Geologic map and section of the Eastern San Gabriel Mountains: Supplement 1 from "Geology of the eastern portion of the San Gabriel Mountains" (Thesis)
The San Gabriel Mountains form one of the units of east- west trending mountain belt of Southern California. They extend from the San Fernando and Santa Clara Valleys on the west to Cajon Pass on the east, and form a topographic boundary between the Mohave Desert on the north and the Coastal plain on the south.
The area, described in this report, occupies a rectangular block, seven miles long and three miles wide, in the eastern portion of the mountains. It lies entirely within San Bernardino County and includes parts of San Antonio, Cucamonga, Hesperia and San Bernardino Quadrangles.
With the exception of the long straight valleys of Lytle Creek and Lone Pine Canyon, the region is steep and rugged. Elevations range from 2700 feet in Cajon Pass to over 9000 feet at the summit of Telegraph Peak.
Most of the mapped area is quite accessible. The main highway between San Bernardino and Los Angeles Playground at Big Pines is located in Lone Pine Canyon. Paralleling this road and one and one-half miles to the southwest, there is a paved road which extends up the North Fork of Lytle Creek to Stockton Flats. During the greater part of the year it is possible to drive from Stockton Flats across the range to Camp Baldy. In addition to these two public highways, there are numerous roads and trails which have been constructed by the United States Forest Service.