274 
ME. PEESTWICH ON THE G-EOLOGY OE THE DEPOSITS 
by the higher chalk hills*. So again in the valley of the Somme, both the flanks of 
the valleys and the lower hills adjoining the valley are covered, but the higher water- 
sheds between the different valleys are free from it. A general section across the 
Somme valley may be represented thus (fig. 16). 
Pig. 16 . — Theoretical section across the valley of the Somme. 
The Loess is therefore, like the high- and low-level valley-gravels, connected with 
existing river-valleys, although the connexion is, owing to its irregularity and wider 
extension, not so apparent ; and it becomes a question whether they may be related to 
the same common cause, presenting two phases connected with temporary changed condi- 
tions. In the first place, such organic remains as are found in the Loess are all common 
to the fluviatile gravels ; but in the latter they are coordinate with a fauna of a more 
freshwater character. Thus there exists in the lower marl, gravels, and sands at Men- 
checourt the same species of Helix , Pupa, and Clausilia as occur in the overlying Loess. 
In the former, however, they are intermingled with Limnaza, Bythinia, Planorbis , and 
other freshwater shells, whereas in the Loess these shells are the exception (see p. 285). 
With the shells in the Loess are also associated the remains of the same species of 
Elephant, Horse, Deer, &c. as are found in the underlying series. 
It is well known that in all rivers subject to floods and carrying down much sediment, 
as, for example, the Severn in its lower course, three forms of sediment will be de- 
posited: 1st, coarse gravel and shingle in the more direct channel through which 
the waters flow with the greatest velocity ; 2nd, sand and fine gravel in those portions 
of the more direct channel where the velocity of the stream is checked from any cause ; 
and 3rd, fine silt and sediment in those parts where the flood-waters out of the direct 
channel remain for a time in a state of comparative repose ; such places are the lee- 
side of the hills, lateral valleys, and plains, and any local depressions or hollows : none 
or little would accumulate in the main channel, as the scour of the retiring waters 
would there prevent its deposition. 
In like maimer I conceive the Loess to be the result of river-floods commencing 
at the period of the highest valley-gravels f, and continued down to the end of that 
* In the valley of the Thames the Loess covers the low-level gravels in thick masses, hut is very scantily 
spread over any part of the high-level gravels, and is rarely to he traced much above them. — March 1864. 
f The Loess may have begun to form even before the fossiliferous high-level gravels, and therefore pro- 
bably when the floor of the original valley was yet higher than these beds indicate. 
