THE BEAVERS OF NORTH AMERICA 63 
to subsist on them, and to the severity of the winter 
in each neighbourhood. Some piles are fully 
thirty-five or even forty feet across and con- 
tain a fairly closely-packed mass of browse and 
wood from five to about ten feet in depth. Very 
little use is made of this store, or what remains of 
it, after the ice melts, for then the beaver prefers to 
cut fresh material for food. The water-soaked 
mass of brush is generally left at the bottom where, 
by rotting, it gradually settles lower and lower, and 
often forms a foundation for a new house or anchor- 
age for the next season’s cuttings. Occasionally trees 
are dropped into the water, more particularly in 
rivers, so that the tops as well as a large part of the 
branches are submerged. The beaver leave them 
there, knowing full well that they can come when- 
ever they wish during the winter and cut off what 
they need under water. It is usually noticeable 
that when a tree is used in this way the animals 
cut' off much of the bark around the thicker portion 
of the trunk, whether it is exposed or beneath the 
water, and they also trim the tree of most of the 
branches which project above water. 
When the lodges are built in places where there 
is a swift current, few trees of any size are ever cut 
below the lodges. Well do the animals know that 
swimming with heavy branches against the stream 
is hard work and usually quite unnecessary, so 
they do practically all their wood-cutting above 
stream and often let the heavier logs drift down 
