SOME GLACIAL WASH-PLAINS. 85 



Sandblasting and gli/ptolithsj — The pel)l)le8 on the sur- 

 face of the wash-plains frequently exhibit the touch of the 

 natural sancll)Iast. Sharply carved glyptoliths have been 

 noted in many localities. The widespread occurrence of 

 these pebbles beneath the soil in New England, in areas 

 where the wind is not now blowing sand, makes it highly 

 probable that immediately after the ice retreated and bo- 

 fore vegetation came in, the barren sandy stretches were 

 for a time in a desert condition. 



Superposition of plains bf/ raised water level. — Phiins 

 may exhibit the phenomenon of superposition in which the 

 outward margin in the case of partial overlap assumes the 

 form of grouped terraces, the lobate margin of the first 

 formed plain extending i)eyond the lobate margin of the 

 overplaced plain. This phenomenon is due to a rise of 

 the water level above the surface of the first plain so that 

 construction begins anew at the ice contact. It is shown 

 in the superposition of a small plain on those which encir- 

 cle Greenwich Cove in Rhode Island. 



It is obvious that the overplacement of plains may con- 

 ceal the initial deposit and result in the formation of a 

 broader plain enveloping a smaller one. The existence of 

 such a buried plain could only be determined on seeing the 

 cross-section wherein the top-set beds of the older would 

 underlie the fore-set beds of the newer plain. 



Boulders generally absent from ivash-plains. — In the 

 town of Rehoboth, Mass., is a broad morainal tract with 

 knob and basin topography, thickly strewn with large boul- 

 ders of the Carboniferous conglomerates. Nearly in the 

 middle of this tract is a small wash-plain with a typical 

 ice-contact on its northern margin. The plain is free from 

 boulders. The ice-contact at the head of the plain shows 



' See Facetted pebbles on Cape Cod, by Prof. W. M. Davis, in Proc. Boston 

 Soc. Nat. Hist, xxvi, 1893, pp. 16o-175; also Post-glacial eolian action in southern 

 New England, by J. B. Woodworth, Am. Journal Sci. for January, 1894. 



