OCR Text |
Show 192 GEOLOGY OF THE HIGH PLATEAUS. smaller than at present. A few large eruptions, however, reach out westward, producing the sinuous course of the boundary which marks their termination. One of these westerly projecting masses separates Bear Valley into two portions, connected by a narrow gorge cut through it by erosion. Overlooking Upper Bear Valley from the eastward stand two conspicuous mountain masses called Bear Peak and Little Creek Peak, of which the respective elevations are 9,870 and 10,040 feet. Although of moderate altitudes, they present, in consequence of their isolation, a very commanding appearance and attract the attention from every point of view in the surrounding country. They are also interesting on account of their structure and the masses which constitute their bulk. The beds which lie at their foundations merit some description. Wherever we examine the contact of the volcanics with the sedimentary beds along the western verge of the eruptive rocks of the Markagunt, we usually find a series of strata composed of finely comminuted volcanic materials. Sometimes it is a fine sandstone; sometimes an argillaceous rock with minute fragments of feldspar and mica; sometimes a calcareous or marly deposit. Often rolled and rounded fragments of notable size are included, and the beds have then a coarse or gravelly texture, the grains being fragments of some eruptive mass so much decomposed that it is. difficult to determine its exact variety. These beds are always well stratified and have clearly been deposited by water, and do not differ from ordinary sedimentary beds, except in the fact that the materials which make up their mass have been derived from eruptive rocks. The individual beds are usually of small superficial extent and small thickness, and are often seen running out with " feather-edges." They always overlie the systematic lacustrine Tertiaries of early Eocene age. Similar formations are found at the northern and southern extremities of the Sevier Plateau and in the East Fork Cation, where they have been more or less metamorphosed. They are exhibited on the west side of Bear Valley and again along the base of the great trachytic wall of the Markagunt in considerable variety. Wherever found they seem to constitute a group by themselves of more recent age than the uppermost Tertiaries of the Wasatch |