144 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
As a part of the drainage system of the Colorado which was fully 
established previous to the period of folding, Dutton cites the House 
Rock Valley, which follows the eastern base of the Kaibab INIonocline 
along its northern part. "Just at the base of the East Kaibab flexure 
is House Rock Valley, which once held a river which has long since 
vanished, though leaving well marked traces of its former action. 
None of the great displacements which traverse these masses had any 
existence at that remote epoch" (Dutton, '82, p. ISS). The valley, 
in common with the rest of the main Colorado drainage, was thus 
considered antecedent to the uplift which developed the monocline. 
Davis ('01, pp. 153, 157) has interpreted this same valley as one pro- 
duced by erosion along a belt of weak rocks after the development of 
the monoclinal uplift. Walcott ('90, pj). (30, 64) cites the flexing of 
the hard, comj:)act limestones now forming the surface of the mono- 
cline, as an evidence that when the folding occurred a considerable 
thickness of overlying strata must have been present. 
The valley is bounded on the west by the Carboniferous limestone 
which rises in the symmetrical arch of the lower member of the double 
monocline. Along the southern part of the valley, at least, the arch 
has not been cut into by the main stream, but everywhere presents the 
smooth, arched surface of the upper Carboniferous limestone, from 
the surface of which the soft overlying beds have been washed away. 
The eastern side of the valley is bounded by erosion cliffs of Triassic 
sandstone. The monocline does not show the same dip at all points, 
nor is its base a straight north-south line. On the contrary, the dip 
is fairly gentle in some places and much stee])er in others, while the 
base of the fold forms a line which cvu'ves distinctly toward the east or 
west in different localities. Along the southern portion of House 
Rock Valley one of these curves is well marked, concave toward the 
east. The cliffs on the east side of the valley curve in sympathy with 
the curved base of the monocline, so that the width of the valley re- 
mains fairly constant (fig. B). It seems clear that House Rock Valley 
has been produced by the erosion of the weak shales Ijetween the 
Triassic sandstone and Carboniferous limestone, after the develo])ment 
of the monoclinal fold had determined the course the valley was to 
follow. 
The small streams which dissect the east-facing slope of the mono- 
cline are characterized in their lower courses by steep-sided gorges 
that open abru])tly on the surface of the Marble Platform. In their 
