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 
