Summary. 
507 
apparently less than half the erosion that marks the oldest drift. 
The southwestward train also stands apparently in closer rela¬ 
tion to the latest drift than to the earlier, and still the interval 
between these as measured by erosion on bowlder and drift 
surfaces is evidently more than 2 to 1. 
SUMMARY. 
The ledge areas under consideration lie in the basin of the 
Crawfish, a small western tributary of Rock river, in southeast¬ 
ern Wisconsin. They occur within the horizon of the lower 
Silurian, adjacent areas showing beds of Cambrian rock. 
The crystalline strata are sharply flexed and folded, resulting 
in extreme alteration in the structure of the rock through dyn¬ 
amic metamorphism. Four different types of quartzite are 
distinguished, the variations resulting from differences in the 
original composition of the beds and varying degrees of meta¬ 
morphism. These differences, though not great, are sufficient 
to distinguish material belonging to the different ledge areas in 
all parts of the bowlder fan. The ledge areas are themselves 
divided into four groups corresponding in the main to these 
four types of rock. 
The quartzite drift distributed from these ledge areas appears 
in separate bowlder trains, covering in part distinct areas, and 
indicating successive drift movements separated by considerable 
time intervals and crossing the area in widely divergent direc¬ 
tions. The earliest of these is a bowlder train which extends 
from the ledge areas westward across the north half of Dane 
county to the drift margin near the Wisconsin. The observed 
amount of quartzite material in this train though small is suffi¬ 
cient to determine the proximate limits of its bowlder fan and 
the area of its distribution covers about 16 townships or 600 
square miles. 
The bowlder train of the next succeeding glacial dispersal 
extends southward from the ledge areas down the Rock river 
valley, and overlapping in part the tributary basins of the 
Pecatonica and Sugar rivers on the west. This is the most 
extended train of the series, its peripheral limits being eighty 
