The Geolofjji of the Coast Ranges. — Lavson. 355 
Post-Pliocene Diastuophism. 
The San Francisco peninsula is composed of two great fault 
blocks, each tilted with a gentle slope to the northeast and 
having a precipitous fault scarp to the southwest. The more 
northern of these blocks extends from Merced valley to the 
Golden Gate, and the southwestern wall of the San Bruno 
mountains overlooking the valley is its fault scarp. The more 
southerly block lies between Merced valley and the coast south 
of San Pedro point. 
The gently sloping back and the precipitous scarp of the 
block intersect in the crest of Montara mountain. The Mer- 
ced strata form part of the sloping back of the Montara fault 
block and dip toward the fault plane which has dislocated the 
two blocks. These Merced beds must obviously have extended 
over the northern extremity of the peninsula prior to the 
faulting. There is now no trace of these beds to be found on 
the back of the block. They have been removed by erosion. 
This complete denudation of the Merced strata from the north- 
ern block and their abundant occurrence on the southern block 
up to an elevation of 700 feet above sea level indicate that 
the northern block is the older of the two. Its more ad- 
vanced sculpture points to the same conclusion. It is believed, 
therefore, that the more northern block was thrown up and 
subjected to denudation before the Montara block came into 
existence as such. The differential throw of the San Bruno 
fault must be at least 7,000 feet.* Then came the Montara 
upthrust whereby the second block was tilted, the axis of up- 
lift being parallel to that of the San Bruno uplift. At the 
close of these orogenic movements, the general altitude of the 
peninsula was much lower than at present, for we have excel- 
lent evidence of the emergence of both blocks in unison with 
the uplift of the whole Calif ornian coast. Two particularly 
well characterized baselevel bench-marks serve to indicate pro- 
nounced stages of this uplift. These are dissected plateaux 
at about 1,200 feet and 700 feet; and, at lower levels, there are 
spread out on the slopes of the two l)locks marine embank- 
ments which give a terraced character to the topography, 
*Thiit is on the assumption that the fault is normal. TIic fact that 
the normal character of the fault is assumed should not be hjsl sijiht of. 
So far as the field evidence is concerned it is possible that an over tlirust 
mii^ht effect the same results. 
