STUDIES FOR STUDENTS. 519 



on the glacier, or more especially in tunnels within or under the 

 glacier, they were constrained by the presence of the ice walls 

 and forced into modes of action that they would not have 

 assumed of themselves on free and open surfaces of the land. 

 The formations of free glacial waters after their issuance from 

 the ice are thus distinguishable from the confined glacial streams 

 of the ice-body itself. The products of glacial waters after their 

 issuance differ from the products of ordinary surface waters in 

 the fact that they were overloaded with detritus in an extraor- 

 dinary and peculiar way, and in the fact that this detritus differs 

 from ordinary land waste. This difference furnishes one of the 

 most valuable criteria in the discrimination of Pleistocene forma- 

 tions. 



IV. Formations produced by floating ice derived from Pleistocene 

 glaciers. — Assuming that the greater mass of the glacial deposits 

 of Pleistocene age were formed by the primary agency of 

 glaciers, it nevertheless remains important to distinguish the 

 secondary products that were formed through the agency of 

 floating ice derived from these glaciers. Just as the waters from 

 the melting glaciers bore away and deposited a certain constitu- 

 ent of the material that had been wrought at first hand by the 

 glaciers, so icebergs bore off and deposited in a fashion of their 

 own another portion. The two classes are secondary and 

 dependent upon the original glacial action, and some of the 

 characteristics of the material must be sought in the original 

 glacial action, but in their final stages the deposits take on 

 character of their own. 



V. Formations produced by shore ice and ice-floes due to low 

 Pleistocene temperature but independent of glacial action.- — The pres- 

 ence of the great ice sheets and the glacial conditions that pro- 

 duced them appear to have given rise to a class of independent 

 ice deposits that may fittingly be put in this classification. These 

 are believed by some writers to attain great importance. What- 

 ever their extent, the)- need to be scrupulously discriminated 

 from the products of both glaciers and glacier -derived icebergs. 



