GEOLOGICAL SKETCH OF THE REGION OF TUCSON. 61 
across the Santa Catalina slopes and leaves a steep bank for miles on the 
north side, as already represented. Similar conditions exist along the 
Santa Cruz opposite Tubac, where the ancient slope from the Santa Rita 
is cut across and shortened with the formation of high bluffs. In the 
deep arroyos trenching the slopes the nearly vertical banks exhibit rude 
stratification of gravels and sand, together with clay-like layers, not unlike 
the succession of gravels, sands, and clays revealed by the boring at the 
Esmond well. 
FORMER LACUSTRINE CONDITIONS. 
The evidences of submergence and uplift given by the detrital slopes 
are supported and strengthened by the phenomena of ancient lakes or 
estuarine deposits. These deposits show the long-continued existence of 
a large body of water withits surface at or about the 4,000-foot contour line. 
The presence of a body of clay in the lower portion of the valley of the 
Santa Cruz at Tucson is evidence of the former existence there of a body 
of comparatively quiet water. It would appear that this deposit was 
anterior to the excavation or erosion of the river valley. It may be 
explained upon the hypothesis of a sink or depression at the intersection 
of the slopes from the Catalinas, the Rincons, Santa Ritas, and the Tuc- 
son Mountains, while the region was submerged. The uplift of the region 
and the advent of the Santa Cruz with the cutting down of its channel 
brought this clay deposit to view. 
The excavation of a deep well for the city water-works passed through 
this clay and showed a heavy deposit of boulders below, marking an old 
channel. The trunk or large branch of a tree was also found, but in the 
condition of lignite, which speedily fell to pieces on drying. 
These phenomena indicate a very ancient river-valley antedating the 
formation of the mountain slopes. 
The fact of the submergence of the continent and its elevation above 
the sea in the late Miocene Tertiary is well known. There have been 
later changes of the relation of land and sea, but generally of less ampli- 
tude. Some of the effects herein attributed to oceanic action may date 
back to the earlier period of emergence. 
The enormous volume of detrital materials filling the valleys and com- 
posing the slopes, whatever their relationship to oceanic distribution, 
bear testimony to long periods of erosion and degradation of the land and 
to eras of greater rainfall than we now have. 
THE SOILS OF THE REGION. 
The soils of the region of Tucson may be grouped in three classes: 
1. The gravelly and sandy washes of the mountain slopes. 
2. The red argillaceous deposits of the older slopes and of Tuma- 
moc Hill and other hills of basalt. 
3. The alluvions of the streams, chiefly of the Santa Cruz and of 
the Rillito. . 
