448 SIDNEY PAIGE 
Regarding the efficacy of sheet floods to erode the land surface 
W. J. McGee says: 
The inference from the character of the sheet flood is consonant with the 
necessary inference from the character of the base level surface. Over dozens 
or scores of square miles in carefully examined localities, hard rocks like those 
of the mountains, and with no sign of decomposition, are planed almost as 
smooth as the subsoil by the plowshare, with nothing either in configuration 
or in covering to indicate that streams have flowed over them, and extended 
consideration has yielded no other suggestion as to the eroding agent than 
that found also in analogy with the observed sheet flood. 
Thus in unequivocal terms the rock-cut plains are assigned to 
an origin by sheet-flood erosion. The question naturally arises 
in the mind of the reader, Was there a stage in the past history of 
the region during which such plains might have been formed by a 
process approaching in kind that of peneplanation ? Again quoting 
W. J. McGee? 
The fourth inference is that the massif [he refers here to an uplifted folded 
area] produced in this way stood at moderate altitude for a long period includ- 
ing approximately the Eocene and the earlier half of the Miocene, that a large 
part of its volume was degraded; that the surface was planed to an approxi- 
mate base level,3 relieved by ridges and masses of the mohadnock and catoctin 
types, usually of harder layers but sometimes marking broad divides, and that 
during this vast period the drainage basins were outlined and developed. It 
is deemed probable that during much or all of this period the precipitation 
was greater than now, so that the district throughout was one of degradation 
and so that the drainage basins were of the normal dendritic type veined by 
rivers occupying broad yet essentially V-shaped valleys; and it is considered 
probable also that the basin-limiting Sierras were less rugged than now. 
The meaning of the above paragraph is a little ambiguous—the 
reference to ‘approximate base level” does not fit in with ‘‘rivers 
occupying broad yet essentially V-shaped valleys.’’ One cannot 
reach a definite conclusion, therefore, as to whether the rock-- 
planed surfaces, referred to above, were dependent upon this stage 
for their beginnings. But considering the language used in describ- 
ing the rock-planation, one is inclined to infer that W. J. McGee 
regards sheet-flood erosions as the primary process in such planation. 
The foregoing description of the Sonoran benches, it seems to 
the writer, applies to the rock-cut surfaces of the Silver City region, 
1 Op. cit., p. 108. Op. cits, Dp. OS. 
3 Underscoring by the writer of this paper. 
