ICE WORK IN SOUTHEASTERN MICHIGAN 203 
defined angle and generally retain the earlier set of strie. The 
most interesting and conclusive of all these evidences of the 
direction of ice movement is that of the so-called ‘‘knobs and 
trails.” " These are now to be seen on the northern and north- 
eastern sides of the quarry, and five years ago also upon the 
southern side. Certain layers of the limestone contain silicified 
Fic. 3.—View of the late Wisconsin glaciation, Corniferous limestone, Sibley 
quarry, showing parallel and intersecting striz and gouges. Looking northwest in 
the direction of ice movement. At the right of the small satchel is a gouge fourteen 
feet long, N. 20° W., which begins asa pin-scratch but gradually broadens north- 
westward, expanding distally to four inches and terminating abruptly. 
fossils and irregular nodules of chert. On account of their super- 
ior hardness these were worn away less rapidly by the ice than 
the limestone in which they are embedded and consequently 
they project above the surface. On the northwest side of many 
of these knobs there are found “trails” of limestone which 
diminish in height gradually to the general surface of the lime- 
GILBERT, Geol. Surv. of Ohio, 1873, Vol. I, p. 539. 
