STRUCTURAL GEOLOGY 
175 
entire extent of the Rocky Mountains; perhaps with greater 
graphic distinctness than Appalachian structure is exhibited any¬ 
where in the whole world. KejyEs. 
Some Prairie Tectonics. Soil conditions on the Great Plains 
give small clue to the geological structures beneath. But there is 
recently revealed information derived from deep-well records that 
points to the fact that the singularly flattened and monotonous 
prairies have not always been so. In northwestern Iowa and 
southwestern Minnesota there persist traces of once larger relief 
contrasts. 
Despite the facts that plain is now the dominant topographic 
expression of the entire region, that the eye has unobstructed view 
in every direction for scores of miles, and that the horizon is as 
unbroken and as straight as the sky-line of a summer sea, there 
formerly existed here a high and mighty mountain range. Extent, 
form and structure of this ancient chain may now be figured 
forth and its characteristic facial expression pictured out. 
This great earth-wrinkle which like Thetis rising from the 
waves sprang out of Mesozoic seas extended from the eastern 
shore of present Lake Superior southwestward far into Nebraska. 
Medially the strata composing this majestic range were bowed up 
more than a mile above the present level of the prairies. In their 
prime these Siouan Mountains rivaled in scenic beauty and grand¬ 
eur the Adirondacks, the southern Appalachians or thfe Juras of 
today. 
As they appear on the general geological map of Iowa the Pale¬ 
ozoic formations are distributed in relatively narrow belts trend¬ 
ing northwest and southeast across the northeastern corner of the 
state. Very singularly it had always seemed these belts abruptly 
terminated at the north very soon after passing the state boundary. 
Far to the north in Manitoba, there is the same narrow belting of 
the same formations and, as in Iowa, the strike is still northwest¬ 
ward, and in line. The Canadian Paleozoic area is separated in 
central Minnesota from the Iowa field by a broad tract of pre- 
Cambrian crystallines. 
These pre-Cambrian rocks form the core of a rather notable 
arch, the axis of which runs northeast and southwest. The ex¬ 
posures of Sioux quartzite constitute the western nose of a canoe- 
