THE TOPOGRAPHY OF CALIFORNIA 577 
EFFECTS OF VEGETATION ON TOPOGRAPHY. 
Sand dunes.—During the summer of 1895 the writer assisted 
in mapping a number of sand dune areas along the coast in San 
Luis Obispo county. In all these areas there seemed to be no 
exception to the rule that where the sand was free from vegeta- 
tion or obstruction, it was piled in ridges at right angles to the 
prevailing sea breezes, and that where patches of vegetation 
grew the dunes became parallel to the direction of the wind, 
and where the vegetation became thicker over the ground, the 
regularity of arrangement of the dunes was more broken. It 
seems that the change in direction of the dune ridges (from 
right angles to parallel with the winds where vegetation began), 
is due to the fact that vegetation once started would check the 
sand from moving at that point and make a shelter for deposits 
to the leeward. This point of the sand dune now being more 
stable, other plant growth would spring up, mainly on the iee- 
ward side, so as to lengthen and increase the elevation of the 
ridge while the unprotected sands at either side would drift 
away, thus forming narrow parallel ridges in the direction of 
the prevailing winds. Ridges fifty to seventy-five feet high and 
400 to 600 feet long, or even longer, were not uncommon where 
the sand dunes are extensive. 
Fiill slopes —While mapping over the area south of San Luis 
Obispo for some twenty or twenty-five miles, it was observed 
that the slope of the north side of the hills was steep while that 
of the south side was gentle. This proved to be almost invaria- 
bly true, no matter in what direction the strata dipped or 
whether the underlying rock was loose sand. 
This part of California, like most of the state, has a dry sea- 
son and a wet one. During late spring, all summer, and early 
fall there is no rain and therefore any shelter from the sun’s 
heat insures thicker and more permanent growth. So the north 
sides of the hills are more thickly covered with vegetation, 
especially perennial growths, and timber is often completely 
confined to them. The roots, leaves, and débris of the vegeta- 
tion would protect the soil from washing away, while on the 
