Vol. XI] DICKERSON—PT. REYES AND SANTA ROSA QUADRANGLES 5gO, 



appears and at Big Rock ranch, the stream canyon is in 

 early maturity, as well as Bolinas Creek which heads in this 

 same vicinity. Gallinas and Novata valleys on the bayward 

 slope exhibit the same general characters. Ponding is lack- 

 ing in both streams and their sidewalls are very steep and 

 meet an alluviated valley due to the last general subsidence 

 in the San Francisco Bay Region very abruptly. It appears 

 probable from these contrasts that Arroyb Sausal and Arroyo 

 Nicasio never had drainage across the entire block during 

 the Pleistocene, but in both cases their upper courses were 

 ponded as an incident of tilting in upper-Pleistocene time. 

 Looking southeastward from the hilltop about one mile 

 west of Tocaloma, an old plateau surface is visible in the 

 direction of Barnaby Mountain. From the same viewpoint 

 the canyon of Papermill Creek (Lagunitas Creek) appears to 

 be cut in this old plateau and since Middle and Upper Lagu- 

 nitas Creek (called Papermill Creek in the Petaluma Quad- 

 rangle) have a greater drainage area than Arroyo Nicasio, 

 ponding is not so evident along its course. Only a slight 

 suggestion of this effect was noted on its tributary, the San 

 Geronimo Creek, near Mailliard Station. 



Elk Creek in Elk Valley of the Sausalito Peninsula has 

 been shown by Professor Holway to exhibit on a small scale 

 essentially the same history of Walker-San Antonio Creek, 

 that is, a Pleistocene stream which once drained the region 

 north and west of Mill Valley Junction in Tiburon Penin- 

 sula had its back broken as a result of tilting, and a portion 

 of its waters were spilled out by one of its headwater tribu- 

 taries. 



There is much that the writer has not discovered in the 

 stream drainage of this block, but it appears evident that the 

 tilting of the San Francisco-Marin Block in upper-Pleistocene 

 time, caused similar results in many of the streams of this 

 block, and that these results are not equal because of un- 

 equal development of the respective streams. 



Petaluma and Santa Rosa valleys were not studied in- 

 tensively, but the geological mapping of these areas and the 

 physiography of their bounding blocks show that they are not 

 simple synclinal valleys. Faulting may have played a part in 

 their development, at least in a minor way, but another cause 



