SOME GLACIAL WASH-PLAINS. 105 



elongated in the direction of the cross-lines, northeast and 

 southwest. Between the lines of ponds are broad, high 

 plains, mainly sloping south westw^ard. The deposit be- 

 tween the Great South chain of ponds and the Crooked 

 Pond series is the most pronounced. The ponds mark ice 

 blocks. The plains mark valleys in the ice filled with 

 detritus. The JNIonument River depression partakes of the 

 character of the northeast and southwest lines of ponds, 

 but has been scoured out by running water. The full 

 interpretation of this interlobate mc^rainal area promises tc 

 throw much light on the toiniation of plains about ponds. 

 The Middlebo rough sheet presents many sporadic plains 

 with lakes and swamps. 



THE WATER-LEVEL OF WASH-PLAINS. 



A stream of water flowing in a trench and scouring its 

 bottom will begin to deposit its load on encountering a 

 deep hole. A sub-aqueous delta with a lobate front and 

 flat top will form in such a place. This deposit will build 

 up to a level at which the velocity of the current for the 

 depth of water is at bottom sufficient to drag to the lobate 

 margin the particles which the stream brings to the place. 

 These particles are hurried along and dropped in the talus 

 at the end. With constant velocity and load, the delta 

 builds uniformly forward. The height of the plain in this 

 case is not directly determined by water-level, but it is 

 indirectly related to it in so far as water-level depends 

 upon the cross-section of the stream, depth of water, 

 velocity, and width of channel. It has not been shown as 

 yet that any wash-plains in this region have developed 

 under conditions similar to those above indicated. 



Streams heavily laden with detritus and pouring out 

 from declivities on to low grounds above baselevel build 

 cones with slopes at angles dependent on relation of load 



