GEOLOGICAL SKETCH OF THE REGION OF TUCSON. 57 
_ Plate 50 represents a familiar example of a flanking slope in full view 
from the Desert Laboratory and from Tucson, on the south side of the 
Santa Catalina Mountains. It shows the western end of the range and 
a profile view of the upper portion of the western slope. The continuous 
slope along the base of the range, being inclined in the line of vision, is 
shortened to the view and is not so distinct, but represents a breadth of 
about 4 miles from the upper margin down to the Rillito, a difference of 
elevation of about 1,000 feet. The course of the Rillito is shown by the 
line of white bluffs on its right bank where it has cut across and eroded 
the slope. 
It is practically a continuous slope, along the whole length of the 
range, for nearly 15 miles, though much eroded, trenched, and cut away 
by floods and by the Rillito. This erosion is well shown by the contour 
lines on the map of Tucson Quadrangle by the U. S. Geological Survey. 
The Tortillita Mountains are also bordered by slopes of great evenness 
and extent on all sides. The angle of intersection of the slope with the 
ridges has apparently the same altitude as on the Santa Catalinas. The 
ridges, however, do not compare in height or bulk with the Catalinas, 
but are formed of similar rocks. 
The Santa Rita Range is similarly flanked by the gravelly deposits. 
The slope toward the Santa Cruz is remarkably regular and is well shown 
by the parallelism of the contour lines upon the map of the Santa Rita 
Quadrangle issued by the U. S. Geological Survey. The length of this 
slope from Helvetia to the river near Garcias Ranch is 13 miles and the 
rise is 1,500 feet, averaging 116 feet to the mile. 
In the region of Tucson the upper or higher margin or limit of the detri- 
tal slopes has a general level or altitude of between 3,500 and 4,000 feet. 
In the Santa Catalinas it corresponds approximately to the 3,500-foot 
contour line, and in the Santa Ritas to that of 4,000 feet. The Catalina 
slope is much reduced by erosion, but the general uniformity of altitude 
of the intersection of the detrital accumulations with the rocky ridges 
is very distinctly visible from all parts of the valley. 
The upper limit of detrital deposits is higher on the east side of the 
Santa Catalinas and of the Santa Ritas than on the west side. It no 
doubt bears a close relation to and was dependent upon the amount of 
erosion and of detritus washed from the mountains. The fan-shaped 
structure often seen in arid regions and the scalloped outline of the inter- 
section with the ridges, due to the greater accumulation of detritus at 
the mouths of the canyons, are not strongly in evidence anywhere in the 
region, the detritus having been very evenly distributed along the ridges 
between the canyons. 
The greatest slope of least grade of the Tucson Valley is that from 
Tucson up to Vail’s Station and Pantano, a distance of approximately 
28 miles and a rise of 1,150 feet, an average of a little over 41 feet per mile. 
