are completed to the Pacific, will induce the best geologists in this country and in Europe to visit them, and the many intricate problems of Rocky Mountain geology must be solved.
* I do not wish to be understood as saying that the Jurassic rocks do not occur, south of the Arkansas, as well as the Permian and Triassic, for there is ample room for their fullest development, but no evidence has ever yet been obtained of its (Jurassic) existence, althongh the country has been so long traversed by explorers. The evidence, so far as it goes, would seem to be against its occurrence at all.