214 IOWA ACADEMY OF SCIENCES. 
On account of the recent breaking away of rock, this level floor 
extends out to the perpendicular face of the escarpment. The 
floor is crowded with striations and grooves, all finely preserved. 
Four different sets of scratches may easily be determined, each 
consisting of perfectly straight, parallel grooves. Some of 
these grooves in the latest series are fully an inch deep, while 
those from the earlier are nearly obliterated. The trends, given 
in the order of apparent age, are as follows: 
1. South 30° 15' east. A-B, Plate xxi. 
2. South 64° east. (7-P, Plate xxi. 
3. South 60° 30' east. E-F, Plate xxi. 
4. South 72° 15' east. G-H, Plate xxi. 
These angles were taken with surveyor’s compass and cor- 
rected 7° for deviation. 
The oldest series is nearly obliterated by the planing of the 
later ice movements but its traces are very numerous, and it 
undoubtedly records a long unwavering movement of the ice 
stream in one direction. The second set shows the most prom- 
inently on account of the greater number of deep grooves, on 
which the later cross movements have made but little impres- 
sion. The scratches of the third and fourth sets are not very 
numerous, but some of them are quite conspicuous. By a study 
of this glaciated surface, something may be learned concerning 
the relative time and continuance of the different ice movements 
in this locality. The rock floor is the hard, compact limestone 
of the Upper Burlington, which, since the passing of the ice 
sheet, has been covered by typical undisturbed drift. The 
markings of the two later movements are of like prominence; 
and which is the earlier is only determined by a study of the 
intersection of two of the grooves. This would indicate either, 
that the grooves formed very rapidly, or else that the planing 
of the rock floor was exceedingly slow. The grooves of the 
two later are not of much greater depth than those of the second; 
but the number of grooves in the second set is more than double 
those of the third and fourth combined. The oldest set of all 
was almost planed away by the ice stream of which the second 
set is the record. This clearly indicates that time was an 
important factor in deciding the number of grooves. In all 
probability the actual periods represented by the third and 
fourth sets were of considerable duration; but each was com- 
paratively short in comparison with the second. It is easy to 
see how these grooves could be cut to a considerable depth 
