THE PLEISTOCENE OR GLACIAL PERIOD. 373 



ridden and generally destroyed or obscured by the further advance 

 of the ice. 



Gradational types, pitted plains, patches of gravel and sand. 1 — Out- 

 wash plains sometimes depart from planeness by taking on some meas- 

 ure of undulation of the sag and swell (kame) type, especially near 

 their iceward edges. The same is often true of the heads of valley 

 trains. The heads of valley trains and the inner edges of outwash 

 plains, it is to be noted, occupy the general position in which kames 

 are commonly formed, and the undulations which often affect these 

 parts of the trains and plains, respectively, are probably to be attrib- 

 uted to the influence of the ice itself. Valley trains and outwash 

 plains, therefore, at their upper ends and edges, respectively, may 

 take on some of the features of kames, and either may head in a kame 

 area. 2 



Occasionally a morainic plain, or stratified drift in the general 

 position of a morainic plain, is affected by numerous sags without 

 corresponding elevations. This topographic type has received the 

 name of pitted plain. The sags, in many cases at least, appear to be 

 intimately connected with the ice-edge, and so to be marginal phe- 

 nomena. 



At many points near the edge of the ice during its maximum stage 

 of advance, there probably issued small quantities of water not in 

 the form of well-defined streams, bearing small quantities of detritus. 

 These small quantities of water, with their correspondingly small 

 loads, did not develop considerable plains of stratified drift, but small 

 patches instead. Such patches have received no special designation. 



When the waters issuing from the edge of the ice were sluggish, 

 whether they were in valleys or not, the materials which they carried 

 and deposited were fine instead of coarse, giving rise to deposits of 

 silt or clay, instead of sand and gravel. 



In the deposition of stratified drift beyond the edge of the ice, 

 the latter was concerned only in so far as its activities helped to supply 

 the water with the necessary materials. 



(3) Beneath the ice. — Subglacial streams seem sometimes to have 

 deposited gravel and sand in their channels. When the waters were 



1 Geol. of Wis., 1873-1880; Davis, Bun. Geol. Soc. Am., 1890, Vol. I, p. 195; 

 Gulliver, Jour, of Geol., Vol. I, p. 803, and Glacial Geol. of N. J. 

 2 Ann. Rept. State Geol. of N. J., 1892, p. 94. 



