294 



CAPE COD GEOLOGY 



chetting so as to leave a series of pits on the beach sand leading to the resting 

 place of the spent pebble. Hundreds of small streams of dry sand run out from 

 incoherent layers to add their contribution to the growing fan of talus that 

 rises between the beach and the top of the wasting cliff. When very high 

 winds are blowing inland, sand and dust are carried up the slope and on to the 

 surface back of the cliff. These seolian deposits at the tops of cliffs form mainly 

 in the prism of slack air motion that lies just back of the edge of the cliff, where 

 vortical movement is at times favorable to deposition. With the secular falling 

 away of the top of the cliff, the horizontally stratified dune sand loses its fore 

 part in the same process and becomes a distinct deposit, a few feet thick, at the 

 top of the cliff. Such a deposit is generally separated from the old glacial 

 surface by a layer of lag-gravel with or without sand-carved pebbles, by a layer 

 of soil, or, more rarely, by a thin bed of shell made by the aboriginal inhabitants. 



DUNES FORMED FROM BEACHES 



The dunes on the beaches differ from the deposits of wind-blown sand 

 because their component sand is sorted by waves. Fine dust, such as is 

 deposited by the wind that blows over the unmodified glacial deposits, is not 

 found in the beach dunes because the finer particles of the dunes have been 

 swept into the sea by the undertow. Furthermore, most of the small particles 

 of the softer minerals in the wind-laid deposits derived directly from the 

 glacial drift are ground down between the pebbles in the mill of the surf 

 and carried seaward with the clay. Along the beach, as in the bared tracts in 

 the interior, there are two contrasted land surfaces — the beach whence the sand 

 is blown (Plate 35, fig. 1) and the dune tract to which the sand is carried by 

 the wind. The beach is in a certain sense a lag-gravel tract in places where 

 scattered pebbles occur near headlands or in front of cliffs from which pebbles 

 are supplied to the beach, but owing to the regular return of the tide and the 

 greater power of the waves to shape and distribute pebbles and sand on the 

 beach, the work of the wind on the beach is limited to shifting the sand and, 

 under favorable circumstances, to producing a paradoxical movement of peb- 

 bles in a direction opposite to that of the wind. The sand-blasted glyptoliths 

 found in the glacial upland lag-gravel tracts are not found on the beach, where 

 the frequent inversion of pebbles and the change in the setting of the surface 

 give no time for sand blasting. 



The principal dunes of the Cape Cod peninsula are formed by the inshore 

 wind on the great flying beaches at the north and south ends of the headland — 



