File:The Quarterly journal of the Geological Society of London (13935627262).jpg

for my present purpose, an advantage over calcareous strata, in that
they are not soluble by water, and the phenomena therefore are less
complicated. As far as my observations extend, I have found that
cylindrical pits and pipes are generally confined to soluble beds, and
that the normal form of the cavities in clays, sands, and gravels is
that of troughs or furrows*. They are usually filled vsdth materials
derived from some neighbouring higher ground, and consequently
generally differing from the subjacent stratum at that spot ; and it is
by the admixture of the contents of the furrows with the material of
the subjacent stratum that the warp, which is derived from the two
conjointly, comes to contain materials mixed in a different propor-
tion from that in the subjacent bed.
These furrows must be important indications of the mode of de-
nudation of those surfaces where they occur ; nevertheless, being
simply the tool-mark of the last agent which has moulded the sur-
face, if we could determine from them what that was, we need not
exclude other and different ones which may have preceded it.
For the sake of a name I shall call the materials which fill these
furrows the " trail." And I will now give a few examples of them
in diagram.
Fig. 1. — Section of a furrow which crosses the Tendring Hundred
Railway, near Great Bentley Church, Essex.
- — — — — — — — . Eailway.
It is 7 feet deep, full of grey sandy and gravelly clay .h), eroded in brown sands
(c). The warp (a) covers it. There is a layer of pebbles at the bottom of
the furrow, and roots have penetrated throughout it. The gravel is more
angular than that of the glacial drift from which it has been derived, many of
the rounded pebbles having been broken up into angular fragments.
At the gravel-pit. Ballast Quay Farm, near Wivenhoe, Essex, two
masses of trail have been left, being too clayey for use. They are
parallel to each other ; one of them is fifty yards long, the other not
much less. Erosion has gone on beneath them, which is shown by
the pebbles of the warp, and also of the trail, lying in festoons.
They are inclined at a small angle to the surface-drainage. They
occur in sandy glacial gravel.

• For examples of these troughs, see figs. 1 and 5. London clay dug for
  

ballast at Bentley Tey, near G-reat Bentley, to the depth of 5 feet, exposed a
section of a furrow ten yards long and a yard wide, running in the direction of