204 W. H. SHERZER 
stone but maintain the breadth of the protective knob. This 
phenomenon is shown quite clearly in Higa dy Asse cule nat. 
row and relatively deep furrows were plowed out in front of and 
at the sides of the knobs, the lateral furrows being prolonged a 
considerable distance backward before they disappear. Of the 
eo 
ae 
fyecs N 
Fic. 4.—‘‘ Knob and trail” phenomena produced by the late Wisconsin ice-sheet, 
testifying positively to the direction of northwest movement. Northern part of Sibley 
quarry. The main central gouge shows “chatter-marks” indistinctly in its lower 
half, while in the upper part of the view there are evidences of “plucking.” A shal- 
low, tortuous groove crosses the left hand portion of the slab, cut subsequently to 
the glaciation by a small stream of water flowing between the rock and the till cover- 
ing. 
various properties imputed to ice for the purpose of explaining 
the movement of glaciers, plasticity under pressure can alone satis- 
factorily explain this phenomenon of “ knobs and trails,’’ with 
their frontal and lateral grooves. 
