1916] 



Dickerson: Tcjon Eocene of California 



399 



Tejon formation. — The only rocks referable to this period are a few 

 isolated patches of light-colored sandstone which occur capping some low 

 hills in the southwest corner of the quadrangle. South and southeast 

 of Merced Falls are two level-topped buttes capped by this sandstone, 

 which rests almost horizontally upon the nearly vertical edges of the 

 Mariposa slates. The basal bed is crowded with angular fragments of the 

 slate and with abundant pebbles of white vein quartz, while the upper 

 beds are composed of a light-colored quartzose sandstone with frequent 

 bands of small quartz pebbles. Marine fossils (Venericardia planicosta) 

 are fairly abundant in the upper bed at the west end of the butte that 

 lies one mile south of Merced Falls. These sandstones are overlain to the 

 west by the light-colored sandstones of the lone formation. The two series 

 are probably not absolutely conformable, as the lone beds transgress onto 

 the rocks of the bedrock series farther north. 



The above-mentioned sandstones, instead of "being overlain to 

 the west by the light-colored sandstones of the lone formation," are 

 in reality stratigraphically higher. These sandstones have been worn 

 away from most of this area and only a few residuals remain. 



After this great erosion, andesitic tuffs and tuff breccias covered 

 all. During the Pleistocene and Recent time much of the andesitic 

 material has been removed, re-exposing the older rocks beneath. 



THE IONE FORMATION IN THE VICINITY OF BEAR CREEK, 

 MERCED COUNTY 



The region south of Merced Falls was first described by Blake 53 

 in the Pacific Railroad Reports. The exploration route was along 

 the old Fort Miller road from Merced Falls to Fort Miller. Blake 

 recognized the unconformity between the sandstone and the under- 

 lying slates and he figured two hills showing the characteristic 

 topography yielded by the middle member of the lone. 



His description of the country south of the Merced is as follows : 



Merced river to Bear Creek, July 22, 18.3 miles. — After leaving the 

 Merced our route lay among numerous isolated hills with flat summits; 

 a group of which is represented in outline in the figure. These hills 

 were estimated to be about one hundred feet in height. They are formed 

 of horizontal strata, and are the remnants of a former plain, the inter- 

 mediate portions having been removed by denudation. The "cap rock" on 

 one of the hills was found to consist of a bed of conglomerate, chiefly 

 of quartz pebbles, underlaid by a bed of light-colored sandstone, and a 

 second stratum of conglomerate. A cylindrical object, like a log of 

 wood, was protruding two feet beyond this layer of sandstone, and it 

 proved to be a part of a fossil tree, with a cross-section like the figure. 



It was somewhat flattened as if by pressure. Its outer portions had 

 evidently been bored into by worms, as cavities similar to those formed 



53 Blake, W. P., Explorations and Surveys for a Railroad Route from the 

 Mississippi River to the Pacific Ocean, vol. 5, part 2, pp. 12-15, 1853. 



