38 The American Geologist. January, 1001. 
tion of the Ozark plateau, while the Uzarkian valleys still oc- 
cur in as characteristic a form and as great a development as 
in the Osage country, they are quite subordinate to the valleys 
above the Lafayette base-level. I wish to have this fact dis- 
tinctly understood so that there may be no confusion as to the 
origin of the term, "Ozarkian." It was derived through the 
fact that the erosion products of the long epoch between the 
Lafayette and the earliest Glacial epoch are so well repre- 
sented in the river valleys of the Ozark plateau, Init even here 
not all the valleys belong to it. Li the Ozark highland south 
of the Missouri line, the Ozarkian valleys are comparatively 
insignificant, and the name would be inappropriate, were it not 
for their fine development in south Missouri on the northern 
half of the Ozark plateau. 
On the War Eagle fork and the main fork of White river 
in Arkansas, the Ozarkian valleys are small troughs, twenty 
to thirty or even fifty feet in depth and several times the width 
of the contained streams, trenched beneath the flat rock-floor 
of much larger valleys. In he Boston mountain region, no 
valleys can be pointed out as distinctively Ozarkian, although 
the bottoms of the deeper valleys on the southern slope must 
reach much below the Lafayette base-level. These valleys be- 
gan to form at the close of the Cretaceous period, and have 
continued uninterrupted to the present day. They had not 
been cut down to a base-level before a new uplift occurred and 
the base-level of erosion was again lowere^l. But no sooner 
do we go out of the mountain region on the south, where the 
Lafayette base-level becomes apparent well up in the hills, 
when we find that here the Ozarkian valleys are quite large 
and deep, being compara])le with those in Missouri. As the 
whole country sinks rapidly to the Arkansas river, these val- 
leys become shallower, sjiread out to a considerable width, and 
finally pass into the southern Arkansas type of Ozarkian val- 
leys, of which the lower trough of the Arkansas river is a 
good example. 
Near Van Buren and Fort Smith, Ark., the highest hills 
near the river (about 200 feet above it) are renmants of the 
main Tertiary peneplain. Midway between it and the river- 
level, arc the depressed areas, sometimes in terrace form and 
sometimes passing inland as gently rolling plains of solid rock 
