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Field Division of Education
The Geology of Rocky Mountain National Park
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The closing of the Cretaceous period, with its great crustal changes and the formation of extensive mountain ranges, saw another great critical period in the history of the earth. Withdrawal of the seas from the continents resulted in a struggle of marine invertebrates and many of the older groups which had flourished during the Mesozoic died out completely. The same is true of land animals and plants; many of the old groups became extinct, and relatively insignificant groups began to show marked evolutionary trends, adaptive radiation taking the places In nature formerly occupied by the extinct forms. With the opening of the Cenozoic we have the introduction of the modern groups. So long as the dinosaurs held their own, and the grasses, cereals and fruits had not become generally distributed, the primitive mammals could only bide their time. However, with the opening of the Cenozoic these necessary environments were an actuality and the mammals quickly took advantage of this opportunity and swept into dominance all over the earth, so that the Cenozoic may well be spoken of as the "Age of Mammals".

During the latter part of the Cretaceous there was a great subordination and extinction of the older groups of plants which had seen so prominent a development during the Mesozoic. The deciduous trees, belonging to the highest order of the plant kingdom, suddenly became very conspicuous and dominated the landscape just as they do today. The indirect effect of the coming of the angiosperms upon the advances of the higher animals can hardly be exaggerated, for they supply nearly all the plant food for the mammals which now dominate all other life upon the earth. Angiosperms provide the nuts and fruits of the forest, the grasses of the prairies, the cereals which furnish fodder and grain for man and his domestic animals, and all the vegetables and fruits that man has cultivated, to say nothing of the flowers that add so much pleasure and inspiration to human surroundings.

Just as the Cenozoic has seen the modernization of animals and plants, so this last short era of geologic time has witnessed the shaping of every feature of the present landscape. With the retreat of the Late Cretaceous seas and the folding of the Cordilleran region, North America assumed approximately its present size and outline. During the Cenozoic the continent has remained essentially emergent, the seas at times have extended ever the coastal regions for short distances but these rarely exceeded 5 or 6% of the continent. The available geologic record, therefore, lies principally in the formations along the coastal areas and the continental formations spread over the land surface, and in features still evident in the modern landscape.

The opening of the Cenozoic era found the Central and Western Colorado region mountainous, due to the mountain making movements of the closing stages of the Cretaceous. The higher portions of the great open folds of the Southern Rocky Mountains were undergoing rapid erosion. During the Eocene epoch the synclinal basins of western Colorado, Utah, and Wyoming caught much of the erosional debris from the surrounding ranges and in them the thick Eocene formations are still preserved. Most of these mountain-rimmed basins had through-following streams and therefore persisted as forested lowlands, in which Cenozoic mammals had their favorite haunts. The sediments of these basins consequently enclose a record of mammalian evolution scarcely equaled else where in the world. During Eocene time most of the sediments from the erosion of the mountains were caught in these basins, for no Eocene sediments are known from the area immediately east of the Front Range. Sediments were undoubtedly being carried eastward by streams draining the Front Range, but the region immediately east of the range must have had an elevation sufficient to prevent the accumulation of deposits here. In all probability this region east of the foothills was undergoing slight erosion.

As the Eocene epoch closed, the intermountain basins became filled with deposits so that during the Oligocene Epoch the streams reached the foothills ladened with sediment. These were spread over the Great Plains as alluvial aprons and flood plain deposits. By the end of Oligocene time the mountains had been worn low, though monadnocks 2000 or 3000 feet high rose in places above the plains.

In Miocene time rejuvenation began with a broad, gentle regional upwarp along the axis of the Rockies. This was in general free of faulting or other marked diastrophism so far as the Rocky Mountain region is concerned, and the uplift was gradual, intermittent and long drawn out, reaching its culmination in Pleistocene time. As a result of this warping, there was little deposition of Miocene or Pliocene sediment within the mountains, though the vast area of stream deposits to the east of them was greatly extended until it stretched from the Dakotas to Texas and from the front of the mountains to eastern Kansas and Nebraska. This regional upwarp of late Cenozoic time gave the Rockies their present height. Erosion has been extremely active since this time and has resulted in the carving of the present features of this grand mountain region. So recent has the last uplift of this area been that the erosive agents have still much to accomplish and we may consider the present mountains as in a young stage of development. The work of the streams in carving out the rugged features of the present mountains has been greatly modified by the work of extensive glaciers which held sway in this region during the great Ice Age. This last great drama of geologic time has left so many of the familiar features of the present landscape of the Rocky Mountains that it is worthy of special consideration.


This is the latest and most recent of the geological epochs and because of the extensive glaciation which gripped the earth at this time it is commonly referred to as the Ice Age. Fully one-sixth of the existing lands were covered with great sheets of ice. So recent was this last episode that the ice sheets have not yet completely disappeared and the desolate continent of Antarctica, like the ice capped island of Greenland, stand as vivid reminders of the past. Aside from Antarctica, the great ice sheets of the Pleistocene were in the northern hemisphere, one centering over the Canadian region and the other ever Scandinavia. The ice covered nearly half of North America, reaching from Alaska to Greenland and southward into the United States as far as the Ohio and Missouri rivers, covering an area of about 4 million square miles.

In addition to these continental ice caps, nearly all the lofty mountains of the world were extensively effected by valley glaciers, and it is this type that is so excellently recorded in Rocky Mountain National Park. Conspicuous evidence of these former glaciers may be found in the lateral, terminal, and ground moraines, the smoothly rounded rock surfaces, polished and striated boulders, steep-walled gorges, U-shaped valleys, and pocket lakes. There appears to be two stages of glaciation represented in the park, - an older one of which little is known and a younger one called the Wisconsin stage, because it coincides in time with the last stage of continental glaciation, which is typically developed in the state of Wisconsin. It is believed that some of the broad open valleys in the lower parts of the park, such as Estes Park and Tahosa Valley were shaped by ice belonging to the earlier of the two stages. They have the broad floors, the steep walls and the perched lakes which characterize glacial valleys, but they lie outside the area affected by the glaciers of the Wisconsin stage,

During the younger or Wisconsin stage of glaciation the ice accumulated in the valleys previously occupied by the streams - and to some extent perhaps by older glaciers - and pushed its way from the high mountains down those valleys to altitudes of about 8000 feet. The areas occupied by the ice are plainly marked and could be mapped accurately with little difficulty. This has not as yet been completely done, however, an illustration showing the outlines of these glaciated areas together with a description of the features developed by these ancient glaciers, is presented in Willis T. Lee's pamphlet on the Geologic Story of Rocky Mountain National Park, Colorado, to which the reader is referred for greater completeness.

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