270 
IOWA ACADEMY OP SCIENCE 
in the ease of the Siouan mountains they were in Comanchan 
time completely leveled and worn down nearly to the level of 
the sea. Unlike the instance of the Siouan elevation the Ozark 
perfectly peneplained area was in Tertiary times again elevated, 
the summit of the great dome attaining a height above the sea 
of more than 2,000 feet. This is the elevated region which we 
see today, modified from its original condition only by the 
trenching of modern rivers. 
To us in Iowa the features of the broad trough and its vast 
economic consequences concerning our welfare are of first im- 
portance. 
This great trough is not a simple flexing of strata on a large 
scale; but a bending of a section already affected by number- 
less foldings and faultings. It is, therefore, a true synclinorium, 
and, withal, one of the most typical known. 
One of the most notable, as well as one of the most unexpected 
features connected with this synclinorium is the presence of a 
number of extensive dislocations. Two of these faults have 
displacement values of 125 and 350 feet respectively. The lines 
of fracture have a northeast and southwest direction — parallel 
to the axial trend of the Siouan and Ozark mountain ridges. 
These faults appear to be members of a system of sub-equally 
spaced stratigraphic breaks, 25 miles apart, that traverse nearly, 
if not quite, the entire width of the state. Since the presence 
of such a fault-system is foreshadowed indications of other 
notable faults are made known at the expected intervals. The 
recognition of these great faults in Iowa where they are wholly 
unlooked for, explains a host of anomalies concerning the areal 
distribution of the Paleozoics beneath the deep covering of till 
that have long puzzled Iowa geologists. 
The geologic date of this regular spaced faulting is fixed by a 
number of circumstances. It appears to be Mid Tertiary, and 
hence coeval with the last uprising of the Ozark dome. This 
.association of the two events is especially significant in that it 
immediately supplies an adequate reason for their presence at 
the points at which they are actually found. In eastern Mis- 
souri the orogenic strain seems to have been relieved mainly by 
a single dislocation; and the famous Cap au Gres fault which 
intersects the Mississippi river at the mouth of the Illinois river 
has a throw of more than 1,000 feet. The faulting is, of course, 
an event long subsequent to those of the Siouan and older Ozark 
