16 
ALLUVIAL DEPOSITS. 
becomes more shallow, and the water overflows its 
banks, till, by degrees, an extensive lake is formed. 
The levels in the vicinity of Lewes constitute an 
extensive marshy tract, through which the Ouse 
winds its way to the British Channel. Tradition, 
ancient records, and the names of several hamlets 
situated upon its borders, testify that in distant 
ages it was covered by an arm of the sea, which 
extended up the country far beyond the town of 
Lewes ; the site of the Cliff being buried beneath 
its waters. During the last century, and before 
the present improved state of the navigation of the 
Ouse, the levels were annually exposed to exten- 
sive inundations, from the overflowino; of the banks 
of the river ; the rliies and other eminences forming 
islands in the midst of the lake. These levels offer 
an instructive explanation of the manner in which 
the change above alluded to is effected. A sec- 
tion of the soil presents the following deposits : — 
1. Vegetable mould. 
2. Peat : a mass of decayed vegetable matter, 
with leaves, and occasionally trunks of trees ; from 
three to five feet thick. 
3. Dark blue clay, provincially termed .svYf, con- 
taining freshwater shells in the upper ])art ; and 
an intermixture of sand and marine shells in the 
lower : from five to twenty-five feet. 
4. Pipe clay ; the detritus of the chalk-marl 
basin, in which the preceding beds are deposited : 
from one to two feet. 
The shells of the silt are, in the u])|)er part, 
Cnjclas cornea, Succinea amphibia, Planorbis cari- 
natiui, Planorbis corneas, Limnea stagnalis, Lim- 
