ROCKY MOUNTAIN A Guide to the Geology of Rocky Mountain National Park, Colorado

AT THE outset we must know a few basic facts which we will use in our examination of the park. The features of the earth's surface, such as mountains, hills, and valleys, have been carved largely by the action of water and ice. Incredible as it may seem, streams can wash away entire mountain ranges. Their rushing waters first cut deep valleys, then widen their floors, then, through the action of tributaries, cut away the divides between, until at last the mountainous surface is reduced almost to a plain. If the process is interrupted by uplift of the land before the plain is completed, and the streams thereby given more fall, the cycle begins all over again with the renewed cutting of deep valleys.

Obviously, streams can work more rapidly in soft rock than in hard. The same stream may excavate a broad plain in easily eroded beds in the same time in which it can cut only a narrow gorge across a mass of granite. The excavation of the plain leaves the granite rising in hills or mountains above the stream-cut lowland.

Rocks may be divided into three great classes: Igneous, or those cooled from a molten state; sedimentary, or those deposited by water, wind, or ice; and metamorphic, or those formed by heat and pressure from the other two. The sedimentary rocks are derived from the igneous rocks through weathering and erosion, and, although originally deposited in horizontal layers, may later be folded into mountains by pressures within the earth. They and the igneous rocks are altered, by these pressures and the intrusion of molten rock, to coarse-grained gneiss and finer grained schist. The schists, in particular, which in Rocky Mountain National Park are formed from ancient sediments, can be distinguished by innumerable sparkling flakes of mica, which lie parallel to each other and cause the rock to split into thin layers.

Life has evolved slowly through the ages. That the remains of plants and animals may have become buried in the muds, limes, and sands, which later hardened into shales, limestones, and sandstones, is evident. As layer upon layer of sediment was deposited, animals which lived in very ancient times were buried near the base of the series, whereas those which lived in later periods were buried near the top. It is possible, therefore, to determine the relative age of certain rocks by the fossils which they contain.

There was a time when the highest forms of life were shellfish, and later periods in which fishes and amphibians developed. All of these are grouped in the era of the Paleozoic or Old Life. The Paleozoic was succeeded by the Mesozoic or Middle Life in which reptiles, including the dinosaurs, were the dominant forms. This was succeeded in turn by the Cenozoic or Recent Life which is called the Age of Mammals.

The eras that have just been named are the ones concerning which we know most, for the record of the rocks is comparatively complete; but they account only for the last one-fourth of the earth's history. Before the Cambrian, which is the lowest division of the Paleozoic, the vast pre-Cambrian eras stretch into the dim past for perhaps 1,400,000,000 years (fig. 1).

Such estimates of the earth's age are not mere guesses. The discovery of radium has given a measure of time. Radioactive minerals undergo a very slow but constant change, the end product being lead. It is possible with such a mineral, which has been only partly changed to lead, to compare the weights of the lead and the unaltered mineral and to estimate the time required to make the change, thus determining the age of the deposit.