156 5 CFA TOLMAN 
able even to the eye, and shows on the topographical maps by the even 
spacing of the contours. The grade increases on approaching the 
rock surface, and in extreme cases, especially where the slopes lead 
up to volcanic hills where undersapping by wind is prominent, it 
approaches the angle of repose for the material of which it is com- 
posed. It is distinguished from the upper margin of lake and ocean 
deposits by the absence of wave-built and wave-cut terraces, and the 
flat upper surface of the top set of the delta, etc. 
Upper limit—The line traced by the contact of the detrital 
material with the rock surface invariably extends up every gully and 
down around every ridge. With lake or ocean deposits a level upper 
line is scarred into the landscape by beach and cliff action. 
Size of material.—The astonishing size of the boulders found in 
the outwash, with other phenomena described later, has led to a 
local popular appeal to glacial action. Boulders up to six feet in 
diameter are found in the Catalina and Santa Rita outwashes near 
Tucson, for a distance of half a mile from the present rock surface, 
and boulders at least two feet in diameter six miles and more from the 
present ranges. In ocean deposits occasional distribution of material 
of this size below cliffs only is noted. 
Shape of boulders—They are generally subangular, although 
some are partly rounded by their tumultuous journey. 
Decomposition of the material—Almost no decomposition of 
either boulders or smaller material was noted in the various bajada 
deposits examined; so much so that grave difficulty is encountered 
when attempting to account for the large amount of calcareous 
cementing material often present. 
Sorting.—Layers of sized and clean sorted boulders, gravel, and 
sand are discovered but more often sorting is completely lacking, 
although stratification is always excellently developed. Pebbles are 
found in a matrix of sand, and boulders in a matrix of clay. In 
extreme cases the latter may not be due to water action. For instance, 
about two miles northwest of Travertine Point, Salton Basin, Cali- 
fornia, talus deposits, remnants of old slopes, were filled in with wind 
dust, and proved treacherous to climb. This is a deposit formed 
under extreme conditions of great daily temperature change and small 
non-torrential precipitation. 
