290 
The American Geologist. 
May, 1896 
the body of water in which they were deposited, causing the 
color to vary in different layers from a dark brown, through 
the various shades of red, to light yellow and gray. Four 
fine sections of this formation are known to me in the valley 
of Yellow creek between this locality and Bolton, 8 miles west, 
besides half a dozen less important exposures. From a study 
of them all, I learn that it is a deposit laid down in an extra¬ 
glacial lake which occupied the valleys of the Pecatonica 
river and Yellow creek and their tributaries, being due to the 
obstruction of the lower portion of the former valley by the 
advance of the Kansan ice-sheet across its mouth. 
Lake Pecatonica and its Stratified Clays. 
An examination of any map of Illinois will show that the 
Pecatonica river, unlike any other Illinois stream of equal 
size, pursues a general east-northeast course from the city of 
Freeport to its mouth near Rockton. This course is directly 
contrary to that which was taken by the advancing ice-sheet, 
and consequently as soon as its border had crossed the Rock 
river valley and occupied the lower portion of the Pecatonica 
basin, a ponding of the waters in the still open portion of the 
valley resulted. As soon as the water level had risen suffic¬ 
iently to permit its outflow through certain transverse valleys 
on the watershed south of the basin, this lake Pecatonica 
maintained a nearly permanent level which I should place 
somewhere between 60 and 75 feet above the present flood- 
plain at Freeport. The mud thrown into the lake from the 
ice-sheet was carried by its currents and everywhere deposited 
over its bottom, following its inclination “up hill and down 
hill.” Glacial pebbles are sparsely scattered through the 
deposit, and at the type section an 8-inch boulder of dolerite 
was found embedded near the base. This small boulder, in 
falling from the surface of the water, indented the laminated 
clay on which it fell, and afterward compelled the next layers 
to curve up over it. Now there is absolutely no way whatever 
in which this boulder and the scattered pebbles could have 
reached this locality except by means of floating ice. Remem¬ 
ber that the country had not yet been glaciated, as the Kansan 
ice-sheet was then just advancing toward its farthest limits. 
These pebbles were brought from the front of the ice-sheet by 
small bergs that broke off along the water front and were 
