14 



University of California. 



[Vol. 3. 



value than a mere guess. Several such cases will be discussed 

 in the succeeding pages, but figures will be avoided. 



In the City of Los Angeles there were observed three Quater- 

 nary formations. The first and oldest is the marine Pleistocene, 

 which was laid down in an approximately horizontal position on 

 the truncated edges of a very thick Pliocene series dipping at 

 angles varying between 30° and 85°, but prevailingly 45°. 

 Great erosion is evident between the deposition of the latest 

 Pliocene and the marine Pleistocene. The latter reached a 

 thickness in places exceeding one hundred feet, but in the 

 northwestern part of the city thinned out to five or ten feet over 

 the submarine shelf near the sea-cliff. Erosion has here pretty 

 thoroughly dissected the bench, and the Quaternary gravel and 

 sand remain only in limited areas capping the principal hills. 

 On the east side of the Los Angeles River, on Boyle Heights 

 and in the vicinity, interstream erosion has given the surface of 

 the marine Pleistocene a rolling topography precisely as on the 

 Red Bluff of the Great Valley where the material is similar. 

 The Los Angeles' River excavated a valley in the marine Pleis- 

 tocene which is about one mile in average width and one 

 hundred feet deep. Subsequently this was partly silted up and 

 the second Quaternary formation deposited. 



This is best developed in East Los Angeles, particularly near 

 the County Hospital, hut it also forms the low terrace in the 

 business portion of the city east of Main Street. It consists of 

 at least twenty-five feet in thickness of indistinctly stratified, fine 

 silty sand (no pebbles) of light brown color. The surface 

 remains flat, except that valleys about twenty feet deep have 

 been eroded in it. The erosion since its uplift has been about 

 equal to that on the Iowan loess of the Mississippi Basin, and it 

 is apparently equivalent to the old flood-plain deposit of the Red 

 Bluff area of the Great Valley. 



The third Quaternary formation is the Modern alluvium, an 

 irregularly stratified, light gray, fine gravel and sand. It is the 

 present flood-plain of the Los Angeles River and tributaries, and 

 forms the floor of the valleys excavated below the surface of the 

 preceding formation. 



After taking into consideration the easily eroded nature of 



