Coal Measures and Limestone in Iowa. — Keyes. 
101 
recession continued more rapidly as the Lower Carboniferous 
period was ushered in, until the water-line reached nearly to 
the present southern boundary of Iowa. The St. Louis epoch 
MISSOURI 
IOWA 
M 
COAL MEASURES 
-«^. 
KASKASKIA 
S7 LOUIS 
^- — — — """" 
AU GUSTA 
s mm ® — > N 
Kl NDERH00K 
Fig. 1. 
represented a period during which there was a general depres- 
sion of the land, allowing an overlap of the St. Louis rocks 
of more than 200 miles. 
At the beginning of the Kaskaskia epoch or the last stage 
of the Lower Carboniferous, another cycle of the great con- 
tinental change set in, pushing the shore-line rapidly some 500 
miles southward to the vicinity of the present city of St. 
Louis. Shore deposits were laid down along the line of the 
Mississippi river in southeastern Missouri and western Illinois, 
with open sea depositions farther southward, and probably 
also west of the Missouri river. While the Kaskaskia beds 
were being deposited south of the mouth of the Missouri 
river agencies of degradation were actively at work over 
all Iowa and northern Missouri. The St. Louis limestone was 
deeply eroded, as is plainly shown in numerous places. The 
depressions, channels and gorges were soon filled with 
clays and sands while here and there thick beds of Carbona- 
ceous matter were buried. 
For the most part the Coal Measures represent a period of 
general though not uninterrupted subsidence. During the lat- 
ter part of the period the waters receded rapidly far beyond 
the boundaries' of both Missouri and Iowa. A long time in- 
tervened before the seas again occupied the Iowa territory. 
This incursion was recorded in the Cretaceous deposits of the 
northwestern part of the state. 
This is in brief, then, a statement of the shore-line changes 
during the Carboniferous in the upper Mississippi valley. 
