34 FIELD GEOLOGY. 



Drift Deposits. There are certain deposits of gravel 

 and clay to which the methods of mapping previously 

 described are scarcely applicable without additional sug- 

 gestions for the student's guidance. He will have made 

 himself acquainted theoretically with the phenomena of 

 the " Glacial Period," but the relics thereof which he will 

 come across in his field expeditions are sometimes very 

 puzzling in their nature and relations. The products of 

 the climatic and physical conditions that then prevailed 

 are, generally speaking, a series of clay beds, sometimes 

 contorted succeeded in places by gravels and sands, 

 as a rule false-bedded and very irregular in their distri- 

 bution and mode of occurrence ; these are in turn over- 

 lain by unstratified clay, enclosing fragments of many 

 kinds of older rocks, and known as " Boulder-clay." 



These glacial deposits may and do occur singly or 

 together, sometimes one may be absent, sometimes 

 another ; but the peculiarity of them all is, not being con- 

 fined to any definite level, having indeed neither true 

 dip nor horizontality. They spread indiscriminately 

 over an old denuded surface, high and low ground alike, 

 capping the hills and filling the valleys, so that over 

 large areas the underlying older formations are com- 

 pletely hidden and this perhaps by a sheet of material 

 which, compared with the older formations, is compara- 

 tively thin and unimportant. Still the " Drifts," as 

 they in common with more recent deposits are called, 

 may be classified, and no geological map can be con- 

 sidered complete from which they have been omitted. 



As these beds, except in rare instances, contain but 

 few, if any, fossils, the frequently scanty items of evi- 

 dence to be obtained from sections, and the inferences 



