196 WALTER HOWCHIN 
until the discovery of subangular erratics, faceted and ice-scratched, 
placed the question beyond doubt. These glaciated stones are not 
at all uncommon, and can be obtained from most of the outcrops 
of the till (see Figs. 2 and 3). 
The Lower Cambrian of South Australia has, in many places 
been subjected to considerable pressure and deformation. These 
diastrophic movements have affected the glacial beds in various 
ways. The mudstone has developed a rough kind of cleavage 
which causes the beds to weather into flaggy masses, at a high 
angle to the bedding plane, while the rock exfoliates in flakes 
Fic. 2.—Glaciated erratic (quartzite) from Cambrian till, north of Petersburg, 
S. Aus. {natural size. W. Howchin, photo. 
parallel to the cleavage. The lateral pressure has produced some 
interesting effects with respect to the included erratics. Those 
which possess unequal diameters have been caused to rotate in 
their beds until the longer axis of the stone has been brought into 
line with the planes of cleavage. This movement of the stone in 
its bed has produced a kind of laminar investment around the stone, 
imparting to the latter a skin of sericitic mica. Some of the stones 
show evidences of abrasion and carry pressure striae. This pres- 
sure striation can be easily distinguished from glacial striae. The 
former occurs as parallel lines, often raised, and covering most, 
if not all, the surface of the stone, and not unfrequently radial 
in direction; while the glacial striae are in the form of single 
