DAVIS: THE PLATEAU PROVINCE OF UTAH AND ARIZONA. 33 
AMPHITHEATERS IN THE Canyon Watts. — Many of the sharp spurs 
_ into which the red-wall cliffs are now cut in the Kaibab portion of the 
canyon are separated by amphitheaters of remarkably regular curvature. 
The explanation for these notable forms that occurred to me after having 
seen them in 1900 (b, p. 178) was fully confirmed in my excursion of 1901. 
The amphitheaters occur only where the area whose drainage falls 
from higher levels over the cliffs is so small that it does not supply 
any large streams even in wet weather. In the absence of large 
streams, all parts of the red-wall cliff in each amphitheater now retreat, 
and indeed for a long time have retreated, at about equal rates. For 
example, the red-wall cliffs of the amphitheaters, Gand J, in the spur 
S 
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rs cut } 
WR pay or UG abe a 
Wy 2 5 a, aw y 
S 
S 
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S 
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ee 
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Figure 11. 
Diagram of amphitheaters and cusps in the wall of the Colorado canyon. 
of Figure 11, receive drainage only from the short slopes of isolated 
lower Aubrey outliers, L and M; all parts of the cliff face therefore re- 
treat about equally. The curved cliff face, G, is an are whose center 
lies near the point, B, this point being the site of a re-entrant in the 
cliff face at that stage in its earlier history when its rate of retreat in the 
re-entrant was but little faster than that on the adjoining convex spurs 
Aand C. The curves of these former convex spurs are likewise arcs 
whose centers are roughly indicated by the apices of the present sharp 
spurs, F and H, that now separate the amphitheaters. In other words, 
amphitheaters are forms into which the acutely re-entrant ravines of 
earlier stages systematically develop as the cycle of erosion progresses ; 
VOL. XLII. — NO. 1 
