To the founders of geology the Mesozoic was known as the "Secondary" time, coming after their "Primary" which included the Paleozoic and older eras and which was followed by the "Tertiary" or last era. Just as the Paleozoic may be referred to as the age of invertebrates because during its time many groups of invertebrates reached the peak of their development only to fall into decline; from the organic standpoint, the Mesozoic may fitly be called the Age of Reptiles, for in the seas, in the air, and on the land, it was dominated by a very diverse and monstrous reptilian horde. The mentality of these reptiles was always of a low order but better things were already indicated by the appearance of primitive mammals in the Early Jurassic.
Conditions during the Triassic in the Rocky Mountain Region were much the same as during the Permian. Triassic rocks are widely distributed through the Rocky Mountain Region from Idaho and Wyoming to Arizona and New Mexico, constituting one of the greatest areas of continental Triassic deposits in North America. Here continental red beds predominate, though marine members of older Triassic interfinger from the west. These deposits are represented in the foothills east of the park, and, with the Permian, make up a thick series of red beds commonly referred to as the Lykins formation. The rocks consist chiefly of shaly sandstone, which in many places is accompanied by thick beds of gypsum. Just as in the Permian, the sea is believed to have entered this region for very short intervals of time resulting in the thin limestones intercalated with the red beds.
The principal sources of the sediments comprising the red beds are believed to have been the Colorado Mountains which had been formed in Pennsylvanian time but which were evidently still being slowly uplifted and eroded. The Triassic period closed with a widespread elevation of the land in the Rocky Mountain Region, and while sedimentation may have continued uninterruptedly from Triassic to Jurassic in limited areas, the general relations of the Jurassic to the Triassic rocks are uncomformable.
Little is known of the land plants of the Triassic. This may be due either to the fact that the arid climates so general over the continents at that time were not conducive to extensive development of plant life; or it may, on the other hand, be due to the red bed being a poor environment for the preservation of plants. From what is known, the forests then were predominantly of conifers much like our modern evergreens, and of cycads. The undergrowth consisted of ferns, tree ferns, and scouring rushes. The chief groups of Paleozoic plants were extinct, or nearly so, for the seed ferns so characteristic of the Coal Measures had largely vanished and the great scale trees are known only by rare specimens. The vertebrates of the land were becoming varied, the reptiles already far surpassing the largest of the amphibians and showing themselves to be adaptive to all conditions on the lands and even reverting to a marine habitat to easily compete with the fishes. Long-snouted phytosaurs, resembling the modern gavials in appearance and habits, were common in the streams. The dinosaurs made their appearance in the Triassic and by the middle of the period outnumbered all other groups of reptiles and held complete sway over the lands. Unlike other reptiles, they were adapted to a running locomotion, since they carried their bodies up off the ground and had the legs under the body and not at the sides. The Triassic dinosaurs wore much smaller than those living in the successive periods of the Mesozoic. They were mostly slender and few reached a length of more than 10 or 15 feet. True mammals appeared just before the close of the period but they were small, insignificant creatures and their remains are among the rarest of all fossils.
Land conditions prevailed over the greater part of the continent in early Jurassic time, the Triassic beds being exposed to erosion. Following this interval of non-deposition, conditions changed and continental deposits once again began to accumulate over the northern Colorado region. These deposits are overlain by marine deposits which indicate an incursion of the sea over this region in Upper Jurassic time; however, their stay in the Rocky Mountain region was of short duration. These deposits are known as tho Sundance formation which outcrops east of the Front Range in Colorado and consist of creamy white to buff sandstone and cherty limestone, also shales and shaly limestone. The marine members of this formation contain numerous invertebrate fossils. After the retreat of the Jurassic seas, fresh-water basins occupied large areas in the Cordilleran zone, and in these were deposited vari-colored marly shales, fresh-water limestones, and sandstones with local conglomerates, all of which intergrade laterally as is the habit of continental deposits. The coarser sediments are commonly irregularly cross-bedded. In these beds no marine fossils have ever been found but more than 150 kinds of terrestrial animals and land plants are known. These include the greatest of all dinosaurs, primitive mammals, crocodiles, fresh water clams and land snails, and land plants. During the Jurassic the Colorado region must have appeared somewhat similar to the present basins of the Amazon or Parana rivers, with low alluvial plains crossed by sluggish streams heading in the distant highlands to the west and carrying heavy loads of mud and sand, especially during flood seasons. Locally, lakes and swamps broke the monotonous topography of the plain. The climate had become more humid and vegetation spread over the landscape profusely.
Reptiles take first place in both sea and land faunas of the Jurassic and of these the dinosaurs were supreme. All of the dinosaurs of the American Jurassic have come from the Morrison formation of the Cordilleran region, which, although named from Morrison, Colorado, extends far to the north into Montana and west into Utah. These are represented by five major tribes. One of the best known American forms, Brontosaurus, reached a length of about 65 feet, while the more slender Diplodocus had a length of 80 feet. The plated dinosaur, Stegosaurus, must have weighed about 10 tons. In contrast to these large forms, others were small and very agile. Among the most bizarre animals of this time were the pterodactyls or winged reptile which possessed leathery wings and naked bodies, somewhat bat-like in appearance. These ranged from minute size with a wing spread equal to that of a sparrow up to species that spanned 3 or 4 feet from wing tip to wing tip.
In the seas ichthyosaurs and plesiosaurs were both at the zenith of their development, the former with a streamline contour and powerful fluked tail.
The close of the Jurassic was a time of great land emergence and the continent was probably larger than it is now. In early lower Cretaceous time a sea covered part of eastern Mexico and from this area it invaded the United States; however, it did not reach Colorado until late. in Lower Cretaceous time. This invading sea gradually extended north-ward over what is now the mountain zone until in early Upper Cretaceous time it joined a great southward advancing arm of the Arctic ocean and with it formed a continuous mediterranean sea the length of the continent. In Colorado, over the area of the present mountains were laid down in conformable succession the Dakota sandstone, Benton, Niobrara, Pierre, Fox Hills, and Laramie formations, making in places a total thickness of Cretaceous sediments, mainly shale, of 10,000 foot or more.
Soon after the maximum inundation of the sea (Benton Time) the northern end of the geosyncline emerged and the great eperic sea began a southward retreat that was hastened by the rapid filling of its basin with sediments pouring in from the rising highlands to the west. The sea lingered longest in an elongate embayment extending northward from the Gulf of Mexico across the western Great Plains states, known as the Lance sea. The final retreat of the sea transformed its old floor into a vast swampy lowland over which the streams spread thick non-marine sediments during the closing stages of the period. In the swamps of this lowland accumulated the vegetation that was to make the vast coal beds of the latest Cretaceous formations of the Rocky Mountain region from Alberta to Mexico.
The closing stages of the Mesozoic are marked by one of the most extensive mountain making movements North America had experienced since the Pre-Cambrian. This orogeny involved a region fully 3000 miles wide, extending from eastern Colorado to eastern Nevada and central Idaho; and from western Alaska to Mexico. At this time the Cordilleran geocyncline which had been the site of so many marine transgressions during the Paleozoic and Mesozoic and which had only recently been covered by the Cretaceous sea, was folded and faulted on a grand scale. Great forces within the earth's crust brought about extensive compression from the west. In the northern portion of the Cordilleran region the effects of this compression were very complex, resulting in great folds and extensive thrust faults causing large portions of the rocky crust of the earth to over-ride other portions generally lying to the east. In the Southern Rockies, the region in which we are most interested, the dominant structures were great open arches such as are so well shown by the Front Range in Colorado, or the Park and Sawatch ranges. Between these upfolded regions are major downfolds or synclinal areas which are still recognizable in the "Parks", namely North, Middle, and South Park, of Central Colorado. Farther northeast in the Plains, the dome-like arches of the Black Hills and the Big Horn arches were formed. To the west of the Colorado Rockies lay the Great Colorado Plateau which was bodily uplifted but without perceptable folding of the underlying rocks. Between the Colorado Plateau and the Rocky mountain region several thrust faults were developed. One of the best known of these is the one lying along the front of the Sawatch Range. These thrusts are even more extensively developed in the Middle and Northern Rocky Mountains. The Lewis over-thrust in Montana has been traced for some 50 miles north and south and has driven Pre-Cambrian strata over the Cretaceous rocks of the Plains. The Absoroka Range east of Yellowstone Park is defined by another such thrust traceable for 125 to 150 miles and having a displacement of at least 28 miles.
Accompanying these throws of mountain-making were great outbursts of attendant vulcanism. Volcanoes are known to have been active in this region throughout the later Cretaceous, and toward the end of the period they spread farther east over the rising areas. It is probable that during the building of the Rockies every state west of the Great Plains had its active volcanoes, for the Cretaceous formations of the Great Plains contain many layers of bentonite (rock made of altered volcanic ash) indicating great showers of volcanic dust. Another phase of igneous activity is represented in the great igneous batholiths of molten igneous rock which welled up from unknown depths, intruding the overlying rocks. The most notable of these were the great Idaho batholith and the Boulder batholith of Idaho and Montana.
The building of so vast a mountain system required a long period of time, even geologically speaking, and it must not be thought of as a great catyclism of nature happening in the course of a few hours, days, or years. The movements are known to have begun long before the close of the Cretaceous and probably took place in various parts of the range at slightly different times. Although the climax of the movements naturally determined the end of the Cretaceous period, this was hardly a point in time but rather a phase in a great diastrophic cycle, and it came long after the uplift had begun. The movements also did not close with the Cretaceous, but some of the dying-out phases were carried over into the Eocene and Oligocene. The drawn out nature of this orogeny has been the source of much controversy as to where the boundary line between Mesozoic and Cenozoic rocks should be drawn in the Cordilleran region.
The boundary line between the Mesozoic and the Cenozoic has given rise to one of the most prolonged controversies In the history of American geology. This has been largely due to the Laramie group which includes several thousands of feet of non-marine deposits which formed over the floor of the CordIlleran geosyncline by aggrading streams after the sea had made a great retreat. The underlying deposits belong clearly to the Cretaceous period and the overlying Fort Union formation and the formations overlying it belong to the Cenozoic. The Laramie contains land plants which are remarkably modern in their aspect and were believed by the early paleobotanists to be of Cenozoic age. Many fine skeletons of dinosaurs are also represented in the Laramie formation, and these are clearly allied with Cretaceous types and are quite unknown anywhere in undoubted Cenozoic rocks. The discovery, in 1912, of the Cannonball marine beds of North Dakota practically settled the question, since these beds, with a fauna of about 80 species of marine invertebrates showing undoubted Cretaceous age, Interfinger with the eastern part of the Laramie group. The Laramie with its great dinosaurs is therefore clearly of latest Cretaceous time, and the overlying Fort Union formation, in which no trace of dinosaur bones occur, contains distinct mammalian fossils which show a definite correlation with the oldest Cenozoic formations.