Itirei' Valleys of the Ozark Plateau. — Hi-r.-<he,i. :Ui> 
removed it would probably be comparable in size to that of 
the Tertiary era, but not of the Cretaceous, This? is the third 
and last step in the formation of the present courses of the 
streams, which, as mapped, may be considered to be the com- 
plex result of three distinct systems of meanders separatetl 
by wide intervals of time. 
Comparative Straightness of the Missouri Valley. 
The Missouri river flows across the northern edge of the 
Ozark uplift, but differs from other rivers of the region by 
being comparatively straight, although above the region of 
the uplift it has a broad valley and meanders on a flood- 
plain. The immediate valley of the river, from near Boone- 
ville onward, varies from one to two miles in width, and is 
trenched below the floor of another valley several times as 
large. At Jefferson City the lower gorge is perhaps about 100 
feet deep, and the rim of the upper gorge or trough lies a 
short distance south of the city and is about 200 feet above 
the river. 
In explanation of the cojuparative straightness of the 
Missouri valley two principal hypotheses may be examined. 
The first is that it is due to a straightening of the river by 
the ice-sheet which once covered northern Missouri and 
advanced to the Missouri valley, yet apparentl}' did not ex- 
tend beyond it in this region ; but the valley was in existence 
previous to the glaciation of its northern side, as its relation 
to the drift shows, and was just as straight in preglacial times 
as now. The other hypothesis is, I believe, the one most 
accordant with the facts. The Missouri river did ^^^not exist 
in any form comparable to its present size until after the 
elevation of the vast Cretaceous beds of the Northwest. 
Hence, while the tortuous valleys of the other streams in the 
Ozark region are lai-gely due to a meandering on a Cretaceous 
flood-plain, the Missouri river did not then exist ; and sub- 
sequently in Tertiary time, when it first began to flow,^it took 
the straightest course across the edge of the Ozark uplift. 
The Lafayette Formation, 
Numerous writers on the geology of soutlieni 3Iissoiiri have 
mentioned the existence of a local drift in llic valleys, which 
is doubtless in large part tiie same deijosit that 1 propose to 
describe as the probable ecjuivalent of the Lafayette forma. 
