GEOLOGICAL STRUCTURE — COAST OR MESA BELT. 501 



which is the remnant of an ancient stream bed whose bottom is now 

 about 100 feet above tide-water, and which is filled by a conglomerate of 

 large bowlders and water-worn pebbles of massive rocks. This conglom- 

 erate, which is cemented by lime and iron, is so much more resisting 

 than the soft clays of the Chico formation that the huge bowlders that 

 fall down as the cliff is undermined by Avave action form a point pro- 

 jecting out several hundred feet beyond the average coastline. These 

 conglomerates are probably of the same age as those which are found at 

 various points in the canyons of the interior, and their formation evi- 

 dently dates back to a time when, after the carving out of the general 

 system of modern drainage, the waters of the ocean reached a higher 

 level than the present and the old drainage channels were partially filled 

 up to the then baselevel. Subsequent erosion, while cutting down to a 

 somewhat lower level and following the same general lines, has often 

 eaten more readily into the softer beds at the sides of these recent con- 

 glomerates and left patches of them still standing, which sometimes 

 form one wall of the canyon a hundred or more feet above its present 

 bottom. 



The modern stream beds from Playa Santa Caterina are almost at base- 

 level for some ten miles inland, at about which distauce eruptive rocks 

 appear from under the Cretaceous and recent beds, and then rise rapidly, 

 reaching an elevation of about 1,500 feet within 15 miles of the coast, on 

 the partly buried slopes of the Coast range. 



Both in the broad valley and on the mesa slopes are relics of terraces 

 which evidence a successive rising of the land above the ocean. 



The lower beds exposed in the bluffs along the coast have a gentle 

 inclination northward and southward from Sandstone point, about three 

 miles north of Playa Santa Caterina, where massive sandstones form a 

 slightly projecting headland. In these sandstones carbonized plant re- 

 mains, too indefinite for identification, were found, and in the cracks of 

 the immediately overlying sandy clays were traces of i)etroleum. From 

 these beds and from calcareous layers about 200 feet above were obtained 

 the following forms as determined by Mr T. ^^^ Stanton : 



Area brewenana, Gabb. Inoceramus, sp. undetermined. 



Baciilifcs cJiicooisia, Trask. Ammonites, sp. undetermined. 



Tessarolax distorki, Gabb. Ostrea, sp. undetermined. 



They correspond with forms found in the Chico beds of California and 

 Oregon. 



From rolled pebbles of impure limestone obtained along the beach to 

 the south of the Playa, which had evidently fallen from the cliffs above, 

 and from a bed of similar composition in place at what was assumed to 

 be about 1,200 feet higher in horizoil, at San Carlos anchorage (collected 



