The Geology of the Northwestern Portion of the Alhambra Quadrangle

The region considered lies between the principal sections of Los Angeles and Alhambra, more strictly it is bounded on the north by the 34°06' parallel, or about the southern limit of south Pasadena, on the west by the 118°12’ meridian, on the south approximately by an extension of Garvey Avenue, and on the east by Fremont Avenue. The area included is about nine square miles. The map used is the Alhambra Quadrangle recently published by the U.S.G.S. The scale of the map is 2000 feet to the inch and the contour interval is 5 feet up to the 500 foot level and above this the interval is 25 feet. The map is quite satisfactory except that the variation in contour interval gives the hilltops an unnatural aspect. The problem was attacked by making a general study of the region with an eye toward becoming familiar with the different materials that were to be encountered, always recording dip, strike, locality, remarks and materials that were studied. The areal map was next constructed by using the principal sandstone as a key bed and correlating the structure of the formations above and below to the structure indicated by this sandstone. The boundaries of the Quarternary alluvium were located principally on the basis of change of slope. The exposures of underground structure in the area are all good and consist principally of road cuts and other artificial excavations. However, in many places natural outcrops as a result of erosion were of considerable value. Also, another somewhat unique circumstance facilitated the problem. Along the northern boundary of the southern branch of the El Sereno sandstones just above Ascot Park and extending to Alhambra Boulevard, the marked contrast in weeds and grasses on either side of the contact between shale on the north and sandstone on the south made it possible to trace the contact with great ease, the accuracy of the procedure being born out here and there by good exposures. The grass in the shale consisted principally of wild barley, etc. In the sandstone in addition to wild barley there was a fairly dense growth of wild mustard. The nearer to the contact the more dense was this growth; also, scattered here and there were cactus plants. No cactus was observed in the shale. Further investigation showed that the sands near the contact were more moist than the shale on the other side. It was also noted that the character of the surface soil was highly indicative of underground material; for example, the shale surf ace after a rain and then a couple of hot days became hard, compact, and cracked, but the sand surface did not crack and remained loose. The capillary action between the sands and ground water, and the compactness of the clay shales account for the contrast in vegetation. It was only in the hills that this feature was observed because of the thinness and "in situ" character of the soil.