HISTORICAL GEOLOGY AND PALEONTOLOGY
It is essential in all historical studies to discover the order in which events have succeeded one another, for this permits an analysis of cause and effect. Although time is continuous it is punctuated by important events, so for convenience we divide it into separate units for easy reference. A study of the rocks over large areas of the earth's crust have led to the discovery of a chronological series of events through which our planet has passed. In working out this history in any particular region the superposition of beds is greatly relied upon to furnish the sequence of events of the time during which they were being deposited. As is well shown in Scotts Bluff, where a series of horizontal beds may be seen, one bed overlying another in succession, the lowest formation is known to be the oldest and succeeding strata, are of successively younger age. In other words each formation has its peculiar group or assemblage of species which is known as its fauna. In regions such as that surrounding Scotts Bluff, where the formations are not seriously disturbed by earth movements, a study of the fossils contained in superimposed beds reveals a faunal succession.
Since the beginning of geologic time no region on the continental areas has been the site of continuous deposition. The rocks in any one region may give a portion of geologic history or several portions, separated by breaks during which time no sediments were being deposited. These fragments of different regions may be pieced together by various means of correlation, either by the use of faunas or faunal succession, by tracing key beds, or by using some inorganic means of correlation. As a result, the major events in the history of the earth have been arranged into a chronologic chart called the Geologic Time Scale.
The only clock the geologist has to go by in estimating the length of the periods of geologic time which offers any degree of accuracy is that of the disintegration of radioactive minerals found in certain rocks. By this means he has estimated the earth to be at least two billions, of years old. An older method commonly used until the discovery of radioactivity was the determination of the present rate of deposition for various kinds of sediments and the application of this to the known thickness of the older formations. Breaks in the record and discrepancies in the rate of deposition introduced great errors in this method, the age of the earth being estimated at 500,000,000 years.
The formations so excellently exposed at Scotts Bluff and in the surrounding area were found to have been deposited during the most recent era of geologic time, the Cenozoic. The almost horizontal beds represent extensive sheets of sandstone, shale and conglomerate deposited in this region as sediments by streams which drained the higher land to the west. Underlying this thick blanket of Cenozoic deposits are older formations representing much of Mesozoic and Paleozoic time. They are hidden from view in the Scotts Bluff region but exposed in the Black Hills and Rocky Mountains where folding and uplift have occurred followed by erosion. From detailed studies of these rocks in the surrounding regions a great deal may be inferred as to what was happening in the Scotts Bluff locality. As a background for the more important events of the Cenozoic which are so magnificently represented in this region, it may be well to review briefly the history leading up to the mere important Cenozoic Era.
The early and middle Paleozoic history of this region is very incomplete. It is believed that the extensive invasions of the sea which extended over a large part of the interior of North America during the Late Cambrian, the Ordovician, the Silurian, and Devonian did not cover this region. (Schuchert, Chas., 1933, pp. 125, 146, 175, 196-197. Darton, N. H., U. S. Geol. Sur. Folio No. 87, Scotts Bluff Folio, states that the Late Cambrian sandstone may underly this region). The Silurian and Devonian are very thin or absent in the region to the north and west, their presence here being doubtful.
Carboniferous limestones, shales, and sandstones are believed to underly all of Nebraska. They are known to the north and northwest about the Black Hills and on the slopes of the Rocky Mountains as well as in Eastern Nebraska where they form cliffs along the Platte River from Ashland to Plattsmouth. The deposits consist of limestones, shales and sandstones and contain thin coal beds in some localities.
During the Carboniferous, widespread transgression of the ocean extended over the Central Great Plains region in which were deposited thick beds of limestone. In the later portion of this period uplift occurred, diminishing the extent of this sea and causing it to shallow so that coarse sediments appear in the record represented by sandstones, sandy limestone and red shale. Emergence was even greater during the Permian when an extensive inland basin was formed across the western portion of the Central Great Plains and extended far to the northwest however, it remained above sea level. In it was deposited great masses of red shale and gypsum, products of an arid climate. These conditions of deposition and aridity continued into the Jurassic.
In late Jurassic time a sea covered this region again and its deposits are known to occur in the Laramie and Big Horn Mountains, in the Black Hills and probably extended beneath the younger formations over Northwestern Nebraska. Mostly fine materials were deposited, but locally, red beds occur indicating the position of old shore lines. In some regions there was a short return to continental conditions during the Middle Jurassic. However, during the Upper Jurassic there was a return of the sea, for we find thick deposits of shale, and thin beds of limestone indicative of deeper water. (Sundance formation)
This incursion of the sea was of short duration however, for it was followed by widespread uplift in the latest Jurassic time. In the region extending from Montana to Oklahoma the deposits consisted principally of clays of the Morrison formation. It is probable that the Morrison formation was deposited over Western Nebraska, but the eastern margin of this formation has not been definitely located. The Morrison is known to contain more than one hundred and fifty kinds of terrestrial animals and plants including the greatest of all dinosaurs.
The Jurassic was followed by a long period of non-deposition and erosion, but in Middle Cretaceous time the seas again inundated the region. The first deposits, formed as the sea encroached over the land, were rather coarse sandstones known as the Dakota Sandstone. This formation underlies the entire central northern Great Plains and is a source of artesian water supplies. Sandstone was followed by a rapid change in the deposition to clays, even more extensive in their distribution than the Dakota Sandstone. These make up the Benton, Niobrara and Pierre formations which record the maximum extent of the seas throughout this region. These clays are finally capped by more sandstones indicating a return of shallow waters and a final retreat of the sea from this region. Marine waters have never since spread over this region. In the wake of the retreating sea brakish water lagoons and bays formed, which in the course of time finally became entirely fresh water. The deposits formed in them represent the transitional state of time between the Cretaceous and Cenozoic.
Extensive crustal unrest marked the closing stages of the Mesozoic Era. The floor of the greater part of the basin which has so recently been covered by the Cretaceous became the scene of folding and thrusting on a colossal scale, resulting in the Rocky Mountains and the Black Hills. This mountain belt extended from Alaska to Mexico, and was approximately five hundred miles wide from east to west in the United States. This great disturbance of the earth's crust is known as the Larimide Revolution. Its effects were not felt in the Great Plains region, for here the older beds still lie in almost a horizontal position.