An Esker Group South of Dayton, Ohio 31 
with foreign rock. Though mixed with sand it is practically free 
from clayey material. Cobbles of flat angular limestone are 
abundant on the surface. At times these cobbles intermingled 
with foreign bowlders of similar size literally pave the surface, 
a result possibly of concentration through the removal of fine 
material by washing. Big granite bowlders are rare. 
Rock weathering. The surface bowlders show varying degrees 
of weathering. The limestone, not being very resistant to water 
action, particularly shows age. Granites sometimes appear fresh, 
at other times are decidedly pitted. If this irregularity is not due 
to their chemical composition, the inference would be that bowl- 
ders representing several different glacial periods have been 
mingled. In a stream-cut south of the eskers many greenstones 
appear. These are described by Chamberlin and Salisbury^^ as 
particularly abundant in sub-Aftonian drift. It may be that in 
these conditions evidence may be found that this area represents 
pre-Wisconsin glaciation and later reworking during the Wisconsin 
period. Such a theory would not necessarily oppose anything 
that has already been conjectured with regard to the history of 
the region. 
Crest-lines. While the crest-lines are sometimes quite hum- 
mocky, the typical esker form is found in all its beauty. Straight, 
even-sloped sections several rods in length may be found, but the 
course usually is serpentine, the crest-line waving up and down 
and from one side to the other of a straight line. Several gaps 
occur, some perhaps artificial; others may be due to constrictions 
in the ice tunnel or various local modifying conditions. Though 
the general course of these eskers is straighter than in the usual 
type, this offers nothing inconsistent with the sub-glacial theory 
of origin; in fact it seems reasonable to suppose that confined 
streams of sufficient size to build up immense ridges of coarse 
material would naturally hold to a comparatively straight course. 
Economic Importance. 
These ridges have great economic value. The supply of gravel 
and sand is practically inexhaustible. The C. C. C. & St. L. 
Loc. cit., p. 384. 
