1866..
FISHER — WARP.
561
subsoil. This I believe to be the sections of channels of drainage,
and that the drainage carries forward some material with it. Some
such action as this may account for the flat-topped elevations, like
the tenons in the framework of a dissected puzzle, which often occur
in or near the line of junction of the warp with the subsoil, espe-
cially (if I mistake not) where the latter contains calcareous matter
and has suffered partial solution. The erosion at the bottom of two
contiguous subterranean channels would leave an elevated ridge be-
tween them, and the superincumbent soil sinking in a general mass
would then flatten the crest of the ridge. By a continuance of such
a process, with slightly shifting directions in the drainage, those
complicated foldings would be formed the cause of which, at first
sight, seems so incomprehensible.
Fig. 8. — Section of a Pit in Victoria Road, Cambridge,
e '°. ^
a. Warp.
b. White sandy brick-earth.
c. Yellowish sandy earth.
d. Fine sandy gravel.
e. Yellowish brick-earth.
/. Fine gravel, more contorted than the
layers above and below.
g. Sand.
The sands contain Bythinia, &c.
Contortions of the kind alluded to, when occurring in gravel-beds,
are sometimes explained by the action of floating ice ; but that cause
can only have operated under water. These contortions have in some
way or another resulted from subaerial causes ; and though I see
difficulties, I think this explanation may be the right one.
In the instances from the Cambridge gravel-pits the percolation
and consequent erosion seem to have taken place in the layer be-
neath the ductile clay, which is folded into such remarkable forms.
Had it been on its upper surface, the layer must have been cut through.
It must be recollected that slight inequalities slowly formed in the sur-
face of the ground by such subsidences would be continually levelled
by the action of rain, herbage, and worms.
The next question which I propose to enter upon relates to the