36 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
predict the pattern which many of the lower ones will in time attain. 
Some of the contours may gain an increased emphasis from following 
the outcrops of cliff-making strata ; thus they catch the eye more readily 
than others which lie on evenly graded slopes ; but all exemplify the 
same general principle. In the absence of strong cliff-makers, the 
high-level contours of spurs and isolated outliers will not assume 
the pattern of acute cusps; they will be rounded at their convex turns 
by the process of soil creeping, just as the crest-line divides are rounded ; 
but in general the high-level contour lines in young or maturely dissected 
plateaus will show large re-entrant curves between relatively sharp 
salients (the sharpest salients or cusps occurring on contours that follow 
the outcrop of a cliff-making stratum), while the contours at low levels 
will show large salient curves between relatively sharp re-entrants (the 
sharpest re-entrants occurring on contours that follow a cascade-making 
stratum in an early stage of the cycle of erosion). 
The Great Terraces. 
REFRESHED CiirF Prorites. — In my former report, it was stated 
regarding the great mesozoic escarpmeuts or terraces, north of the 
canyon: “The sharpness of the cliffs is highly suggestive of aridity, 
but a relatively short arid period would suffice to sharpen the cliff pro- 
files, even if they had been somewhat dulled by a previous humid 
period” (b, p. 188). This opinion needs a supplement concerning the 
change in the appearance of the escarpments that would accompany the 
Figure 12. 
The changing profiles of retreating escarpments. 
transition from the subdued forms appropriate to a late stage of the pla- 
teau cycle, to the refreshed forms appropriate to the present young 
stage of the canyon cycle. 
In the later stages of a cycle of erosion in a region of nearly horizontal 
structure many escarpments may have retreated far back from the 
main waterways. The graded floor of the peneplain of degradation, 
A B, Figure 12, may then ascend gently to such an altitude at the base 
