•548 The American Geologist. December, 1S95 
cour.ses irrespective of their existenee. The theory which 
8eenis most in aeeorchinee with the known facts is that which 
attributes the crooks to the former meandering of the streams 
on a lowland plain. I have observed that the valleys endeavor 
to follow the lowest portions of the ancient Cretaceous valleys 
or basins, which, as we have seen, supplied the conditions for 
the development of a most perfect system of meanders. The 
old Tertiary valleys are not nearly so crooked as are the 
streams at the present day. In general, the windings of the 
Cretaceous streams, as revealed by the Tertiary valleys, were 
of greater magnitude than at any subsequent period, and were 
in proportion to the size of the stream. It is assumed that, 
following the post-Cretaceous elevation of the Ozarks — the 
nu)St profound elevatory movement which has ever affected 
them, — the streams trenched their valleys in the positions' 
Avhich they hatl formerly occupied on the peneplain. After 
cutting to the new Tertiary baselevel they spent a long period 
in undermining the valley walls, until they had formed a new 
but much more circumscribed flood-plain, about which they 
meandered somewhat like the streams of the present day. 
As indicated b}' the lower troughs of the vallej's, this late 
Tertiary system of meanders was less in magnitude than the 
previous Cretaceous system. This may have been due either 
to the more limited area of the flood-plain or to a diminished 
supply of water, probably in part to both these conditions. 
We have now reached the second step in the production of the 
crooks in the present rivers, for the lower or (^iiaternary 
valley winds about within the very crooked Tertiary valley. 
The post-Tertiary elevation appears to have been suthciently 
rapid to cause the streams to cut the new valleys exactly or 
nearly under their old courses. In the progress of time the 
streams had again cut to their new baselevel, and have since 
widened their valleys sufflcientl}' to allow the formation of a 
new system of meanders, which, however, is very imperfect as 
compared with the previous ones. The streams are flowing 
from side to side of the valley, first undermining one blult' 
and then the other, forming high mural precipices on the 
outer side of the curves and a comparatively gentle slope on 
the inner side. It is a system of meanders confined to too 
limited an area to give it free play, and were the rocky walls 
