GEOLOGY OF MOUNT DESERT. 67 



understanding of this exceptional form, unless it can be 

 explained as a glacial modification of a mountain range 

 previously serrated by transverse notches of moderate 

 depth, down from which lateral ravines descended to the 

 lowlands, north and south. It has therefore been sup- 

 posed that the more rapid flow of the ice through these 

 preglacial passes gouged them out as deep as the open 

 lowland on either side, or even deeper. 



There are few well marked examples on Mount Desert 

 of the curious deposits of gravel and sand, elsewhere com- 

 mon enough in New England, that were formed during the 

 closing stages of the ice period. Many of the mainland 

 valleys are half clogged with heaps, ridges, or plains of 

 these loose materials, lying upon scored rock or upon a 

 varying sheet of till ; their origin being ascribed to streams 

 that ran from the waning ice sheet, discharging their load 

 of sand and gravel in the open spaces along the ice border. 

 The absence of such gravels on Mount Desert would imply 

 that during the disappearance of the ice sheet the streams 

 from the mainland avoided the island, and followed by 

 preference the lower districts east or west. The surface 

 deposits of drift are, however, frequently of a gravelly 

 nature, especially at altitudes above that of the clays later 

 described, and below elevations of four to six hundred feet ; 

 and these may be ascribed to the wash of streams and 

 currents as the ice melted away. Above the head of Bass 

 Harbor, they attain an uneven form, characteristic of the 

 gravel mounds or kames of the mainland ; but this is 

 exceptional on the Island. 



Postglacial History. 



What with the deepening of the transverse valleys and 

 the irregular deposition of the till over the rocky floor, 

 we find the drainage of the Island peculiarly embarrassed 

 since the disappearance of the ice. In preglacial time, 



