GEOLOGY. 
63 
ORIGIN. 
Most of the terrace deposits are probably of marine origin. This 
is proved in the case of the most typical deposits by the presence of 
the pholas borings already mentioned. The deposition was carried 
on in shallow water and much of the material was derived on the 
spot from the wearing away of the shore line of Monterey shale, the 
fragments of which were not always subjected to much polishing 
before being deposited and protected from agencies of erosion. 
These deposits give undeniable evidence of a great uplift of the 
coast during Pleistocene time. It seems most probable thbt the 
terraced surfaces resulted from marine planation along gradually 
rising shore lines and that the formation covering them represents 
the beach and shore deposits. The rise of the land was probably too 
rapid and the amount of sediment too small to allow much off-shore 
extension of the deposition. The material that may have been 
deposited in the deeper places determined by the depressions in the 
topography has since probably been largely removed by erosion. 
The terrace deposits themselves have been extensively eroded and 
in many places are left as mere remnants. Some of them have no 
doubt been subsequently added to by wind-blown sand. 
It is probable that some of the terraces and horizontal Pleistocene 
deposits along valleys have been formed by streams. Most of the 
valley fillings were probably laid down in this way. At the mouths 
of some canyons, as along the western side of Graciosa Canyon, 
Pleistocene deposits have been built up in the shape of detrital fans, 
which have since been carved into flat-topped, steep-sided blocks by 
recent streams. 
DUNE SAND. 
The prevailing northwesterly wind from the ocean has amassed 
great deposits of sand in places along the coast. The process has 
probably been going on all through the Quaternary period and it is 
hard to distinguish the older of the eolian deposits from those partially 
or entirely of marine deposition. The line of contact of these forma¬ 
tions as mapped is arbitrary. 
The greatest mass of dune sand occurs at the northwest end of the 
Casmalia Hills, where the gradual slope down to the sea from an eleva¬ 
tion of about 1,200 feet is covered by loose, yellow sand of probable 
eolian origin. This drifts about incessantly and is probably still in 
the process of collecting, being supplied from the long, low, open 
shore to the north and held in check by the bulwark of the Casmalia 
Hills on the south. This deposit has a thickness of several hundred 
feet. At its base along the coast is exposed a basal layer of large 
bowlders and horizontally stratified sand. The original slope of the 
