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Show ALLUVIAL FORMATIONSâ€"THOUSAND LAKE MOUNTAIN. 279 These alluvial masses are partly conglomeritic in texture, especially near the borders of the lava sheets and at the foot of the plateau slopes. Towards the middle of the valley they become finer, shading into sandy or fine gravelly deposits. They are instances of the formation of conglomerates upon a considerable scale by the alluvial process, but under conditions somewhat different from those disclosed in Sevier and Grass Valleys. The included fragments upon the western and southern portions are always volcanic and exceedingly varied. The debris derived from Thousand Lake Mountain on the eastern side consists mainly of fine quartz sand coming from the decay of the Jurassic and Triassic sandstones of that structure. In the northwestern part of Rabbit Valley a few exposures of Tertiary beds are found beneath the terminal trachytic sheets. Upon the eastern side of the valley are still better exposures upon the lowest slopes of Thousand Lake Mountain. In the latter locality they are soon cut off by the great fault, and reappear nearly 4,000 feet above, beneath the lava cap upon the summit. Below they abut against the Lower Trias. Laterally they run beneath extensive outpours of basalt, which, though not of very modern origin, are still comparatively recent. THOUSAND LAKE MOUNTAIN. Thousand Lake Mountain is an exceedingly interesting object. The name was given by the Mormons who pasture flocks in the valley below. They derived it from a group of pools of glacial origin upon the summit. Structurally and morphologically it is a small plateau, in some respects very similar to the other and larger members of the district, but possessing, also, features peculiar to itself. The country to the west of it is thrown down by a profound fault forming the depression of Rabbit Valley, The country to the east of it is the inner region of the Plateau Province, from which thousands of feet of strata have been removed by the grand erosion of Tertiary time, while the mountain itself has been left like a gigantic butte or cameo upon the border of the region. Upon its southern flank the Fremont River has cut a wide passage, which has separated it from its mighty parent, the Aquarius Plateau. Upon the summit is a lava cap from 400 to 500 feet in thickness and |