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Show GEOLOGY. 11 composed chiefly of west- dipping limestone, separate the Lake Mountains from the East Tintic Range- the succeeding highland mass to the south. The northern end of these hills is capped by horizontal basalt with which light pumiceous tuff is associated. The East Tintic Range, a complex mass of sedimentary and igneous rocks, forms the southwestern border of Utah Lake basin. As in Rock Canyon, the sediments consist of Cambrian quartzite and Carboniferous limestone in juxtaposition, indicating the absence of the Ordovician, Silurian, and Devonian. The main structure of the sedimentary rocks is synclinal, but these constitute a relatively small part of the outcrops, igneous rocks, rhyo-lite, andesite, monzonite, and basalt occupying most of the region. These are of both extrusive and intrusive origin, and are of Tertiary age. The low spur of the Tintic Mountains known as Long Ridge, which lies south of Goshen and connects with the Wasatch- save for a narrow Pleistocene strip south of Santaquin- consists of andesite in its southern part, while southeast- dipping Carboniferous limestones outcrop in the gorge of Currant Creek. T. ATE GEOLOGIC HISTORY. The above resume implies for this region a complex geologic history which need not here be discussed. A statement of late geologic events will, however, add to a clearer understanding of the valley deposits in which the underground water is stored. TERTIARY HISTORY. After many thousands of feet of sediments had accumulated in Paleozoic and Mesozoic time, during which the general region was occupied by oceanic waters, profound continental uplift occured in early Tertiary time. Since then the ocean has not invaded the interior of the continent and during Tertrary time much of the Cordilleran region is believed to have been occupied by a number of lakes in which a considerable thickness of rocks accumulated. During the Eocene, according to the geologists of the Fortieth Parallel Survey, a great freshwater lake occupied the Wasatch Mountain area, and toward the close of this epoch the mountains were finally uplifted and the relative depression of the Great Basin originated. The late Tertiary witnessed the formation of several lakes whose positions were determined by different crustal movements, and these lakes persisted with varying relations into the Pleistocene epoch. The end of Tertiary time was marked by further earth movements that divided the Great Basin area into two main depressions, following the bases of the recently uplifted Wasatch Mountains and the Sierra Nevada. In Quaternary time the bordering mountains were occupied by glaciers, and enormous lakes accumulated in the marginal depressions of the Great Basin. The two largest of these have been named after early explorers. Lake Lahontan covered an immense area in western Nevada and Lake Bonneville occupied a considerable part of western Utah and extended into adjacent parts of Nevada and Idaho. QUATERNARY HISTORY. The existence of Lake Bonneville is borne witness to by a number and variety of facts, chief of which are the remains of shore lines and shore deposits, and the great thickness of sediments that accumulated in the lake and that now constitute the valley floor. At its greatest extent the water of Lake Bonneville was approximately 1,000 feet above the present surface of Great Salt Lake. This large body of water abutted against the adjacent highlands and the outline of the lake was intricate. Deep bays and jutting promontories marked the shores, and lone mountains, partly submerged, stood out as islands. The area considered in this report formed part of one of these bays. This- was divided by a close stricture into an outer bay and an inner, the outer covering the valley of the Jordan River and the inner spreading over Cedar, Utah, and Goshen valleys and a part of Juab Valley. In the inner bay the Goshen Hills made two islands, and the Pelican Hills constituted one large and several small islands. Small estuaries occupied Emigration and Little Cottonwood canyons, connecting with the outer bay, and the inner bay sent an estuary into Provo Canyon. a a Gilbert, G. K., Lake Bonneville: Mon. U. S. Geol. Survey, vol. 1,1890, p. 103. |
