Page:The New International Encyclopædia 1st ed. v. 17.djvu/235

ROCKY MOUNTAINS. 211 ROCKY MOUNTAINS. them of laige size and iciiiarkablc character. In beils of siiniUir age, consisting largely of volcanic dust, at Florissant, Colo., immense numbers of fossil insects have been obtained, and near Green River in Wyoming soft sliales are crowded with the remains of tishes. Fossil plants, particularly of Lower Cretaceous, Jurassic, and Tertiary times, are also abundant. Valuable coal seams "of Cre- taceous and Tertiary age occur at many locali- ties. One of the most remarkable facts concerning the geological structure of the Rocky ^Mountains is the presence of a series of abrupt folds along their eastern border in which the horizontal strata, several thousand feet thick, underlying the CJreat Plateaus, are bent upward. Remnants of these same beds spared by erosion occur in several of the ranges to the west of the Front range, at an elevation of 5000 or GOOO feet above the portions not afiected by mountain-building forces. At many localities along the east base of the Front range, from Xew ilexico northward far into Canada, the abrupt folding of the rocks is shown by the nearly vertical position of the eroded border of the strata remaining. At times the folds were overturned eastward, so that the beds in their eroded basal portions dip westward. In northern Montana a still more intense move- ment resulted in the fracturing of the rocks in an overturned fold, producing a nearly horizontal fault or thrust plain, in connection with which, as reported by Bailey Willis, Algonkian rocks were carried seven miles eastwanl and rest on Cretaceous strata. In general the various ranges composing the Rocky ilountain chain are due to upward folds or anticlinals in sedimentary and igneous rocks, and the elevation of plastic magmas now repre- sented by granite, gneiss, schist, etc., in their central portions. In general, also, as shown by the north and south trend of the longer a.xes of the folds, the direction in which the force acted which caused the rocks to bend was east and west. The principal movements which upraised the mountains occurred at the close of the Jleso- zoic, as is shown by unconformities between Mcso- zoic and Tertiary beds. The upheaval of the mountains ■n-as followed by erosion. Nearly all of the scenic features which now attract the eye are due to the work of streams and glaciers which have deeply sculp- tured the upheaved mountain blocks. The broad valleys, including the parks of Colorado, etc., are due to the upraising of their bordering moun- tains; but the cations, such as the Yellowstone, Arkansas, Colorado, and other streams fiow through, are the result of abrasion by the debris- charged rivers themselves. The infinitely varied secondary valle.vs and canons and the multitude of gorges, gulches, amphitheatres, and other similar incised features of the relief are due to erosion, while the countless mesas, buttes. pin- nacles, etc.. which rise above the general level of the surrounding country are remnants of ancient uplands spared by the erosive agencies. Erosion or earth sculpture has also brousrht out the characteristic features of the Black Hills in which the more resistant rocks stand in relief and the weaker beds underlie valleys, and has given to the several regions of Tjad lands' their urfique topography. In addition to the numerous ranges due to lateral pressure and consequent upward folding there are many elevations due to volcanic agencies. Mountains built by volcanic eruptions are uiuncrousin Arizona and .ev .Mexico. Tothi» class belong San Francisco -Mountain and Mount laylor. situated farther east, in sight of wliiili there are a large number of ■volcanic necks' ex- Jiosed by the removal of the craters which once inclosed them. Fast of the Front range in New Mexico, and well out in the (ircat I'lateaus. there are a number of conspicuous volcanic craters, of whi(j^ the leading example is Mount C'apuliii. -loO feet high above the surrounding plain, and with a crater on its sunnnit nearlv a mile in diameter. The Spanish Peaks, in southeastern Colorado, furnish admirable examples of the deep erosion of large volcanic mountains. Colorachi. Wyoming, and .Montana are almost wliollv with- out recent volcanic craters, but in W extern Wyn- niing and extending across southern Idaho are the basaltic lavas of the Snake River Plains, one of the mo.st wonderful exhibits of its kind in the world, associated with which there are numerous volcanic craters. In the region of Yellowstone Park there are gieat accumulations of rbvolitic lava, of older date than the basalts of Idaho, but still retaining some of their volcanic heal, as is made manifest by the numerous hot springs and geysers. -Associated with volcanic eruptions is tlie injection from below of molten or plastic magmas into the rigid rocks composing the outer portion or 'crust' of the earth. These in- trusions in part occupy fissures and form dikes, but at times were forced between stratified beds and produced intruded sheets of igneous rocks, perhaps many scores of square miles in area, and under other conditions formed cistern-like in- trusions termed laccoliths, which raised the rocks above into domes. In the Rocky ilountains there are numerous examples of each of these varieties of igneous intrusions, many of which have been laid bare by erosion. Of these the most remarkable are the laccoliths forming the Henry Mountains in southern Utah, where several intrusions in previ- ously horizontal rocks elevated domes measuring three to five miles in diameter and from a few hundred to fully 7000 feet high. These moun- tains furnished the tv-pe of a class of uplifts not previously recognized. Other similar laccolithic mountains occur in southwest Colorado, and about the Black Hills in South Dakota, and have l)een recognized elsewhere. Perennial snow banks and miniature glaciers occur in the mountains of Colorado and on the Teton range in Wyoming. In nortliern Montana small glaciers are frequent, and in the Canadian Rockies form a conspicuous feature in the mag- nificent scenery. The best known is perhaps Illicilliwaet glacier, near (Jlacier House, on the line of the Canadian Pacific Railroad. Other glaciers occur in the higher portions of the moun- tains throughout .lherta. The glaciers are all of the .lpine type, and from Montana northward are remnants of great ice sbeets which covered the mountains during the Clacial epoch. Many of the more conspicuous features in the North Rockies, such as the deep, steep-sided valley, with rounded or U-shaped bottoms, numerous lakes and side alcoves from which the slrcjims descend in cascades, are due to the former gla- ciers which flowed away from the several ranges. The summit portions of the Big Horn, Teton, and other ranges in Wyoming are glaciated, as is also a large area in the region of great mountains in Colorado. Nearly all of the numerous and fre-