1 T-t The American Geologist. September, 1894 
ing this the highest point, so far as known, reached by this 
formation within the state of Illinois. 
St. Peter sandstone. This formation, as the map shows, 
underlies about ten square miles in the valley of the Elk Horn 
creek and its tributaries. Beginning in the valley of the 
main stream a short distance southwest of Foreston, it fol- 
lows the creek to Brookville on the Carroll county line. North 
from Brookville it occupies the valley of a smaller creek for 
several miles, and at two or three points passes into Carroll 
county. From the main body of the outcrop near the Free- 
port and Sterling road, one broad arm runs west three and a 
half miles reaching a half mile into Carroll county. The exact 
boundary of the area is very sinuous, as it is dependent on 
the ravines and valle} T s and on small faults which have been 
detected in various parts of the tract although obscured by 
the overlying loess. 
The St. Peter sandstone has been too often described to re- 
quire a minute description here. There are, however, some 
features which maybe peculiar to this region. The iron con- 
cretions, "like broken iron pots," found in the Wisconsin and 
Rock river outcrops, are absent. The entire mass has been 
stained with oxide of iron by percolating waters and is 
brightly colored, red and yellow predominating. The lowest 
exposed strata are heavy-bedded and tolerably solid; but the 
great mass of the formation is laminated and loose. There is 
a six foot stratum of light green shale near the top, which is 
persistent throughout the area and is overlain by one or more 
heavy-bedded, partially lithified strata of white or light gray 
sandstone, constituting the top of the formation. About the 
center of the northwest arm of the outcrop there is an expos- 
ure in a bluff of •)."> feet thickness of sandstone. This locality 
is interesting because of the beautiful false-bedding. The 
strata dip rapidly in all directions. One portion may dip 
smith as much as 30°, and the sandstone immediately above it 
may dip north, east or west at the same rate. This confused 
stratification could not have been produced except near the 
surface of the sea. and from the fact that the dip is in all di- 
rections, it is considered very probable that here was a very 
shallow spot, perhaps at times a low island or sand bank in 
the midst of the Silurian sea. Other places also show false 
