Lawson. I 



Post-Pliocc 'tie Diastrophism . 



121 



base-level, and are now corrading, forming flat-bottomed valleys with 

 very steep stream-cliffs on either side. Beneath the valley bottoms 

 are the eroded Pliocene or older soft sandstones, but the surface is 

 commonly strewn with pebbles accumulated from the gravels of the 

 upper terraces or the original delta plain, and from the occasional 

 conglomerate beds which are intercalated with the sandstone. Thus, 

 in the stream topography of the coastal slope of San Diego County, 

 there is revealed to the most cursory inspection abundantly demon- 

 strative evidence of a Pleistocene elevation of the coast of 800 feet. 

 The only question which can arise is whether or not this is the full 

 measure of the uplift. The evidence seems to indicate that it is. 

 The wonderfully striking contrast between the bare granite hills and 

 the even plain seems to force upon one the conviction that there are 

 no Pliocene formations above the level of the original gravel plain. 

 It is possible, however, that this assumption may be at fault. If the 

 stationary condition of the coast at which the original delta gravel 

 plain was developed were of very long duration, many times longer 

 than that of the interval since the inception of its uplift, another 

 possibility might obtain. 



The marine Pliocene may have been thicker and have buried 

 more of the granite slopes than present appearances suggest, and, 

 being an incoherent sand, have been practically wholly removed 

 down to the level at which the gravel plain was developed. One 

 fact, quite insufficient in itself to warrant any conclusion, but still 

 suggestive, lends support to this possibility. This is the discovery 

 of a few water-worn pebbles on a granite hill near Cajon, at an ele- 

 vation of 900 feet above tide. Some notches, also, which are 

 observable in the profiles of the granite hills, may possibly be ancient 

 sea-cliffs, but the writer was unable to satisfy himself that such was 

 their origin, since they lack the horizontal extension which charac- 

 terizes shore features. The writer attaches little weight to these 

 vague suggestions of an uplift exceeding that of which the summit 

 of the gravel plain affords us the measure, and is of the tentative 

 opinion that the demonstrative uplift of 800 feet is the full measure 

 of the Pleistocene uplift of this part of the coast. 



Marine Terraces. — Corresponding to the stream terraces are cer- 

 tain well-defined wave-cut terraces. The lower of these are some- 



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