76 THE MAMMOTH CAVE. 
limestone. The floor is covered with yellow 
sand. 
It requires a rise of only five feet of water in 
Echo River to overflow Great Walk; and that 
depth is sufficient to allow boats to float between 
the lake and the river. There are times, we 
are informed, when Great Walk is filled with 
water from the floor to the ceiling. Extraor- 
dinary as the statement may appear, it is not 
an uncommon occurrence for the water to rise 
to the height of sixty feet in Lake Lethe, at 
which times the iron railing on the terrace 
above the Dead Sea is entirely submerged. 
This great rise of water is produced by freshets 
in Green River. 
