117 
converted into a lake and another layer of silt and débris is slowly 
being deposited, which in time will completely entomb the tus- 
socks and include them, together with other derelict material, in 
a constantly thickening layer over the bottom of the lake. 
nthe Bulletin of the Garden (Vol. 1, No. 2, January, 1897), 
in an article on ‘The Glacial or Post-Glacial Diversion of the 
Bronx River from its Old Channel,” Professor Jas. F. Kemp 
describes the locality as follows: ‘From the entrance to the 
gorge a swampy depression extends westward to the railroad and 
has all the characteristics of an old abandoned channel. The 
railroad has crossed it by an ane and culvert. Just east 
of the culvert there is gneiss but a few feet below the soil and at 
this point the old stream evidently surmounted a reef.” 
Fic. 30. Theoretical section of bed of lake. a, gneiss; 4, gravel and bowl- 
ders; ¢, mud and silt; d@, water. 
If we consider this description, together with the more obvious 
surface features, we may therefore regard the present lake as 
having had its origin i in a stream flowing through a channel of 
bare gneissic rock, Which subsequently became choked with 
gravel and boulders and converted into a swamp, in which vege- 
tation lived and died and whose remains accumulated as a black 
organic mud; and finally came the construction of an artificial 
dam, flooding it with water and resulting in the deposition of the 
silt, svhich is now taking place 
is designed to wae these successive stages and the 
ie of their deposits, in a theoretical section through 
