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Field Division of Education
The Geology of Rocky Mountain National Park
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The formation of the earth was followed by long eras, the history of which is veiled in the darkness of antiquity. Enormous groups of ancient rocks, lying in tangled confusion below the Paleozoic, form an impressive record of these early times. When these rocks lie in their normal order of superposition, the local history is not difficult to interpret. However, as is very often the case, these rocks have suffered intense deformation during the great length of time since their formation, and have been intruded by igneous masses. They are like a manuscript that has been scattered and torn, leaving us only "internal evidence" as a clue to the order in which the recovered portions should be fitted together. Moreover, the almost general absence of fossils leaves us with no secure means of correlating portions of the record now widely separated by erosion or by intervening areas of younger rocks. We might easily compare the early records of geologic history, so beset with difficulties, to the bare inklings of early human history. Written documents tell clearly the story of the last two thousand years of man's development, but back of the last five thousand years stretch the millennia of forgotten civilizations known to us only through the ruins they have left. The scattered implements of Paleolithic man record chapters in the history of civilization no less real because they are not yet fully understood. They are too disconnected to permit a full synthesis of pre-history; however, by means of constant research in every part of the world, the veil of time is being pushed back more and more into antiquity.

Pre-Cambrian time was very long, being estimated at 1500 million years, or three quarters of all known geologic time. It is not certain how many eras of time are represented; however, two have been generally recognized: the Archeozoic and the Proterozoic.

The entire region of Rocky Mountain National Park is underlain by Pre-Cambrian rocks; and all the main peaks, with the exception of Specimen Mountain, consist of various phases of this complex. These rocks are covered over in many of the valleys and lower basins by glacial debris, recent stream deposits, and soil; however, numerous outcrops exist in the bare rocks of the steep walled valleys and gorges. Three important phases have been recognized, the oldest being a schistose complex known as the Big Thompson Schist. This has been intruded by at least two granitic intrusions, an older Longs Peak Granite, and a younger Mt. Olympus Granite. The Big Thompson Schist has been described by Fuller (Fuller, M. B., 1926) as a remnant of a former extensive blanket of highly metamorphosed sedimentary rocks that stretched from an unknown shoreline at the east of the present foothills, westward across the site of the front range to beyond Middle Park. This formation is believed to have been originally a fine grained, highly silicious sandstone with little conglomerate and limestone at the east. These graded westward into fine grained shale.

The second important phase of the Pre-Cambrian rocks of Northern Colorado, the Longs Peak Granite, was pushed up as extensive, intrusive masses into the sedimentary beds. It formed huge stocks and batholiths which fingered into the strata as dikes and sills. The third phase, the Mt. Olympus Granite, was intruded before the Longs Peak Granite was entirely crystalized at all points, and some time before the metamorphic processes induced by the Longs Peak Granite were complete. The Mt. Olympus granite intruded mostly into the region immediately to the east of the Longs Peak Granite.

From this fragmentary and greatly distorted record of Pre-Cambrian time it is difficult to interpret much of the oldest history. We would perhaps be safe in assuming that in this very long time the seas spread over this region many times depositing muds, sands and limestones to extensive thicknesses. These wore subsequently folded and distorted by mountain-making movements and intruded by masses of molten magma which solidified into granite. Toward the close of the Pre-Cambrian and during the early and middle part of the following Cambrian, the granites and metamorphosed sediments deeply eroded.

Practically nothing has been found in the way of fossils from the metamorphosed sediments of this region. Luckily, however, not all Pre-Cambrian rocks are as barren of fossils, and although life was probably not as abundant at this time as during later times, numerous indications of it have been found. These consist of simple forms of plant life such as algae and the lower groups of animals such as the protozoans, sponges, perhaps some of the limey corals, and the worm-like creatures which represent the highest type of life represented at this time. This is not a very impressive array of creatures but it is significant in indicating that organic life had not evolved very far in these early times. The ancestors of numerous other groups of animals must have been in existence at this time; however, it is believed that they may have been lacking sufficient hard parts to become preserved In the rocks, the soft tissues being easily destroyed.

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