12 



GLACIAL GRAVELS OF MAIKE. 



to the full force of the wind, are blown up and over the ridge, soon to be 

 followed and covered by other grains. When the wind changes, these sand 

 grains may all be blown back again. As the di-y winds in Maine are most 

 frequently from the Avest, the net result of this movement back and forth is 

 an unsteady march eastward. In places the dunes have traveled from 1 to 

 3 miles wp and over hills 200 to 300 feet high. Often a layer of sand is 

 left on the ground passed over by the main dune, and then the vegetation 

 characteristic of a sandy soil appears. Thus, in western Maine a growth 

 of white pines on high hillsides is almost always found on a dune of blown 

 sand or on ground passed over by one. 



It is fortunate that blown sands so often leave a trail behind them, for 

 the foremost or principal dune thus becomes gTadxially smaller and its 

 power to do mischief is lost unless other dunes follow and overtake it, 



which may happen if the sand 

 is very abundant at the place 

 where it began to blow. A 

 large proportion of the dunes 

 now overgrown with vegeta- 

 tion have traveled away from 

 the sand plains where they 

 originated into regions once 

 covered by till, clay, or gravel. 

 In most cases it is possible 

 to distinguish blown sand from that deposited in its present situation 

 by water, even when both are covered b}' vegetation. The blown sand 

 will be found at A^ery irregular elevations and on Avestern slopes, except 

 where it has been blown up the western slope of a hill and over its top and 

 has come to rest on the eastern slope. Blown sand contains no A^ery large 

 pebbles, and is not oA^erlain by bowlders. The dunes form rounded ridges, 

 domes, or terraces, and their forms are such as to be recog-nized at once by 

 the practiced eye. Usually the country to the Avest of a dune is coA^ered 

 Avith more or less sand, a sign that the dune has passed oA^er it. These 

 features are sufficiently different from those shown by AA^ater-deposited sand 

 in similar situations to enable us usually to distinguish them. Fine sand is 

 the only material subject to Avind transportation on a large scale, yet each 



Fig. 1.— Stratification of wind-blowu 



