MctainorpJiisin of Glacial Deposits 3 



pactness is manifest in the angle at which the cHlT-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 constituted the proverbial "hard pan" of well-drillers. Further- 

 more, 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 

 contact 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. i). But 

 the relation of the inequalities precludes subaerial erosion ; the ir- 

 regular 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 

 varying form silt to rather coarse sand. I have examined many 

 exposures, both modified and unmodified, which show jointing and 

 faulting (fig's. 3, 4, 5). In no case was I able to show conclusively 

 a displacement of more than three inches, and this maximum dis- 

 placement was always in the water-laid drift. It is quite impossible 

 to measure movement along a fault-plane involving only till. On 

 the theor)^ that every joint is a fault, ^ we may assume a displace- 

 ment even though it caimot 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-planes there has taken place either a secondary alteration 

 or a deposition from percolating water (fig. 2). In some cases this 

 secondary deposit has weathered away more rapidly than the wall 

 material ; in others, less rapidly. 



4. WeatJicring. — Leaching in a relatively short time removes 

 carbonates, especially from surface deposits. Only at a consider- 

 able distance from the top do we often get evidence of carbonates in 

 the superjacent drift. This leaching by ground w^ater is the first 

 step in the cementation process always going on at lower horizons. 

 The bluish compressed drift invariably shows the presence of cal- 



7 G. F. Becker, Bulletin of the Geological Society of America, vol. T\' (1S93), p. 73. 



