14 The American Geologist. January, 1895 
which preceded it. That the Valley loess of this region was 
not deposited like the ordinary tlood-plains of great rivers, is 
evident from the way in which the strata dip down the sides 
of the valleys. It was formed by the sedimentation on the 
bottom of a long and narrow, branching lake, of current-car- 
ried glacial silt. But the lower portion of the Pecatonica 
valley received a very heavy deposit which, on completion, 
had a nearly flat surface. This has since been extensively 
eroded by the stream, and a system of terraces has thus been 
formed. 
Beyond the Pecatonica basin, along the bluffs of the lower 
portion of the Rock river, and at various places along the 
Mississippi river between Dunleith and Rock Island, quite 
extensive deposits of a valley loess, apparently synchronous 
and similar in mode of formation with that of the Pecatonica 
valley, have been observed. Here, also, this loess has been 
carved into a rough terrace system. The immediate valley 
of the Mississippi was largely filled up with a thick deposit of 
loess which may have formed a nearly level plain 50, 100 or 
150 feet above the present river level. It is more probable, 
however, that, as in the Pecatonica valley, the surface of this 
valley loess, when its deposition was completed, was a shallow 
trough, the deposits along the bluff's being considerably higher 
than along the center. The Mississippi loess consists basally 
of an irregularly stratified bed of brown sand, gradually 
growing finer upward and passing through a typical loess into 
the weathered clay of the upland loess, which the older geolo- 
gists scarcely recognized as loess. 
Upland Loess. 
This is a bed of light brown massi-ve cla}^ of glacial origin, 
which generally conformably overlies the Valley loess. The 
line between them is sometimes sharp and distinct, but often 
the one passes into the other by insensible gradations. In the 
city of Freeport there is a sudden change from the blue sandy 
clay and quicksand of the upper portion of the Vallej^ loess to 
the light brown, stiff, unweathered portion of the Upland 
loess. This is commonly known as "hard pan," although this 
name belongs more properly to another formation. It con- 
tains numerous concretions of iron oxide or limonite in the 
form of balls and pipes, but has no calcareous nodules. The 
