Produced by Gary Martens and Laurie Saikin
This, the closing period of the Reptilian age, is well represented among the rocks of Nebraska. It is remarkable, however, that no equivalent of the European lower cretaceous has yet been found in the West. The cause of this was stated above. During the whole of the period represented by the lower green sand of the European cretaceous, the entire Rocky Mountain region was still dry land. Whether its utmost height was reached at the close of the Jurassic, or whether it continued to rise far into the cretaceous, can only be conjectured. The weight of evidence, however, is in favor of the former view. In Europe, the lower and middle cretaceous were periods of subsidence, and, therefore, it is possible that they were here also. This subsidence continued over the Rocky Mountain area, and partially over the adjoining plains, until the entire region reached the sea level. The Gulf of Mexico encroached on the land, and gradually extended northward and northwesterly to the Arctic Sea. In Nebraska, its eastern boundary is on the west side of the Permian south of the Platte, and north of that river along the west line of the carboniferous. It crosses the Missouri above Fort Calhoun, and takes in Northwestern Iowa and much of Minnesota. As Dr. White found isolated patches of the lower cretaceous in Central Iowa, it must formerly have been much more extensive. Large tracts of it have evidently been removed by erosion.
Divisions of the Cretaceous.--Nowhere in this country is the cretaceous so well represented as in the West. Its typical divisions are seen to the best advantage on the Upper Missouri. The following are the divisions of Meek and Hayden, beginning from below: Dakota, Fort Benton, Niobrara, Fort Pierre and Fox Mills group. Those who hold that the Laramie group is cretaceous add it to the above. Since the rocks of this period have been studied in the mountains by the United States Surveys, two of the chiefs, Drs. Hayden and Clarence King, have agreed on a slightly different division. They retain No. 1, or the Dakota group, as the basal member of the series. The next two, however, viz., the Fort Benton and Niobara, they call the Colorado group. The Fort Pierre and Fox Mills are grouped together under the name of the latter. King also prefers to unite the Fort Pierre to the Colorado. The former divisions, however, appear to me the best, as they completely outline the cretaceous in Nebraska and the Upper Missouri.
The Dakota Group.--This group was so named by Hayden, because of its great development southwest from Dakota City. Beginning from below, it consists largely of whitish clay, from a few inches to four feet in thickness. Then follow various thicknesses of conglomerate and cencretionary sandstone, yellowish coarse sandstone, red ferruginous sandstone, containing impressions of leaves, petrified wood, etc. This last is from thirty to seventy feet thick. Toward the west, this group dips beneath later deposits, and therefore its real breadth cannot be ascertained. I have, however, traced its exposed surface from east to west, over a breadth of from sixty to ninety miles. It is found mainly in the following counties: Dakota, Wayne, Winnebago and Omaha Reservation, Burt, Washington, Cumming, Stanton, Colfax, Dodge, Douglas, Sarpy, Saunders, Butler, Seward, Lancaster, Cass, Gage, Jefferson, Saline, and occasionally in the counties bordering on these. Southwesterly, it has been traced to Texas. It dips out in numerous places as the basal member of the cretaceous in the mountains. It covers a large part of Northwestern Iowa and extends far into Minnesota.Origin of the Dakota Group.--As already observed during, at least, the progress of lower cretaceous times, Nebraska, with a large part of the Rocky Mountain region, was a land surface in process of slow subsidence. By the time the middle cretaceous began, this subsidence had reached so low a level as to admit the Gulf of Mexico, which gradually spread over the area where the sediments of the Dakota group are now found. These sediments are a shallow sea and beach deposit. Existing shallow seas are laying down similar materials, as, for example, the North Sea along the Belgian coast.
Fossil Leaves of the Dakota Group.--As early as 1859, Dr. Hayden had obtained impressions of dicotyledonous leaves from the rocks, which he subsequently named the Dakota group. They were remarkable for their modern aspect, as most of the genera to which they belonged are still represented in our existing flora. The collection of these leaves has steadily gone on until the present time, some of the most important being made by Meek and Hayden in 1856 and 1857, and again in 1865. Prof. Newberry also engaged in this work in another field. Prof. Marcou and Capellini also added to the number, as well as Prof. James Hall, Lesquereux and Prof. Mudge. In 1874, the Hayden Surveys published Lesquereux's Dakota group cretaceous flora, which combined all the previously published descriptions with a giant deal of original matter, and gave a full description of all these leaf impressions that had been discovered up to that time. Thus far, there have been found in this group 170 specific forms, among which are six ferns, seven cycadae, ten conifers and 150 dicotyledonous angiosperms, or true flowering trees. Of the conifers, several species belonged to the giant cedar family (Sequoia) and at least one was a glyptostrobus, similar to the one now growing in Japan and China. One palm flourished here at that time. But the great bulk of the vegetation was modern and made up of exogens (outside growers) or true flowering forms. Among these flourished at least five species of cottonwood and four populites closely allied to the former. There were also six species of willows, eight species of oak, two species of beach, six species of buttonwood, one species of fig-tree, two of spicewood, seven of sassafras and two species of cinnamonum. Magnolias and tulip trees, which are among the most magnificent of all modern trees, were then abundant. Buckthorns, walnuts, sumachs, apples and plums have left their remains in this group and must have flourished during those times.
Origin of the Flora of the Dakota Group.--No geological question is more involved in doubt than the source or origin of the flora of the Dakota group. So far as known, it is entirely disconnected from all antecedent types. There are only two ways in which we can account for the sudden appearance of this Dakota group flora. One is, that it appeared without any connection with antecedent types. It involves the theory that by some fiat of Nature's God it was spontaneously and suddenly produced. Few naturalists now accept this view. They regard the vegetable world as a connected chain. They are, therefore, in this case driven to use the "scientific imagination" and suggest the following explanation we have already seen that throughout the unnumbered centuries of the latter Permian, Triassic, Jurassic and Lower Cretaceous, Nebraska was an extended land surface, and covered by a colossal vegetation of which no memorials have been preserved The peculiar animal life of this time flourished here as elsewhere. Now, it is conceivable that during these long periods, whose length is simply incalculable, vegetable life underwent many changes, because the conditions of climate and environment changed many times. The transformation, therefore, from primitive types was gradual, all the intermediate links of which have been lost, and the last factor, the flora of the Dakota group alone preserved.
Climate of the Dakota Group Epoch.-- Many of the genera of plants of the Dakota group epoch are still flourishing in Nebraska, Kansas and even in Minnesota. Prof. Heer has also published a memoir on groups of cretaceous plants from Greenland, whose facies resemble that of the Dakota group. It seems probable that the Greenland fossil cretaceous flora was cotemporaneous with that of the Dakota group, and if so, a similar climate prevailed from Southern Kansas to the Arctic circle. However that mar be, little difference can be detected between the fossil vegetable forms in Kansas and Minnesota, and therefore a temperate climate must have prevailed over this entire region during Dakota group times, not greatly different from the one that now exists in Nebraska. It was, judging from the presence of some species, only slightly warmer than our present climate. It was colder, however, than the preceding Triassic and Jurassic, and also colder than the climate that subsequently prevailed in the Niobrara cretaceous, and during Eocene and Miocene Tertiary times.
The preceding epoch was brought to a close by the change produced by a further subsidence of the region where its deposits are found. Where shallow seas and extended sea beaches and flats, full of low islands, had obtained, now rolled deeper waters and quieter seas. The deposits formed during these times have been called by Hayden the Fort Benton group. They are dark gray laminated clays, sometimes alternating near the upper part with seams and layers of soft gray and light-colored limestone, filled in many places with marine shells. In places in Nebraska, this group contains seams of impure lignite, coal, and other carbonaceous matter. It lies conformably on the Dakota group below, It is so friable that, whenever it is left exposed, so far as I have observed it has disappeared. In many places, where deep sections have been made by canons and railroad cuts through the Niobrara group, which lies above its deposits are almost invariably present, and often in notable thickness. One of the finest of these exposures is seen below the mouth of Iowa Creek, in Dixon County, along the Missouri Bluffs. Below Milford on the banks of the Blue, and at other points in Seward County, in deep sections, it also makes its appearance. Hayden has observed its deposits in places to be 800 feet thick. As the materials are of a kind that are slowly deposited, it must have been a long epoch. The numerous low islands that had existed in Nebraska during the previous epoch had now mostly disappeared beneath the deepening seas. Some land surfaces existed in Southeastern Nebraska, but no such vegetable memorials have come down to us as marked the preceding epoch. Marine life, however, was abundant. Meek has described from this group five species of Inoceramus, mollusks distantly related to the oyster, and nine species of chambered shells, some of which were of great size and beauty. He also described many other molluscan forms. The seas swarmed with fishes, and reptilian life was abundant.
A still further subsidence of the continent, especially toward the north and west, inaugurated the Niobrara group epoch. Hayden gave it this name because of the great development of its deposits below the mouth of the Niobrara in Northeastern Nebraska. Here its deposits consist of an impure chalk rock, varying from a grayish white to a Pink bluish and yellow hue. Below the mouth of the Niobrara many of the chalk bluffs are several hundred feet high, with a perpendicular face often excavated beneath by atmospheric agencies. These chalk rocks are seen through Knox, Cedar, in many places in Dixon County, and in places on the Lower Republican. Elsewhere the deposits, especially those beneath the stratum of chalk, are mostly of an impure limestone, which often shade imperceptibly into a silicate of lime. This stratum is often called the Inoceramus bed, from the immense numbers of this mollusk which frequently compose it. Under the Inoceramus bed there is in many places toward the southwest a stratum varying from a few inches to fifteen feet in thickness, of an impure, yellowish, silicious limestone. According to Prof. Mudge, it is the characteristic feature of this group in Kansas. It can be observed at Milford, in Seward County, in places in Harlan County, and at many other points between these stations. Lately, a chalk bed of this deposit was found near Red Cloud, in the Republican Valley. It is pure white, soft, easily worked, and contains little besides carbonate of lime, and a small amount of iron carbonate, but not sufficient to color it. Judging from microscopic and chemical tests, it is as pure as the best European chalks.
The Niobrara is the most widely extended of all the cretaceous groups in Nebraska. In Southern Nebraska, from the western line of the Dakota group to Harlan County--where it is overlaid by the Plioceno--it is over one hundred miles wide. In North Nebraska, from Dakota County--where it begins to overlie the Dakota group--it extends westward for over one hundred and fifty miles. It was mostly a period when deep seas overspread a large part of the area now covered by its deposits. Southeastern Nebraska was a land surface during this epoch. The eastern border, at least, of the present cretaceous area was the eastern shore line of the interior sea of the time.
Vegetable Life of the Niobrara Group Epoch.--The diatoms and desmids which abounded in some strata in the European chalk were sparingly represented in the Niobrara group seas. I have only in a single instance found a few diatoms under the microscope in some chalk obtained below the mouth of the Niobrara River. The specimen was overlaid by a portion of the skeleton of a fish which seems to have protected the silicious matter which had accumulated end which contained the diatoms.
The peculiar impressions of geologically modern leaves (Dicotyledons) which characterize the Dakota group are wanting in the Niobrara. Different seas now prevailed, and, as is evident from the fossil animals to be noticed, hereafter, a warmer climate. Fossil wood, however, is abundant, both petrified and agatized. Microscopic determinations have detected many species of trees, the majority of which were conifers of araucarian type, the balance being deciduous trees, with a few cycads and zamias. The Dakota group vegetation that characterized this region in early cretaceous times had retreated, and a southern flora that had culminated in Jurassic times had largely taken its place.
Animal Life of the Niobrara Group Epoch.--The chalk of Europe, which was mainly made of rhizopods, contained them so abundantly that, according to Ehrenberg, there were millions in a cubic inch. They were also abundant in our own chalk seas. Some specimens which I examined under a compound microscope, from Cedar County, contained them almost in equal abundance. Spiculae of spenges often occur. Molluscan life was specially abundant. One of its limestone layers is known as the Inocaramus bed, from the abundance of the shells of that genus. One species closely related to the Inocerami was twenty-seven inches in diameter. There were also great numbers of anomias, oysters and chambered shells. The seas swarmed with fishes, some of which, like the Protheus molossus, were among the most rapacious that ever existed. The abundance of shark remains testifies to the great number of fishes that must have existed to supply food to these rapacious animals. In the chalk of Knox County, through a vertical thickness of 100 feet, every foot of rock contains more or less of scales, bones and teeth of fishes.
Reptiles.---At least forty species of reptiles have been described by Leidy, Marsh and Cope, from the Niobrara group of Kansas and Nebraska. They ranged in size from twelve to seventy-five, and one hundred feet in length. One tortoise had a spread of expanded flippers of fifteen feet. Here have also been found the largest of flying reptiles. One of these, a Pterodactyle, had a spread of wing of from twenty-three to twenty-five feet.
Birds.--Nothing is more remarkable about this remarkable age than the peculiarities of its bird life. Like other vertebrate forms of that epoch, the birds were highly reptilian. Eleven species have been described by Marsh from these deposits. He has made from them two orders. One of these orders had teeth in continuous grooves, and saddle shaped vertebra. The other order had teeth in distinct sockets, and fish typed or bi-concave vertebra. The former was represented by a form called Hesperornis, of ostrich size and diving fish-eating habit. The latter order is represented by a form which Marsh calls Ichthyornis. It had great powers of flight, was aquatic, and also lived on fish. This bird disputed, in those times, the empire of the air with huge dying reptiles (Pterosaurs), to whom it was more or less distantly related. It is evident, therefore, that there was a most vigorous life during Niobrara group times. During the earlier and middle portion of this era, the Niobrara Ocean was connected on the west with the Pacific. Later, the sea bottoms were raised up along the Rocky Mountain chain, giving access and egress alone from the gulf on the south, and the Arctic Ocean on the northwest. A slow process of elevation continued on the east as well as on the west, contracting this ocean to ever narrower limits. A reverse movement was now going on from what was taking place early in its history. Then it was in process of subsidence, now it was in process of slow elevation. When sand-bars eventually were thrown across the channels of moving waters, much of its life was imprisoned and gradually destroyed. The most vigorous species and individuals would last the longest, but all eventually had to submit to the inexorable fate of final extinction.
The Niobrara group epoch had come to a close by that process of elevation that had already been long going on along the eastern cretaceous region. A trough containing oceanic waters was left in Northeastern and in Southwestern Nebraska. Whether it was continuous diagonally over the State has not been definitely ascertained, owing to the thickness of the superincumbent deposits. This trough, or the materials which filled it up, and which constituted the Fort Pierre deposits, are exposed in places in Knox and Hitchcock Counties. The materials in Nebraska consist of thin beds of brownish sandstone, underlaid by plastic clays, calcareous shales, sometimes containing sulphuret of iron, and, more rarely, carbonaceous matter. Gypsum in the form of selenite is often present. Only the lower members of this group are present in Nebraska. The rarity of organic remains in Nebraska in this group indicates that during its formation the waters were not connected with the ocean. It was losing more water by evaporation than it was receiving, and hence the deposition of gypsum which always occurs under such circumstances. The life that, at its opening, swarmed in its waters, was rapidly exterminated. Elsewhere, especially on the Upper Missouri and in the Rocky Mountain region, it was very different. There an abundant animal life filled its waters, but, as those remains occur beyond our boundary, they are not here discussed.
No rocks of this group occur in Nebraska, and hence the discussion of its remains is omitted. From the middle of the Fort Pierre group epoch to the, close of the Fox Mills, Nebraska was again an extended land surface. During this time, in the Rocky Mountain region, from 500 to 3,000 feet in thickness of sediments were deposited. During this long epoch, Nebraska was covered with forests and savannas. In the forests, cycads, zamias, araucarian pines and some deciduous trees flourished. The land swarmed with an abundant animal life, among which dinosaurs were the dominant type. Lowly mammals of marsupial type were struggling for existence, but already giving promise of that prodigious development which was to mark this class in the next era.