St. Peter Sandstone in y. W. Illinois. — Hershey. 175 
bedding, dipping, however, more decidedly in one given direc- 
tion. Resting upon the upturned edges of these layers, there 
is generally one thick horizontal layer of sandstone near the 
top of the formation. Evidently the subsidence was in 
progress which resulted in the deposition of the Buff lime- 
stone. As already intimated, the thickness of this formation 
in this area is only from 75 feet to somewhat over 100 feet r 
while in all directions from it in northwestern Illinois the 
thickness is not less than 200 feet. It attains an elevation 
above sea level of nearly 900 feet, a little less than in the Rock 
riveroutcrop. 
The Buff limestone. This underlies a narrow strip skirting 
the sandstone on its southern and western side. It is a mod- 
erately thick-bedded, buff or light brown dolomite, argilla- 
ceous, and nearly free from fossils. Its thickness is about 20 
feet, thinning out to the northeast. It passes, by insensible 
gradations upward into the Blue limestone and downward into 
the St. Peter sandstone. Hence its lower portion is very sandy 
and sometimes shaly. Its designation as Buff limestone is 
deceptive, for under cover it is just as frequently blue as the 
Trenton above it. Perhaps the term Pecatonica limestone 
would be more appropriate, as it is best exposed in the Peca- 
tonica valley, near the Wisconsin line, and northward. 
Trenton or Hive limestone. This surrounds the sandstone 
outcrop and is exposed in four quarries and a number of 
smaller excavations. When drilled through, it is of a deep 
blue color; but generally on outcrop it is a thin-bedded, light 
blue-gray or light brown, very fossiliferous limestone and dol- 
omite, with a few very thin layers of shale. In the vicinity 
of the small fault marked on the northern portion of the map, 
it is characterized by the vast amount of fossils with which 
it is thickly packed. 
What makes this locality doubly interesting is that, besides 
the small fault above mentioned, it shows an unconformity or 
hiatus between the St. Peter sandstone and the Trenton lime 
stone. The very fossiliferous thin-bedded Trenton limestone 
rests direct]} 7 on the sandstone, the Bull' limestone, winch sep- 
arates them around the south and west boundary of the area, 
being here absent. The upper stratum of the sandstone is a 
loose, unstratined mass, about two feel thick, which is com- 
