GLACIAL SEDIMENTS, 291 



but little when we study them north and sotith of these moraines. Local 

 advances and retreats of the ice might be expected during the decay of 

 the ice-sheet, but they are to be regarded as minor incidents of one 

 Glacial period rather than distinct periods worthy of a place in geological 

 chronology. The moraines that correspond to the outer terminal moraines 

 of the second ice-sheet of the Northwest are to be sought for in the 

 Gulf of Maine, not along the present coast. The so-called interglacial 

 pei'ibd of the Northwest was longer than the intervals between the retreats 

 and readvauces of the ice in Maine, so far as the known facts Avarrant 

 conclusions. 



The significance and explanation of the fact, if such it be, that there 

 was but one glaciation are left for future investigation. 



GLACIAL SEDIMENTS. 



Under the term "till," as here used, is included all matter transported 

 to its present position by glacier ice. The term "glacial sediments" denotes 

 all matter transported to its present position by streams of water from the 

 melting ice. No doubt ice movements contributed to the transportation of 

 kame matter. Yet clearly we have in case of the glacial sediments a form 

 of transportation that the till did not undergo. While the till is glacier 

 drift simply, the former are glacial drift plus water drift. 



RELATION OF WATER TO THE GLACIER. 



Energy reaches the glacier in various forms. Radiant energy comes 

 to it from the sun and other bodies. Part is reflected, part is radiated and 

 lost, and part is absorbed, which is but another way of saying that it is 

 transmuted into molecular motion and is expended in doing work within 

 the ice, such as melting it, raising its temperature, or aiding its flow. 

 Molecular heat is communicated to the ice from surrounding bodies and 

 performs the same kinds of work as radiant . energy . Most of the radiant 

 heat comes from the sun, most of the molecular heat from the air and the 

 summer rains or from the earth beneath the ice. The chief sources of heat 

 act from above, and there the most of the melting and other direct actioi'i 

 of heat takes place. The glacier is one form of heat engine. From the 

 time that heat aids in cementing the separate snow crystals into clear blue 

 ice up to the time that it resolves the ice back again into granules and 



