66 
IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
posure in Macoupin county Udden had sharply veered to the 
eastward towards Greenville and Vandalia he might have found 
abundant outcrops extending that far beyond his last. On the 
other hand had he traveled southwestward he might have caught 
up the outcrop again running from a point east of Alton south- 
eastward to the south line of Madison county where he actually 
did pick up the thread again. This belt of country extending 
from Alton eastward well merits close inspection anew. 
The period at which this great rent in the earth’s crust took 
place is a matter of some moment. That it was subsequent to 
the close of the Paleozoic era is clearly indicated by the fact 
that the Coal Measures are fully involved and abut Cambric 
sandstones. That it probably was closely associated with the up- 
lifting of the Ozark dome is shown by the circumstance that it 
is parallel to and near the margin of the broad arch ; in fact 
it seems as if the movement were a part of the same orographic 
disturbance which involved the Ozarks but that the great arch 
was not able fully to sustain itself on the north and dropped. 
The Early Tertiaric peneplanation of the region effects both the 
Ozark surface and the region covering the fault area. The 
faulting thus doubtless took place about the beginning of Ter- 
tiaric time. 
fi 
Des Moines. 
