Produced by Gary Martens and Laurie Saikin
The closing centuries of the Miocene were the opening ones of the Pliocene epoch. At this time, the great Miocene lake of the plains underwent further subsidence, but gently and gradually. The convulsions of the mountain regions were not shared by the plains. There is no trace on the plains of the intervention of a period of dry land, as some have supposed. The Miocene became the Pliocene lake by subsidence, and extension northward, southward and eastward. In places, it became much deeper than it had been. "On the west, it now even reached the foot-hills of the Colorado Range." The Pliocene in Central Nebraska overlies the Niobrara cretaceous; in Southwest Nebraska, it lies on the Fort Pierre cretaceous.
The Pliocene of Nebraska was called the Loup Fork group by Meek and Hayden, and the Niobrara group by Marsh This last designation is not allowable, as that name is used to designate one of the cretaceous groups. Cope has recognized two divisions of the Pliocene in the West, namely: The Loup Fork group and the Equus beds. This latter is the upper Pliocene of King and the post Pliocene of other geologists.
Thickness of the Pliocene Beds.--Along the foot-hills of the Colorado Range, the Pliocene beds average nearly two thousand feet in thickness. They thin out eastward, probably because the mass of materials was obtained from the mountains, the greater part of which was precipitated along or near its western shores. In Nebraska, Kansas and Dakota toward the east, the Pliocene beds become thinner, until they ran out entirely. It is certain, however, that originally they were much thicker than at present. Being the upper rocks at the time, they must have been subjected to an enormous amount of erosion during the subsequent quaternary age. The monuments of this erosion are still visible in many places. In Township 10 north and 26 west of Sixth Principal Meridian, there is a Pliocene peak, nearly three hundred feet high, that represents the original level of these deposits. In 13 north, 51 west, there are limestone cliffs seventy-five feet high, and similar ones all over this region, in far separated, isolated spots. The top of all these rocky cliffs, whose strata are horizontal, represent where the general level of the Pliocene once was. Perhaps the most remarkable monument of the original level of the Pliocene in Nebraska is at Scott's Bluffs, and at Chimney Rock, on the North Platte. These have long been noted landmarks. Chimney Rock is about one hundred and fifty feet high. The strata here and at Scott's Bluffs are horizontal, and therefore the general level of the country must have been as elevated, at least, as the top of these crags. No doubt much material has also been removed from the top of these old monuments, as they have been subject to erosive agencies ever since the commencement of the glacial age. From two to four hundred feet, therefore, must have been removed from the general surface of the Pliocene deposits of the plains. Notwithstanding the immensity of this erosion, a considerable thickness still remains.
Elevation of the Pliocene.--At Chalk Bluffs, the line of separation between the Miocene and Pliocene is 6,000 feet above the sea level. Near 41°30´, the Pliocene reaches an altitude of over 7,000 feet. In the valley of the Loup Fork, the contact plain between the Miocene and Pliocene approximates to 3,000 feet. There is, therefore, a gradual sinking eastward of the contact plane between the Miocene and Pliocene.
Eastward Barrier of the Pliocene Lake.--It has been a question what barriers on the east held in the waters of the Pliocene lake of the plains. Two theories have been suggested. One is, that the whole western shore line, with the mountain chain against which it abuts, and the present incline toward the east, was low enough, during Pliocene times, to hold the waters of the lake. This theory, however, is irreconcilable with the known facts concerning the elevation of the Rocky Mountain system during the Tertiary epochs. Evidently, this region near the eastern shores of the lake, arid on the south, was once elevated into a rim, and it was the sinking of this border, toward the close of the Pliocene, and the transference of the geosynclinal of the continent to the Missouri and Mississippi Valleys, that helped to bring the Pliocene to a close. The Pliocene materials at least were deposited in a broad level lake between the meridian of 98°and 105´, and subsequently the whole area of subsidence toward the east, accompanied by slight continued elevation toward the west, was transformed into an incline from the base of the foot-hills eastward. This eastern rim was a comparatively low plateau, without lofty mountains or ridges, and the streams which flowed from it were of insignificant size.
Materials of the Pliocene Beds of the Plains.--Near the mountains, the materials of the Pliocene beds are exceedingly coarse, and where they are in contact with the foot-hills, they are composed of conglomerates made up of water, worn pebbles, feldspar and quartz in masses, and some small pieces or chips of all the archaean rocks which are represented toward the west. The fragments are of all sizes, from a shot to a man's head, and even larger. The coarser conglomerates are from the upper beds beneath which there are often much finer materials. The erosion of the upper strata has, in many places, cut through the coarse conglomerates and widened the bed below in the finer sediments, producing overhanging rocks. At Scott's Bluffs, near the western line of Nebraska, there is a fine exposure of the Pliocene rocks, which are here made up of sandstones, marls and whitish and yellowish white clays. Along Lodge Pole Creek, the Pliocene rocks have assumed more the form of bluffs. Here, and occasionally on the Upper Republican, the thin, marly members sometimes contain thin masses of jasper-like rocks, which occasionally contain dendritic markings, produced by oxides of the metals. Among these, moss agates are occasionally found. On the Niobrara and Loup Rivers, there is, in many places at the top, an immense amount of loose or at least incoherent sand, or loosely compacted sand. The decomposition of these Pliocene beds in these regions has produced the famous sand hills. Next below, are beds of compacted gravel and sand. Then come calcareous and arenaceous concretions, combined with or inclosed in whitish and yellowish grits Greenish and greenish gray sand comes next. Arenaceous marl, shading from deep yellow to dull red, lies below the last. At the bottom is observed a grit of yellowish hue, often highly calcareous, and sometimes containing limestone more or less concretionary, from one to seven inches thick. Whenever the strata are followed to any distance, they are observed to vary a great deal in character, and often within a few hundred yards. The least variation exists in the great marl beds. Sections on the south line differ very much from sections on the north line of the State. The most silicified strata contain more or less of calcareous materials. The smooth water-worn pebbles of the conglomerates are composed of many kinds of rock, metamorphic and archsean materials being the most abundant. In places in Southwest Nebraska, the cap rock of the Pliocene is an intensely hard silicious stratum from two to ten feet thick. It varies from something akin to quartzite, to flint, hornstone and chalcedony.
Polishing Powder--lnfusorial Earth--Geyser Flocula.--One of the most remarkable of all the deposits of this Pliocene lake of the plains is a peculiar, flour-like material that appears in beds of greater or less thickness and extent that occurs on the Republican, the Loup, Niobrara and other sections. When I first examined it under the microscope, eight years ago, a few diatoms were collected from which circumstance it was regarded as probably of the character of tripoli. Since then, in many specimens that have come under my observation, a diatom has rarely been found. In almost every specimen examined, however, great numbers of the forms that Ebrenberg called Phytolitharia were detected. The most conspicuous of these are triangular in shape, with one edge convex and the other concave, or the opposite. They cover, under a microscope magnifying 300 diameters, the space of about one-eighth of an inch, and of incalculable thinness. Under such magnifying powers, the specimens are translucent. The chemical analysis of this earth is, however, very different from tripoli. It is a silicate of the alkaline earths, and generally of soda, magnesia; lime or potash. Sometimes one and sometimes several of these alkalies are present. Color, from light gray, snow white, green slid yellowish. To the touch it seems much like flour. When used as a polishing powder, no grit or scratching is detected, even under a microscope. Along the Republican, it is found in almost every county. A bed of this earth, which may be regarded as a type of all the rest, of great thickness and an exposure of one-fourth of a mile long, exists in Furnace County, about eight miles southeast of Arapahoe.
Origin of this Flour-Like Earth.--Near or in many of these beds, all over the Pliocene region of the plains, are found many extinct geyser tubes, and sometimes old geyser basins. Of these I obtained at least thirty between Arapahoe and the west line of the State. I have also found them in the Loup region and on the Niobrara. As some of these geyser tubes had their exit in the Fort Pierre group, on the Upper Republican, it is probable that they commenced their work in the Cretaceous period, and were in operation all through the long centuries of the Eocene, Miocene and Pliocene epochs, and far into the Quarternary. A similar bed exists on Oak Creek, which was deposited in interglacial times. Nebraska, and at least Northern Kansas, in fact, was a great geyser region all through the Tertiary period. It far exceeded in the number and magnitude of its geysers the Upper Yellowstone region and Iceland at the present day. Few memorials of those old extinct geysers are visible at the present time, owing to their being covered up by the superincumbent Quaternary deposits, but enough remain to show that a prodigious number must have existed in at least Pliocene times. It is probable that this flour-like silico-alkaline earth owes its origin to these old geysers. It is well known that hot alkaline waters dissolve silica. When, therefore, the geyser streams holding silica and alkalies in solution were poured into this old lake, they were precipitated, on cooling, to the bottom. Chemical analysis shows the identity of this material with the geyserite from the Yellowstone.
Length of the Pliocene Epoch.--The great amount of erosion to which the Pliocene rocks have been subjected, and the great thickness of the beds yet remaining, especially along the base of the mountains on the west, indicate that this epoch was of long duration. It probably endured through as many centuries as the preceding Miocene.
Life of the Pliocene Epoch--Coniferous trees, some of them of araucarian type, the giant cedars (Sequoias), were abundant in Nebraska during the Pliocene. A juniper, a cypress, were also here. In the lower portion of these deposits, sabal palm remains, along with the fig-tree, occur. The sweet gum, locust, cassia, sumach, walnut, tulip-tree, cottonwoods and oaks all flourished during those times.
Animal Life.--A large turtle abounded. The remains of a large eagle, found in the Loup Pliocene, indicates the presence of an abundant bird life. The mammalia were still the dominant class. Of the horses, at least four genera and fifteen species flourished. With the three-toed horses were now associated a one-toed horse, equus excelsus, the most perfect--at least the most modern--that had up to that time appeared. It differed only in trifling details from our present horse, and extended through the Pliocene into interglacial times. The horses which had commenced in the Eocene with the dawn horse family, and abounded all through the Miocene, seemed to culminate in the Pliocene. In most of the genera, the lateral toes had disappeared, but still retained, as the modern horse does, the splint bones as memorials of the original structure of the family. The rhinoceros, which was cotemporary with the horses during these times, was about the size of the one now living in India. The elephant family was represented by at least two genera and two species. Mastodon mirificus has not yet been found outside of the Nebraska Pliocene. The elephant described by Leidy, from the Niobrara, was larger than any that has yet been found in rocks of any epoch. A tapir, a hog, antelope and deer were also abundant. The oreodons, so abundant during the Miocene, had, however, dwindled to three species. To make up for the decadence of this family, the camels appeared in extraordinary numbers. The most characteristic genus was Procamelus, represented by at least five species. One species of this genus was about the size of the existing camel. The largest of all the family, whose remains were found, Leidy has named Megalomeryx Niobrarensis, from the locality where its remains were obtained. The molar teeth had affinities to those of the llama and sheep. It was probably about one fourth larger than the modern camel. A still more curious, comprehensive, camel-like animal, described by Leidy, was Merycodus necatus. Its molar teeth combined the characters found in the sheep, camel and deer. It was about as large as the latter animal. Two species of bison and a bear-like animal lived here at the same time. Of the cat family at least one species approached in size and ferocity the existing Bengal tiger. There were four species of the dog family, one of which was one-half larger than the largest wolf of modern times.
Not the least remarkable feature of Pliocene times was the presence in extraordinary numbers of those animal forms which are regarded as most useful to man. And yet it is doubtful whether man had yet appeared. At least no undoubted monuments of his presence in the world during Pliocene times have yet been discovered.
Let us now, if we can, form some picture of the character and physical condition of the Tertiary ages. Take, for example, the middle Pliocene. Had we been in existence then, and started westward on a journey from some point near where the Missouri now flows, much of the peculiar life of the times would have been observed. The climate was congenial in an eminent degree. The great Pliocene lake caused a much moister atmosphere than exists at present. Groves of Sequoias, like the present gigantic trees of California, the glyptostrobus of China and Japan, the cypress, the date and the palm, were interspersed with magnificent savannas. The songs of ten thousand birds, many of them of the most beautiful plumage, would have greeted our ears. At some places, herds of thousands of Oreodons would have been encountered. Bisons, similar in form to our buffaloes, would have been seen cropping the grass. At other points, might have been seen herds of elephants and mastodons quietly proceeding toward some streamlet, or lakelet, to indulge in a bath. Vast numbers of many species of camels would have been seen reposing at midday on a gentle hillside under the shade of sequoias or cypress. More curious than all, thousands of Hyperions, those wonderful three-toed horses, along with many kinds of one-toed horses, of all sizes, would sometimes have made the earth tremble under their tread. When, at last, in such a westward journey, the shores of the great Pliocene lake would be reached, its borders would have been a marvel for the life represented there. A rhinoceros might have been seen wallowing in the mud near the shore. Thousands of waterfowl would have been riding the gentle waves. Elephants, camels, oreodons and horses might have been seen there slaking their thirst in the streamlets flowing into the lake. Life would have been observed everywhere--the hum of insects and the song of birds in the air--life in the trees, in forest and glade, on land and lake. Most of it, too, was happy life. It is true, some unfortunate ruminants would fall victims to the gigantic wolves and cats of the time, but the carnivora were not the rulers of the land. Grass and leaf and seed and fruit eating animals were the rulers of the Pliocene world in Central North America. It was a physical paradise, for violence, rapine and murder were the exception and not the rule. Violence, indeed, has existed in every geological age, but in Pliocene America, herbivorous life was so dominant that it could successfully defend itself against the carnivore, and the latter evidently obtained most of their prey by stealth and by picking off the aged and infirm. Animal life is generally happy when it is left alone, and this was specially the case during American Pliocene times. It is a grave reflection on humanity that, not the Creator, but man's injustice and inhumanity, produces most of the misery which we see in the world. This is, however, but a very faint picture of these happy Pliocene times which lasted for untold ages. But every one can, for himself, by "the aid of the scientific imagination," form such pictures of the wonders of that old-time world.
Close of the Pliocene Epoch.--There is evidence that the Pliocene epoch only gradually came to a close. The lake of the plains was probably partially drained, and a large part of its surface became dry land long before the last centuries of the Pliocene had ended. The eastern border of the great Pliocene rim commenced to descend and gradually left out the water until much of this great lake of the plains became dry land. There is also evidence of increasing cold in the deposits of this lake through their upper sections. The southern shores of the lake were probably rising at the same time, which helped to intensify the growing cold. Glaciers were forming toward the far north, and conditions of climate similar to the present were introduced. Arctic conditions gradually began to prevail in temperate latitudes. The flora and fauna as gradually migrated southward, and many species were exterminated. The distinct fauna that appeared during the next or Quaternary period originated some time during this epoch of transformation of Pliocene into Glacial times. During the closing centuries of the Pliocene, convulsive movements occurred farther west, among which were vertical displacements of mountain masses and lava outflows, but so far as observed nothing of so violent a character occurred on the plains.
We have seen how the Pliocene epoch closed its remarkable history. Its closing centuries were the opening ones of the Quaternary period. The plains were dessicated before the Pliocene had entirely passed away. Following this condition of excessive dryness came one of great humidity and a much lower temperature than the previous or present age. The snows of winter eventually accumulated too rapidly to be removed by the summer's warmth. This finally resulted in the glaciation of the plains of Nebraska. A thick mantle of ice extended south of the southern line of the State, and, according to Agassiz, at one time to the thirty-sixth parallel. Thus was inaugurated the Glacial epoch of the Quaternary period. The following is the order of epoch of the Quaternary period in Nebraska: A Glacial, or Drift, Forest Bed, a second Drift, Loess and Terrance epoch.
The monuments of this epoch are undoubted. along the Missouri River, from Fort Calhoun to the Kansas line, wherever the superficial deposits are removed and hard limestone constitutes the surface rock, they are worn as smooth as mirrors, except diamond-like scratches which cross them in a direction averaging 17 degrees east of south. Below Plattsmouth, at one point, they run 27 degrees east of south. Twelve miles east of Lincoln, the direction of these scratches is 13 1/2 degrees east of south. The ice in glacial times seems to have moved more easterly than later. Similar markings are made by existing glaciers and all geologists now refer them to the same origin. The materials lying on the country rock over the State are of glacial origin. First, we have in a multitude of placer beds of blue clay, from two to sixty feet thick. Above the latter, or where it is wanting, on the country rock, modified drift gravel and clay, or simply sand or gravel, occurs. Next above are gravel and bowlders of various sizes and shapes and generally water-worn. Sometimes the blue clay has intercalated between its layers thin beds of sand and pebbles. This blue clay is a characteristic feature of the earliest deposits of the Quaternary, over a great part of Southern Nebraska and also in places in Northern Nebraska. Where free from sand, it is practically impervious to water. In most of its characters, it resembles the English till, which Zeike shows was formed beneath glacial ice. A bed of ice 3,000 feet thick moving forward at the rate of a few inches or few feet per day would crush and pulverize everything beneath it On the final retreat of the glaciers, this fine, impalpable mud, made by this mighty crusher of ice, in part accumulated at the lower end, and in part was carried away by the rushing streams to be deposited in quiet waters. In some such way, the glacial clay became somewhat irregularly laid down over the State. The gravel, small and large bowlder deposit above the blue clay is the most widely distributed glacial deposit of the State, though in vertical thickness it is much less than the Loess. In a few townships, at isolated spots, it comes to the surface, but generally it is deeply buried beneath later deposits. Sometimes, where it is exposed at the surface, it is so mingled with later materials as to escape the attention of any except the most observing. It ranges in thickness from a few inches to seventy-five feet. Above this true drift material, modified drift occurs in many places and conspicuously in some of the counties in Southeastern Nebraska. It has been remodified, and, therefore, somewhat stratified by water agencies. Its materials are generally comparatively fine and so mingled with Loess and calcareous matters as to become an exceedingly fertile soil. The pebbles disseminated through this modified drift, and the true drift below it, are largely made up of metamorphic materials, and must have been brought from beyond the State. Variegated agates, cornelians, jaspers, chalcedonies, etc., are frequently found over them.
Old Forest Bed.--Above the preceding glacial deposits there is found in many counties in the State, in deep cuts, a black soil from a few inches to ten feet thick. It closely resembles the present surface black soil of the State. In places in the eastern part of the State, it contains a very large quantity of silicified wood. Many of the specimens which I examined, microscopically proved to be coniferous. This bed in the Loup region yielded fewer specimens of wood. In this epoch, therefore, Eastern Nebraska must have been covered with forests that were at least in part coniferous, and Western Nebraska partly with forest and partly with prairie. Along with the coniferous trees there grew here cottonwoods, maples, willows, walnuts, oaks and other deciduous trees. A huge buffalo and elk, deer, giant beavers, mastodons, the hairy elephant, and at least one species of fossil horse flourished here at this time. The mammalian life that ruled in these old forests was gigantic in character. The thickness of its black soil deposit, and the remains of its abounding animal life, indicate that, compared with the cycles of human history, it was a very long epoch.
Second Appearance of Glacial Drift.--On top of the old forest bed materials, and where these have been removed, on top of the siliceous clay door of the forest bed, occur gravel, sands of various degrees of fineness, bowlders and bowlder clay. In places, the bowlders of various sizes constitute the principal portion of the overlying materials. Sometimes these bowlders are marked with parallel striae, and beds and piles of them occur of enormous thickness.
Kames.--On the Logan, Elkhorn, on tributaries of the Republican and Loup, and in other places, occur long rows of sometimes gravels and sometimes sands, very distinct from the Pliocene sands of the Niobrara. The Loess deposits to be described hereafter abut against them, but often their tops have been blown over the Loess to such an extent that even geologists have been deceived into the belief that they are of very recent origin. They, however, antedate the Loess, as is evident from the fact that they extend beneath the latter. I am not sure whether any of them exhibit any true marks or stratification. They bear some resemblance to the Kames of Scotland and Asars of Scandinavia, and to them they are for the present referred, though doubtfully.
Calcareous and Silicious Materials.--Between the deposits which are doubtfully referred to the Kames, and lying on their flanks, occur, in many places, great beds of fine silicious matter, which, in places, is rich in iron, mostly in the form of a sesquioxide. It is often mistaken for Loess, whose character it approaches, but is slightly coarser and a shade darker brown, and varies also more in character. It can be studied to good advantage in the Republican Valley, from Harlan County westward. From Plattsmouth to the south line of the State, it often shades insensibly into Loess. Here sometimes beds of sand take its place. Often this deposit is highly calcareous, and sometimes it shades down into the drift below it. In such cases, the drift pebbles and bowlders are covered with calcareous matter.
The most remarkable of the deposits at this horizon are the strata of calcareous and other alkaline matters that are found in the upper portion of these siliceous beds. The amount of alkaline matter ranges from 10 to 90 per cent, and the beds vary in thickness from a few inches to fifteen feet. Between Nebraska City and Brownville, along the Missouri bluffs, are some fine exposures of these alkaline beds, though they are more mingled with sand and gravel than farther west. The calcareous concretions found here are, however, exceedingly abundant and beautiful. Samples are common which measure from one to five inches in diameter. Inside they are sometimes partially hollow, and, portions of the mass being separated and loose, they rattle, on being shaken after drying. In Saline County, there is a thin, almost pure snow-white layer of this calcareous matter. Farther west, in Webster, Fillmore, Hamilton, York and some other counties north, as well as south of the Platte, this alkaline material occurs at this horizon at various localities. It differs greatly in thickness and extent of beds, and in the proportions of the alkalies present and silicious materials with which it is combined. It has sometimes been used for mortar and plastering, and from the people has received and the name of natural mortar. It does not, however, avail for outside work, as the rain softens and gradually removes it. The alkaline material is largely composed of lime and magnesia. Along the Missouri, the alkaline concretions are almost wholly composed of it.
Resume of Geological History between the Glacial and, Loess Periods.--We have seen that the retreating ice sheet of the Glacial period left in its path huge beds of blue clay and other Drift materials, which in their upper portions were modified by water agency. The land was flooded, and over the great lake or interior sea thus formed, icebergs floated and dropped their loads of sand, gravel and bowlders on the bottoms, and where they were stranded left this debris in enormous heaps. This period of depression and floods was followed by one of slow elevation, when the waters were drained off and a new forest bed was formed to the shores of the retreating lakes, or to the foot of the glacier mass. As the period of glaciation was a time of very great relative humidity, this must also have been the character of the climate all through the flood and old forest bed period. The ice sheet again advanced and destroyed these magnificent forests before it. Newberry, who first directed attention to this old forest bed, found no evidences of this period of glaciation in Ohio. Here, however, it is clear. The second advance of the ice sheet obliterated, in many places, this old forest bed. The retreat of this ice sheet was accompanied by another period of subsistence, the land was again flooded and a lake again occupied the plains. During the ages that this body of fresh water abutted against the ice sheet in the north, it received icebergs from the retreating glaciers. The materials left by the glaciers were remodified in their upper portion, and new matters also laid over them by the torrents and icebergs. When the ice sheet retreated from the shore lines, finer sediments began to be laid down. Fine sand now took the place of gravels and bowlders, and, as the waters contracted in volume, the calcareous matter held in suspension began to be precipitated. There is no evidence that this great lake was dessicated previous to the beginning of the next, or Loess period. It was only reduced to smaller dimensions. When, at last, Central and Eastern Iowa became dry land, and the ice sheet had retreated to the head-waters of the Missouri and Yellowstone, the Loess materials began to be laid down on the floor of the old lake bed. So important are these Loess materials in historic and economic geology, that they will be discussed in greater fullness in the next section.