ALLUVIUM DOWNS. 



285 



rents sweeping past their mouths, and depositing 

 the materials in the eddy formed by the meeting of 

 the currents. Sometimes the surf throws up a 

 sandbank so as to block up their mouths, and thus 

 converts them into fresh-water ponds or lakes. 

 Where the materials which form the barrier are 

 coarse gravel and pebbles, the water filters through 

 at ebb tide, and remains near the tide-level ; but if 

 they are fine sand, the water accumulates till it 

 overflows the obstacle, or has a sufficient head to 

 excavate a channel through the barrier and escape. 

 In this manner almost every bay, inlet, and marsh 

 on the north and south coast of Long Island have 

 either had their outlets blocked up entirely by the 

 materials deposited or so nearly as to leave only 

 narrow entrances. The only exceptions are where 

 they have been protected from the sea by the long 

 sandy islands. In this way extensive tracts of 

 beach, marsh, and salt meadows have been formed 

 within a comparatively short time ; and it is not 

 uncommon to meet with persons on the island who 

 will tell you of many acres having been deposited 

 during their lifetime, and of seeing ships sail where 

 now there is land, some feet above tide-water level. 



Dunes or Downs. — These are low hills of loose 

 sand, which have been piled up by the wind like 

 drifting snow-heaps, and, like them, are frequently 

 changing their size and position ; so that, in some 

 places, productive lands are buried by the moving 

 materials, while in others they are uncovered by 

 their removal. These hills are very common along 

 the coast in the southeastern part of Massachu- 

 setts ; and near Cape Cod they are 60 or 70 feet 

 high, and of a snowy whiteness. They gradually 

 move towards the west; and a series of these dunes, 

 several miles long, now threaten the village and 

 bay of Provincetown, and large quantities of beach- 

 grass have been transplanted to their ridges for the 

 purpose of arresting their progress. These hills of 



