DAVIS: THE GRAND CANYON OF THE COLORADO. 183 
Relation of the Inner and Outer Canyons. — In the third place, the 
position of the inner canyon along the middle of the esplanade is, as 
was suggested to me several years ago by Mr. C. H. White (then one of 
my graduate students, now instructor in mining at Harvard), singularly 
significant of a single period of erosion. This may be understood by 
considering the two alternatives in order. If the esplanade represented 
a mature valley floor, broadened by the lateral swinging of the river 
during a lower stand of the land, rather than by the rapid wasting of 
the weak lower Aubrey layers in contrast to the persistence of the 
strong Red-wall group, then when renewed uplift revived the process of 
canyon cutting, the river must have occupied an irregular path along 
the esplanade, and the inner canyon would have been cut at one place 
near the northern wall of the outer canyon, and at another near the 
southern wall. This condition is actually illustrated in the valley of 
the Rhine; here a narrow inner gorge is incised in the flat floor (espla- 
nade) of a broad trough which in turn is eroded beneath the bordering 
uplands ; a structural origin for the trough is inadmissible because the 
rocks are greatly deformed; and, moreover, river gravel and silt still 
cover the floor of the trough. The narrow gorge is intrenched irregu- 
larly along the trough floor, sometimes turning so far to one side that a 
continuous descent leads from the high upland directly to the river, while 
a correspondingly broad mid-level floor remains on the other side. The 
same relation occurs in the valley of the Moselle, whose young gorge 
meanders conspicuously from side to side in the floor of the mature 
trough. The valleys of the Lot and the Dordogne in southwestern 
France exhibit similar features, except that the younger or inner mem- 
ber of the composite valleys are here opened wide enough to have scrolls 
of incipient flood plain on one or the other side of the river, while flood 
plains are as yet only just begun in the gorges of the Rhine and the 
Moselle. 
If, on the other hand, both the outer and the inner canyons of the 
Colorado are the work of a single cycle of erosion, and the esplanade is 
of structural origin, then it is necessary that the walls of the outer 
canyon should retreat symmetrically on either side of the inner canyon, 
and that the inner canyon should bisect the floor of the esplanade thus 
produced. This case would correspond to that of all one-cycle valleys 
in horizontal strata, where the symmetry of the benches and slopes on 
the two walls results from their essentially equal retreat under the 
weather. Innumerable illustrations of such symmetry may be seen in 
the side canyons of the Kaibab section, where the Red-wall cliffs are 
