140 
East 
Uinkaret 
Shivwits 
West 
Marble Platf»: 
BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
es 
[1 
Ficure 10. 
strict, from the lower lands west of the Grand Wash fault to the Echo flexure. 
somewhat exaggerated. Modified from Dutton’s section (c). 
Vertical scale 
East and west section of the Grand Canyon di 
the roughly concentric attitude of the escarpment 
of horizontal Eocene on the northwest with the 
escarpments of the swell itself (a, pp. 19, 20); 
but it nevertheless seems permissible to class this 
dome-like uplift with the Waterpocket and Esca- 
lante flexures as having been formed in what may 
be called the Cretaceous-Tertiary interval, and 
as having been, like the flexures, greatly eroded 
before it was buried by Eocene deposition. True, 
the unconformity thus implied is nowhere pre- 
served, for the Eocene strata have now receded 
to a distance of about forty miles from the centre 
of the swell ; but some indirect evidence for their 
original unconformable extension over the eroded 
swell is found in the small amount of adjustment 
of two transverse streams to the domed struc- 
tures, as will be more fully stated further on. 
The Kaibab and the Echo (Paria) Flexures have 
been described as about contemporaneous with 
the broad uplift that introduced the erosion of 
the Grand canyon (c, pp. 192, 205), but they seem 
to be of older origin if one may judge of the date 
of their deformation by the recession of the Trias- 
sic cliffs that stand in association with them. This 
is shown as follows: The Triassic (Vermilion) cliffs 
have retreated at least twenty-five miles around 
the northern end of the Kaibab since its uplift- 
ing; and the same Triassic cliffs have retreated 
to close coincidence with the line of the Echo 
(Paria) flexure since its production, from what- 
ever irregular line of front they had previously 
when the strata were horizontal. But, on the 
other hand, the upper Aubrey cliffs, enclosing the 
outer gorge of the canyon in the Kanab and Uin- 
karet plateaus, have retreated only two or three 
miles since the beginning of the canyen cycle. 
Surely then the Kaibab arch and the Echo mo- 
nocline must be older than the beginning of 
the canyon cycle. Moreover, Walcott has called 
attention to the flexure of the east Kaibab mono- 
ee 2 ee eee 
