THE METAMORPHISM OF GLACIAL DEPOSITS A75 
glacial deposits, we have here-a great variety in texture. The till 
of some exposures is very fine, and quite free of even small bowlders; 
other exposures contain many, and large, erratics. More uniformity 
in texture, however, is found in the water-laid drift belonging to this 
study; usually, it is fine, even silty. 
All these deposits apparently show the effects of great and long- 
continued pressure. They are dense in structure. This com- 
pactness is manifest in the angle at which the cliff-faces stand, not 
infrequently overhanging; also in the tendency of bowlders, show- 
ing on the surface of the cliffs, to hang even after more than half 
their mass has been exposed. In some cases I was able to satisfy 
myself, by tracing this hard horizon back from the cliff, that it con- 
stituted the proverbial “hard pan” of well-drillers. Furthermore, 
I have seen several dug wells being made, in which case there could 
be no doubt about the identity of this compact horizon and the bluish 
till. 
3. Obvious physical alteration.—In several cliff-exposures the con- 
tact between this hard deposit and the superjacent drift is a series 
of sags and swells representing either an irregular deposition of the 
subjacent material or its unequal erosion later (Fig. 1). But the 
relation of the inequalities precludes subaérial erosion; the irregular 
surface is either genetic, or it was produced by the erosion of over- 
riding ice. 
Contortion and folding is observed particularly in the water-laid 
deposits (Fig. 2). This alteration has been studied in material vary- 
ing from silt to rather coarse sand. I have examined many exposures, 
both modified and unmodified, which show jointing and faulting 
(Figs. 3, 4, 5). In no case was I able to show conclusively a dis- 
placement of more than three inches, and this maximum displace- 
ment was always in the water-laid drift. It is quite impossible to 
measure movement along a fault-plane involving only till. On the 
theory that every joint is a fault,’ we may assume a displacement 
even though it cannot be measured. In all exposures of till thus 
altered, the joints are nearly vertical, and in systems (Fig. 6). In 
the water-laid deposits this characterization is less clear. It should 
be stated, furthermore, that along most of the joint-planes or fault- 
1G. F. Becker, Bulletin of the Geological Society of America, Vol. IV (1893), p. 72. 
