104 CARBONIFEROUS LIMESTONES 
A short distance beyond this section, the underlying limestone rises again towards 
the northwest to the height of twelve or fifteen feet, for a distance of four miles, 
when, at a sudden bend in the river, in latitude 42° 31’ 44”, before it enters and 
meanders through the open prairie, the limestone pitches beneath the water level, to 
the northwest, and is overlapped by a mass of dark, bluish-gray, argillaceous shale. 
Here fragments of coal occur in the debris, though no regular bed is seen; the 
slide of the argillaceous layers and vegetation may conceal it from view. A quarter 
of a mile above, the limestone is again in place, skirting the bed of the river, and 
continues in low walls of five to ten feet, either on one side or the other, for four or 
five miles. The current of the stream becomes sluggish as it traverses low, flat 
land, the banks being usually only four to five feet above the river, and, even when 
it skirts the higher ground, these are not above five feet in elevation. 
After leaving the last limestone exposure, we continued to ascend the Iowa for 
fifteen or twenty miles, when we finally arrived at a barren region of drift knolls 
stretching away as far as the eye can reach, destitute of timber, except a narrow 
belt skirting the Iowa River. The hollows between the drift knolls are wet and 
marshy, and the summit of the hills strewn with gravel and boulders, intermixed 
with a thin, poor soil, that only supports a scant growth of stunted herbage. 
' KNOBBY DRIFT REGION OF NORTHERN LOWA. 
The corps whose duty it was to explore, in 1848, the southern and western 
tributaries of the St. Peter’s River, observed towards the heads of the Mankato and 
Lesueur Rivers, a country of the same character which we encountered high up on 
the Iowa. It was hence inferred that these barren drift knolls extend beyond the 
northern boundary of Iowa, covering the whole water-shed that gives rise to these 
streams, as well as to the Iowa and Red Cedar Rivers. Seeing, therefore, no object 
to be obtained by a farther advance up the Iowa, and finding that our stores of 
provisions were barely sufficient to carry us back to the settlements, I determined 
