STUDIES FOR STUDENTS. 533 



in respect to the method of formation of the osars and in respect 

 to the position and restraining functions of the ice sheet at the 

 time of their formation. 



(3), Overwash aprons. — When terminal moraines grew to 

 constitute notable barriers along the border of the mer de glace, 

 and when the ice pressed against these moraines so as to obstruct 

 the transverse flow of the glacial drainage along their inner bor- 

 der, the waters derived from the ice crept over the moraines in 

 numerous small streams, which deposited gravel, sand, and silt 

 on the outer flank of the moraine. These deposits are often 

 distributed along the moraines for great distances and constitute 

 a fringe of assorted material to which Shaler has given the apt 

 name "apron." These constitute one of the most satisfactory 

 demonstrations of the marginal character of the moraines and of 

 the relations of the ice to them. The material varies widely in 

 coarseness according to the conditions of formation, and a struc- 

 tural sub-classification may be based upon it embracing [a) 

 gravel — (<£) sand — (V) silt-aprons. Immediately next to the 

 moraine the material is sometimes exceedingly coarse, constitut- 

 ing little less than a boulder belt. At the other extreme of the 

 series the silt sometimes forms a clay deposit and sometimes it 

 takes on that peculiar assortment which constitutes loess. 



The class of aprons here described are dependencies of 

 definite terminal moraines. There were, however, tracts of 

 assorted material formed by waters outflowing from the ice 

 where no definite terminal ridging took place. Such forms may 

 be designated outwash aprons in distinction from overwash 

 aprons. This class is usually made up of sand or silt. In the 

 latter case there are gradations into the great flanking tracts of 

 loess which appear to have arisen in the manner indicated on 

 very low slopes with prevailing slack drainage. 



(4). Pitted plains {in part). — Both the osar deltas and the 

 overwash aprons are characterized in certain regions by a sur- 

 face marked with numerous depressions, sometimes symmetrical 

 (kettles), sometimes irregular, with undulatory bottoms and 

 embracing knobs and sub-basins, giving the surface an expres- 



