POST-KANSAN EROSION. 1 
M. M. LEIGHTON. 
Visitors to the Maquoketa river gorge below Monticello in 
Jones county have been impressed with its rocky walls of the 
Niagaran formation which rise in places 100 to 125 feet above 
the stream. Overlying this rock formation are drift and loess. 
Crags, turrets, and chimney rocks, similar to the rugged fea- 
tures of the valleys of the driftless area, appear here and there. 
In fact, the characteristics of this gorge are so nearly like those 
of the valleys of the driftless area that in the report of the 
Geology of Jones County the gorge is considered to be pre- 
glacial in age. 
During the investigations of the Iowan Drift, Dr. Win. C. 
Alden and the writer found that this valley Is Pleistocene and 
'probably post-Kansan in age rather than preglacial. Well 
records reveal that there is a deep preglacial or pre-Kansan 
valley underlying what is now the Langworthy ridge to the west 
and that this buried valley reaches depths considerably below 
those of the Maquoketa. gorge. These well records also revealed 
the fact that the present Maquoketa gorge has been cut through 
what was formerly a high rock divide, as shown in figure 1. 
It is thus clear that the gorge is Pleistocene in age. Is it 
post-Nebraskan or post-Kansan? Both the Nebraskan and Kan- 
san drifts are known to extend to the southeast of this 
locality, as shown by their superposition in the Chicago, Mil- 
waukee & St. Paul Railway cuts near Delmar Junction. In view 
of this and in view of the absence of any evidence that the gorge 
has been glaciated, it seems that the gorge is probably post- 
Kansan in age. In the bottom of the valley are the valley- 
train terraces of Iowan age, which indicate that the Maquoketa 
river had .completed cutting this valley by Iowan times. At 
the close of the Kansan epoch this region was apparently a flat- 
tish plain, a new surface made by the heavy deposition of Kan- 
san drift on the pre-Kansan topography, with the consequent 
filling of the former valleys and burying of the divides. On 
ir This and the following papers are published w r ith the permission of the 
Director of the Iowa Geological Survey. 
