96 LANDS OF THE ARID REGION OF THE UNITED STATES. 
plateaus have been carried to the sea, but in the western districts the 
materials carried from the mountains are deposited in the adjacent valleys, 
so that while the mountains are composed of rocks of great age, the rocks 
of the valleys are of recent origin. In that geological era known as the 
Glacial epoch the waters of a great lake spread over these valleys, and the 
mountains stood as islands in the midst of a fresh-water sea. For the history 
of this lake we are indebted to the researches of Mr. Gilbert. It had its 
outlet to the north by way of the Shoshoni River and the Columbia to 
the North Pacific. These later beds of the valleys are in part the sedi- 
ments of Lake Bonneville, the great lake above mentioned, and in part 
they are subaérial gravels and sands. 
The Wasatch system of mountains is composed of abrupt ranges 
crowned with sharp peaks. The several minor ranges and groups of 
peaks into which it is broken are separated only in part by structural 
differences, since ridges with homogeneous structure are severed by trans- 
verse valleys. The drainage of the whole area occupied by the Wasatch 
Mountains is westward to the Great Salt Lake. The streams that head in 
the western end of the Uinta Mountains and West Tavaputs Plateau cut 
through the Wasatch Mountains. 
Great Salt Lake and its upper tributary, Utah Lake, exist by virtue 
of the presence of the Wasatch Mountains, for the mountains wring from 
the clouds the waters with which the lakes are supplied. 
Walled by high ridges and peaks, many elevated valleys are found. 
In the midsummer months these valleys are favored with a pleasant, 
invigorating climate. Occasionally showers of rain fall. Vegetation is 
vigorous. The distant mountain slopes bear forests of spruce, pine, and 
fir; the broken foot hills are often covered by low, ragged pinon pines 
and cedars; and the flood plains of the streams are natural meadows. 
About the springs and streamlets groves of aspen stand, and the streams 
are bordered with willows, box elders, and cottonwoods. Now and then 
a midsummer storm comes, bringing hail, and even snow. When the 
short summer ends, the aspen and box elder foliage turns to gold flecked 
