STRATIFIED DRIFT 95 I 



more or less independent of topography, much as water may be 

 forced over elevations in closed tubes. It is not to be inferred, 

 however, that the subglacial streams were altogether independ- 

 ent of the sub-ice topography. The tunnels in which the 

 water ran probably had too many leaks to allow the water to be 

 forced up over great elevations. This, at least, must have been 

 the case where the ice was thin or affected by crevasses. Under 

 sucii circumstances the topography of the land surface must 

 have been the controlling element in determining the course of 

 the subglacial drainage. 



When the streams issued from beneath the ice the condi- 

 tions of flow were more or less radically changed, and from their 

 point of issue they followed the usual laws governing river flow> 

 If the streams entered static water as they issued from the ice,, 

 and this was true where the ice edge reached the sea or a lake,, 

 the static water modified the results which the flowing waters 

 would otherwise have produced. 



STAGES IN THE HISTORY OF AN ICE-SHEET. 



The history of an ice-sheet which no longer exists involves- 

 at least two distinct stages. These are (i) the period of 

 growth, and (2) the period of decadence. If the latter does 

 not begin as soon as the former is complete, an intervening 

 stage, representing the period of maximum ice extension, must 

 be recognized. In the case of the ice-sheets of the glacial 

 period, each of these stages was probably more or less com- 

 plex. The general period of growth of each ice-sheet is 

 believed to have been marked by temporary, but by more or 

 less extensive intervals of decadence, while during the general 

 period of decadence, it is probable that the ice was subject 

 to temporary, but to more or less extensive intervals of 

 recrudescence. For the sake of simplicity, the effects of these 

 oscillations of the edge of the ice will be neglected at the 

 outset, and the work of the water accompanying the two or 

 three principal stages of an ice-sheet's history will be studied 



