LOCATION AND TOPOGRAPHY 185 



doubtless be thrown on the subject by a study of the region at the 

 southern end of the San Joaquin valley, where the axes of several of 

 the ranges appear to focus. 



Topography 



The San Gabriel mountains are divided into two more or less distinct 

 ranges by streams which flow in easterly or westerly directions approxi- 

 mately parallel to the length of the chain. The most prominent of 

 these streams is the San Gabriel river, which cuts nearly through the 

 center of the group from south to north, and whose east and west 

 branches separate the major portion of the southern or Sierra Madre 

 range from the rest of the mountain area. The Middle Fork of Lytle 

 creek, immediately west of Cajon pass, and Tujunga canyon, north of 

 La Canada, nearly complete the separation of this southern border range 

 from the northern mass. The Sierra Madre range is long and narrow, 

 averaging only about 6 miles in width. Its sides are precipitous and are 

 cut by short, deep, narrow canyons. The steep southern face of the range 

 probably represents an ancient and bold fault scarp, which, how r ever, has 

 been considerably altered by erosion since its origin. The average height 

 of the Sierra Madre is something over 6,000 feet, its highest point, Cuca- 

 monga peak, however, reaching an elevation of 8,911 feet. Great detrital 

 fans, varying in area and depth with the size of their parent streams, 

 stretch out from the southern base of the Sierra Madre over the San Gabriel 

 valley, which borders the mountains on the south. North of the Sierra 

 Madre is an elongated and complex range of ridges and peaks which cul- 

 minates near the eastern end of the group in San Antonio peak (" Old 

 Baldy "), elevation 10,080 feet. The slopes in this northern area are 

 more gentle and the relief less sharp than in the southern, the outlying 

 hills of the former grading off into the buried valleys of the Mojave desert, 

 which bounds the San Gabriel mountains on the north. 



The Verdugo mountains and the San Rafael hills are isolated portions 

 of the San Gabriel mountains lying south of and separated from the 

 latter by La Canada, a valley averaging nearly 2 miles in width, which 

 runs for several miles parallel to the western end of the Sierra Madre 

 range. At the eastern end of the San Rafael hills and La Canada lies 

 Pasadena, while 9 miles southwest of the latter is Los Angeles, the chief 

 city of southern California. 



The rocks described in this paper were collected for the most part 

 during reconnaissance trips over the following territory : Mount Lowe 

 railroad to summit of mount Lowe, Rubio canyon, Eaton canyon, Big 

 Santa Anita canyon, Mount Wilson toll-road, Sierra Madre-Mount Wilson 



