578 NOAH FIELDS DRAKE 
south side of the hills the almost barren ground would lose 
much of its soil at every freshet. Over most of this area erosion 
is lowering the hill tops about as fast as stream corrasion lowers 
the creek beds. So the vertical distance between hill top and 
stream bed remains nearly the same, while the horizontal dis- 
tance on the north side of the hill becomes less because the hill 
top recedes to the north as its south side is the principal one to 
be trimmed back by erosion. Such a process would in time 
almost produce a bluff. Suchsteep hill-sides may be seen 
immediately east of Arroyo Grande along the south side of Tar 
Springs Creek, San Luis Obispo county. 
Topography as modified by rainfall—tn southeastern Cali- 
fornia where it is especially arid, there is almost no vegetation 
to hold the soil and disintegrated rock in place, so the weathered, 
loose, rock-fragments are soon blown to low ground leaving the 
rocks of the hillsides exposed in rough angular forms and the 
topographic outlines strongly fixed by the character of the rock 
mass or strata. 
In the humid portion of the state, however, vegetation holds 
the soil from washing or drifting away and as the rocks con- 
tinue to disintegrate, the hills are covered and rounded off by 
this mantle of soil and débris. Besides this reason for the roll- 
ing character of the topography, the following causes may be 
added: The sedimentary rocks as a rule are soft and much 
broken by numerous fissures, faults, and folds, so hard and fast 
lines or horizons to resist weathering do not often exist. The 
igneous rocks are usually great masses and necessarily show 
their characteristic rolling topography. 
Taken as whole probably the most characteristic feature of 
the topography of the Pacific Coast is the rolling nature almost 
everywhere seen where relief is shown. This feature is espe- 
cially impressive to one familiar with the bench, bluff, and flat 
topped mountain topography of the Mississippi valley region. 
NoauH FIELDS DRAKE. 
STANFORD UNIVERSITY, 
California. 
