6oo PRINCIPLES OF STRATIGRAPHY 



4. Glacial sand plains. Whenever the ice holds up a tempo- 

 rary body of water between its front or sides and the margins of 

 the valley or basin in which it lies, opportunity for the formation of 

 glacial deltas or sand plains is given. These may be formed around 

 the margins of the lake by streams coming from the land, when 

 they partake of the true delta to be described beyond. Such deltas 

 have been recorded from glacial lakes in New Jersey and else- 

 where. The marginal sand plains of the Upper Hudson Valley 

 were deposited in lakes bordering the ice sheet which occupied 

 the center of that valley. Sand plains formed at the front of the ice 

 of material carried by streams from the ice abound in New Eng- 

 land, especially in eastern Massachusetts, where they have been 

 fully described.* A characteristic feature of these sand plains is 

 their correspondence in height to the elevation of the outlet, which 

 determined the level of the temporary lake. Not infrequently a 

 number of successive sand plains is formed corresponding to a suc- 

 cession of lake levels determined by progressive uncovering of lower 

 outlets. In New England these plains are generally highest in the 

 southern portions of the old northward sloping valleys and become 

 progressively lower toward the north. In Lake Bouve, an extinct 

 glacial lake in the Boston Basin, eight series of such sand plains are 

 known, each series lower than the preceding one on the south. 

 ( Grabau-23 :5(?o.) A special example of rather striking character 

 is seen in the sand plains of Wellfleet, Eastham, and Truro on Cape 

 Cod, which were deposited in a lake held in an embayment in the 

 eastern ice lobe, and dammed in part by the terminal moraine of 

 that ice lobe, across which the discharge of the waters occurred. 

 In this case the highest series of plains lies between the two lower 

 series. By the melting of the ice these plains were left without any 

 immediately surrounding land barrier, as isolated sand masses run- 

 ning northward into the ocean from the eastern end of the terminal 

 moraine. (Grabau-24; Wilson-60 rj^-dd, pis. 34, 36, j/, 38.) In 

 form the isolated sand plain, revealed on the melting of the ice 

 and the drainage of the temporary lake, consists of a surface slope 

 which is gently forward from the center of origin ; a steeper frontal 

 or delta slope, joining the surface slope abruptly, and having a 

 lobate or scalloped outline, and a still steeper back slope often with 

 concavities between sharp cusps. This latter surface is due to 

 slumping after the supporting ice front against which the delta was 

 built had melted away. Its slope, therefore, represents the angle 

 of repose of the material under the conditions of formation. The 

 gentle surface slope represents the subaerial part of the delta, while 

 * See papers by Crosby, Grabau, Clapp, etc., cited in Chapter III. 



