Early in Cambrian time the seas invaded the continental borders and advanced slowly toward the interior. They did not reach Colorado until nearly the close of the period, and as a consequence, no lower, middle, or early upper Cambrian rocks are found in the state. In later Cambrian time the seas invaded northern and central Colorado, the waters coming in from the northwest. The waters of the sea found the surface of decayed rock much as one finds the surface of the lower granitic areas of today, strewn with broken rock, sand, and clay, grading downward into the unweathered bedrock. The rivers which formerly carried the sediments beyond the region into the oceans bordering the continent, now dropped them in the shore waters of the invading sea. The waves worked over the residual surface material, and the sediments brought in by the streams, sorting them and spreading them over the sea floor in layers or strata of conglomerate, sandstone and shale. In some parts of the sea the shore waters were clear and made a favorable habitat for animals and plants. The animals took lime from the waters, made it into their shells and other parts, and as the animals died, these limy parts accumulated on the bottom. Century after century these accumulations grow, and the waves broke and ground many of them into a mud-like mass and spread a mixture of limestone mud, shells, and shell fragments over the sea floor in layers which afterward consolidated into limestone strata of the upper part of the Cambrian formation.
Cambrian rocks have not been recognized in the boundaries of the park, but they are known to occur at numerous points along the eastern slope of the Front Range in southern Colorado. There they consist principally of sandstone with some shale and a little limestone. They also occur bordering the Pre-Cambrian masses in the San Juan Mountains, along the White River Plateau, and in the Uinta Mountains. The formation is generally thin, but in the Uinta region it reaches a thickness of 1,200 feet.
Lands during Cambrian time must have been scenes of barren desolation, for rocks of this age bear no direct evidence of terrestrial life of any sort. Moist lowlands may have been clothed in feeble vegetation more like sea weeds than any of the plants which we know growing on the land today. Animals had not yet learned to breathe air, and there is not a trace of land animals or of inhabitants of rivers and lakes. The seas, however, swarmed with a great variety of invertebrate animals; the dominant types were trilobites, animals somewhat resembling the modern horse-shoe crab.
For some time after the opening of the Ordovician period the land areas continued to decrease and the seas to advance. Much of the land surface had evidently been worn down to such an extent that the slope to the sea was gentle and the rivers were able to carry but little coarse material. Some sand and clay was deposited but limestone seems to be the predominant type of deposit. Early in Ordovician time the seas occupied the Cordilleran trough and extended into Colorado from the northwest, much as in the Cambrian. Consequently the Ordovician sediments are represented in some places along the Front Range of Colorado and have been divided into several distinct formations, the oldest being the Manitou limestone, followed by the Harding sandstone and the Fremont limestone. The Harding sandstone is a very important formation, since in it have been found the remains of very primitive fish. For a long time these were the oldest known evidence of this group, but recently somewhat similar forms have been reported from the Cambrian.
The shallow seas still remained the principal arena of life during this period. The Ordovician deposits of this region or any other, have yielded no proven record of either land animals or plants. The invertebrates continued to dominate the marine waters, the most common groups being the brachiopods, the now extinct graptolites, bryozoa, true corals, crinoids and the cephalopod, Endoceras. The most notable advance in the life of this period is seen in the occurrence of very primitive fishes. These remains were first discovered in the Harding Sandstone near Canyon City, Colorado and were announced by Charles D. Walcott in 1891. The remains consist of bony plates which show close affinities to well known forms occurring in Silurian and Devonian rocks, and clearly belong to the order of fishes known as the Ostracodermi. Strange as these fish look, they are yet related to the living hagfishes (Cyclostomes).
It is believed that the seas retreated from what is now the mountain region of Colorado in late Ordovician time, and there is no evidence of a return to this area until late in the Devonian. Colorado and much of the surrounding territory was land area during late Ordovician, all of Silurian and the greater part of Devonian times. It is known from a study of Silurian rocks elsewhere that the climate was mild and equable over North America even far north in the Arctic circle, for coral reefs built up by organisms very sensitive to cold flourished there. Invertebrates continued to dominate the marine life. It is in Silurian rocks that the earliest remains of supposed land plants have been discovered. These consist of a few fragmentary stems, some of which bear small bract-like leaves. It is possible that the land surface existing in Colorado at this time was clothed by these primitive land plants occupying perhaps the lowlands along the rivers and shores but probably far from being profusely distributed. Fishes undoubtedly lived in the streams; and scorpions and thousand legged worms (millipeds) are known from the upper Silurian rocks and may have been the first animals to inhabit the lands.
No early Devonian rocks have been found in Colorado, and therefore there is no direct evidence than any part of the state was covered by the sea during early Devonian time. In middle and late Devonian time the seas spread eastward from the deeper portion of the Cordilleran geosyncline, an embayment entering western Colorado. Formations which were deposited as sediments in this Colorado embayment are now known in the San Juan, at Salida, at Glenwood Springs, along the White River plateau, and elsewhere. Although the Devonian sea extended eastward at least to the Front Range in the southern part of the state, it is doubtful whether they ever extended over the region now occupied by Rocky Mountain National Park. Since deposits of this age are lacking in the foothill region east of the park it is very likely that this region remained a rather low land during all of Silurian and Devonian time.
The abundance, size and variety of fishes during this time has led to the Devonian being known as the "Age of Fishes." Over 100 species and more than 40 genera of fresh water fishes are known. It was from a slab of Uppermost Devonian strata from Warren, Penn., that Professor Beecher many years ago found what appears to be a crude footprint. If this be such, it heralds the most momentous step in the whole advance of organisms from the lowly amoeba to man, namely the emergence of our ancestors from the water to the land. This footprint is believed to have been made by an amphibian and is the first evidence of the existence of this group. The transitional stages between this form and the lower organism from which it was developed are not known but it has been suggested that the amphibians were developed from one of the groups of fishes (Crossopterygii) which are known to have had organs which might readily serve as a primitive lung, and fins containing skeletal elements quite readily homologized with those of the amphibian limb. Those "lungfishes" which were abundant throughout the Devonian period, lived in regions subject to seasonal rainfall alternating with periods of droughts. During the latter the shrinking water-holes brought death and destruction to great numbers but there were always some holes which did not go dry. Here the stagnation and crowding put a high premium on the ability to gulp air into the swim-bladder, where the supply of oxygen led to the spread of minute bloodvessels and the gradual perfection of lungs, through countless generations. The Crossoptorygii with their stout fine could forsake their pools, perhaps in the cool of the night, and flounder about on the banks on short forays in search of food. Once the lungs had reached a certain stage of efficiency and the fins had been modified into stubby limbs, the metamorphosis was complete and air-breathing land vertebrates had arrived.
During the Devonian a varied, rich, and luxuriant vegetation existed, and fossils of all the higher cryptograms are found. Many of them were represented by large tree-like forms, and the first forests appear. Ferns, clubmosses and horsetails were represented by very large species which reached tree proportions. Fossil logs of primitive evergreens are also found in late Devonian strata, but these plants were more like seed-bearing ferns than like the conifers of today.
It is believed that for a very short time at the close of the Devonian the sea withdrew from the Colorado region only to be shortly followed by another invasion even more marked than the preceding. It is probable that most of the state of Colorado was covered by the early Mississippian sea, this being the most complete submergence of the Colorado region since Pre-Cambrian time. The Mississippian-series is represented in the Castle Rock quadrangle, by 85 feet of strata referred to as the Millsap limestone; however no rocks of this age are known farther north in the foothills of Eastern Colorado so that it is difficult to say whether this sea spread over the region now occupied by the park. If it did, the material deposited was eroded away before the subsequent Pennsylvanian period. Mississippian deposits are found in many places in Wyoming and are known as the Madison limestone.
Although the life of the Mississippian was clearly evolved from that of the Devonian, the Mississippian faunas were given a distinctive character by the decline of such groups as the corals and trilobites and the groat expansion of others like the echinoderms, the lacy bryozoa and spiny brachiopods. Fishes were locally abundant, though less varied than during the Devonian. Land animals left an indisputable record in the form of numerous footprints, though actual bones are still unknown in America. Doubtless many of these record the tragic search for water as the vanishing mudholes gave way to barren flats during the summer droughts.
Long before the end of Mississippian time a general rise of the Colorado area began resulting in a general withdrawal of the seas from the Colorado region. Disturbances of the earth's crust became active at the close of the Mississippian and during the Pennsylvanian causing large areas in Colorado, Wyoming, Utah, Arizona and New Mexico to be extensively uplifted to form low mountain ridges. These mountains have commonly been referred to in the past as the Ancestral Rocky Mountains, but since these mountains are not a part of the present Rocky Mountains it seems less confusing to call them, as suggested by Schuchert, the Colorado Mountains. Early in the Pennsylvanian there began another period of subsidence allowing the seas to slowly creep over the land, the waves assorting the materials they found on the surface, carrying the finer sands and clays into deep water to make sandstone and shale, and leaving the coarse material as a conglomerate. The transgression of the sea was so extensive, as is shown by the present distribution of Pennsylvanian rocks, that the recently uplifted Colorado Mountains stood as island masses surrounded at times by the sea. These land areas were undergoing vigorous erosion and the streams were carrying great quantities of waste material into the surrounding lowlands and out into the sea. The shoreline in the Colorado area was probably very uneven; in some regions there were extensive bays which were being rapidly filled with coarse deposits laid down as deltas at the mouths of streams. Some of these delta deposits are very extensive and were evidently laid down in continental basins wholly cut off from the sea; other deposits seem more likely to have been formed along river courses and flood plains, and in fresh water basins.
The Fountain formation, so well represented in the foothills along the Front Range, was formed during this time. It consists chiefly of arkose, red sandstone, grit and conglomerate, being coarse grained, crumbling, and mottled with gray and various light shades of red. The material is irregularly bedded and varies greatly from place to place, having many of the characters of fluviatile material. The formation is 5000 feet thick near Colorado Springs but thins toward the north extending into Wyoming. It is believed that this formation represents a great aluvial cone with its apex in the mountains in Central Colorado and its base toward the north.
At the top of the Fountain formation is a persistent sandstone member laid down in shore waters. It is cross-bedded, thinly laminated and decidedly quartizitic. It varies slightly in color but is generally pink. This is the Lyons formation.
From a study of the present distribution of the Pennsylvanian deposits in this region and the character of the deposits, the relative extent of the seas and the position of the important land areas are fairly well known. These are shown by Heaton. (Heaton, R. L. 1933).
Invertebrate life of the Pennsylvanian seas was not only prolific but varied. Spiny brachiopods and one type of foraminifora, the fusulines, were extremely important, in some cases forming large. bodies of limestone. Corals were simple cup forms but not very abundant. During this time, land insects were numerous and reached their largest size. Many are known which exceeded 4 inches in length, and one form found in the Coal Measures of Belgium had a wing spread of 29 inches. Cockroaches, scorpions, spiders, and centipedes were common. The amphibia were sprawling creatures, mostly, only a few inches long. However, some forms are known which exceed ten or more feet. A few reptiles of small size appeared during part of the later period, but their remains are very rare.
The Permian may be said to have brought to a climax conditions which began in Pennsylvanian time, not only in the Rocky Mountains area but throughout the continent. The shrinking seas gave way to widespread continental areas. The abundant rainfall of the early Pennsylvanian was succeeded by Permian aridity which greatly reduced land waters and caused scarcity of food. The seas which spread into the Southern Cordilleran geosyncline covered parts of western Colorado but in eastern Colorado semi-continental conditions initiated in the Pennsylvanian continued into the Permian. Here, deposits of red, brown, and chocolate sandstones, shales, and sandy shales with local thin calcareous or limy beds were deposited. Cross bedding and flow and plunge structure are very common as well as abrupt changes in the character of the material, the texture and color. These red beds in Colorado have been called the Lykins formation.
The life of the early Permian is so much like that of the Pennsylvanian that it is hard to fix the boundary line between the two. Permian time, however, is marked by rising lands and greater aridity which shut out the continental seas. These conditions were generally adverse to life and the struggle for existence was intense. All forms of life were greatly reduced in numbers and many forms disappeared entirely. Mountain-making movements of the earth's crust were very marked; for it was during this time that the principal movements occurred, producing the Appalachian mountains of eastern North America. The Urals of Europe, and the Variscan chains were completed across southern England, Germany, and Northern France. In the Orient there were also folding and thrusts along the arc of the Japanese Islands. The widespread aridity and other great changes wrought in climate by these major movements in the earth's crust had marked effect upon the vertebrate life. The amphibians which had dominated the land during the warm and humid Mississippian and Pennsylvanian, and which were dependent upon bodies of water for at least a portion of their life history, found that environment becoming very scarce. However, the relatively insignificant reptiles of the late Paleozoic were increasing greatly in number during the Permian. Before the close of the period they had undoubtedly mastered all the land, and heralded in a time when they were to reign supreme.