MOUNDS NEAR THE ESTUARY OF THE THAMES. 215
the steadings of temporary "lake dwellings" used by early seafaring
folk. The degree of salinity of the water in the neighbourhood,
again, tells against their having been used as salt works; and the
absence of any signs of the action of fire in the mounds at Sea Salter
would seem to indicate that some, at least, have not been used by
man, but are of natural origin.
The question as to what that origin has been, is not so easily
answered. One possible explanation I would suggest is that a
few of the mounds may be due to the pleating of masses of half-
DIAGRAMS TO ILLUSTRATE Three POSSIBLE Modes OF FORMATION OF THE MOUNDS AT
Sea Salter.
Fig. I. The alluvium (A) deposited around an old outlier of London Clay (E).
Fig. 2. Mound of upheaval, formed by the slipping of the Tertiary strata (B and C) in the
direction of the dip, as shewn by the arrow.
Fig 3. Cone of deposition of alluvial mud. piled up around the orifice of an old spring. In
this case the spring is supposed to burst forth along the line where the plane of saturation of the
Tertiary sands was intersected by the floor of the old estuary.
consolidated alluvial mud consequent upon the sapping and con-
sequent slipping of masses of that formation so troublesome to
engineers, the London Clay. I have seen ridges of much the
same character thrown up before a slipped mass of London
Clay on a railway. Ridges of much the same kind were thrown
up along the Settle and Carlisle Railway in 1872, when a heavy
embankment was thrown across a peat moss where Hawes Junc-
tion now is. The pliant mud and peat were squeezed up on
each side of the embankment and thrown into a series of parallel