A Notable Ride. — Calvin. 375 
less area where the topographic forms are the resultant of ero- 
sion acting on indurated rocks of varying degrees of hardness, 
to an area of comparatively inconspicuous topographic forms 
developed by erosion of a body of drift Avhich was originally 
left by the retreating ice fields, with a surface approximating a 
plane. It was not a true plane, however, but a drift plain, di- 
versified by numerous undulations which were due to two facts 
— the ice deposited more material in some places than in 
others, and the mantle of glacial detritus was not in all cases 
sufficient completely to disguise the high ridges and the deep 
valleys of the pieglacial topography. The thickening of the 
Kansan drift along its ultimate border has given rise to a well 
marked ridge which curves so as to trend northeast and south- 
east from Peosta. The drift plain is inclined very gently to 
the west, and all the drainage streams, for some miles back 
from the drift border, take a westerly or southwesterly course. 
A new type of topography now engages the attention of the 
observer. The landscape stretches away in an unbroken plain 
to the far horizon; but the surface of the plain, as in all the 
Kansan-drift areas of Iowa, is carved into a dendritic system of 
miniature hills and valleys. Compared with the driftless area 
which was traversed only a mile or two east of Peosta, the visi- 
ble effects of erosion are very small. Except along the perma- 
nent streams the rain sculpture has affected only the drift; it 
has not exposed any of the indurated rocks. Making allow- 
ance for the original undulations of the plain, it may be said 
that the symmetrically rounded swells of the surface, in strik- 
ing contrast with the tumultuous and disorderly arrangement 
seen in the area of eroded Maquoketa shales, all rise approxi- 
mately to the same level. The Kansan drift in this region is 
everyw'here overlain by loess. Mature erosional topography, 
on a small scale, was developed on the Kansan surface before 
the loess was laid down upon it. Subsequent erosion of the 
friable loess has modified the curves of the original prohle lines 
to some extent, and so the surface features which present 
themselves between Peosta and Epworth, the next station west, 
have been classified under the name of loess-Kansan topogra- 
phy. The region affords as typical examples of this kind ot 
suiface carving as can be found scores, or even hundreds, of 
miles kack from the terminal margin of the Kansan ice. 
