Land Forms of the San Gabriel Mountains, California

LAND FORMS OF THE SAN GABRIEL MOUNTAINS, CALIFORNIA Joseph E. Williams San Francisco Junior College, San Francisco, California Introduction The transverse mountain system of Southern California which begins at Point Arguello and extends 240 miles eastward is divided into four distinct ranges. From west to east the mountain ranges and the passes separating them are the Santa Inez Range, the Santa Clara River Pass, the Santa Susana Range, the Saugus-Newhall Pass, the San Gabriel Range, the Cajon Pass, and finally the San Bernardino Range on the east. The San Gabriel Mountain area, which lies 25 miles NE of Los Angeles, is the field of this study. The Spanish settlers gave the name "Sierra Madre" to the range, but at some time later than the founding of the San Gabriel Mission on the San Gabriel River alluvial fan the mountain became known by the name of the Mission . The boundary of the area is considered to be the contact zone between the mass of igneous and metamorphic rocks that constitute the mountain mass and the many alluvial fans deposited at its base. This contact is sharp; the mountain face rises abruptly for four or five thousand fçet above the surrounding fans, which in many places have coalesced into a piedmont plain (1). In a few places where streams reach a local base level some recent alluvium has been deposited. Up to the present time no Tertiary or other stratified material has been found in the mountains. In plan the mountains approach a trapezoid having an area of about 1200 square miles. The longer diagonal, from Newhall to Cajon Pass, is 60 miles; and the shorter, from Vincent to Pasadena, is 25 miles. The NW limb of the trapezoid is formed by Soledad Canyon and the Santa Clara River. The SW side is the Tujunga-La Canada fault valley. On the south side is the piedmont plain, stretching from Pasadena to San Bernardino. The San Andreas fault zone separates the San Gabriel Mountains from the Mojave Desert on the NE. The purpose of this study is to apply the inductive method of research to the present-day landforms and trace their genetic development. Are the flat surfaces of the ranges remnants of old peneplains? What roles have inner structure, climate and vegetation played in modeling this mountain landscape ? Did glaciation change the contours of the valleys? Previous Work Most of the work on the San Gabriel Mountains done heretofore has been concerned with their tectonic history and structural geology. The most extensive work has been done on the western half of the range by William J. Miller and his associates at the University of California at Los Angeles (2). The discovery of a large body of almost pure white anorthosite in the form of a batholith or large laccolith is particularly significant for the structure of the entire range. 16 1941of Pacific Coast Geographers17 In 1902, Hershey published a paper on the crystalline rocks of Los Angeles , Kern, Ventura, and San Bernardino Counties (3). In 1905 Arnold and Strong examined a few of the crystalline rocks north of Pasadena (4). Kew (1924) referred to the entire crystalline mass as a "basement complex" consisting of a metamorphic series intruded by granite (5). Noble (1927) published a report of the work done on the San Andreas fault zone which bounds the NE side of the area (6). Hill (1930) made a detailed study of the structure of the Little Tujunga Canyon area (j). None of the above works has studied the valleys, landslides, nivation depressions or other surface features in detail. Furthermore, no comprehensive geologic study has been made of the Tertiary strata surrounding the San Gabriel Mountains. Until this work is completed, it appears difficult to arrive at conclusive opinions regarding the Tertiary history of the mountain mass itself. Due to this lack, the present paper attempts to analyze only the Quaternary developments, and to conjecture possible Tertiary conditions. Geologic Base The material which constitutes the San Gabriel mass is described by Miller as "a great block of pre-Cretaceous metamorphic and igneous rocks, thousands of feet high, bordered on all sides by Tertiary and Quaternary deposits." Further, to quote Miller...