40 The American Geologist. January. 1901. 
depth, but in places increasing to fifty feet, which it has ex- 
cavated in the bottom of the Ozarkian valley. This canyon is 
scarcely anywhere wider than the stream, which is itself con- 
tracted to a swift mountain brook, and its walls are mostly 
perpendicular precipices, along which the rocky strata are 
finely exposed, showing that structure has had nothing to do 
with the existence of this canyon in the bottom of the vastly 
larger Ozarkian valley above. It has the appearance of ex- 
treme recency of inception. It is crossed by rock-ledges over 
which the stream cascades, and is obstructed by large blocks 
which have fallen from the walls. There is a freshness, a 
youthfulness about it which I have only observed heretofore 
in post-Glacial rock valleys in the Wisconsin drift area. 
This recent canyon of probably Modern age (it is form- 
ing today) gives out before the Arkansas river is reached. It 
is a feature exclusively of the middle course of the Little Ce- 
dar (and similar creeks flowing down from the Boston moun- 
tain). The smooth valley floor above it could not have been 
formed while the stream had its present rapid rate of descent. 
This indicates a tilting of the valley toward the Arkansas 
river. This, with the presence of the canyon, postulates a very 
recent diflferential uplift of the Boston mountain region. It 
will be remembered that earlier in this discussion we found a 
remarkable monocline in the main Tertiary base-level be- 
tween the Boston mountain and the Arkansas river. The La- 
fayette base-level partakes in this unusual deformation almost 
to the same extent as the Tennesseean. It was probably dur- 
ing the early part of the Quaternary era that this abnormal 
slope was mainly produced, but the testimony of the tiny rock- 
canyons of Modern age indicates that a movement of a similar 
nature has occurred in very recent times and may be in prog- 
ress today. Certain peculiarities in the streams of southern 
Missouri, and the fact that there has been a wholesale change 
in the places of emergence of the springs in times very, very 
recent, once led me to believe in a pronounced uplift of the 
whole Ozark plateau in the Modern epoch (if it is not in 
progress today), and now these modern canyons of west-cen- 
tral Arkansas come to light to corroborate the idea, at least as 
applied to the Boston mountain region. 
The preceding generalizations on the history of the phys- 
