THE SUBSIDENCE AT LEXDEN. 3
Mr. Fisher considers that there must be a vacuity somewhere
beneath the subsidence. It cannot, he says, be in the gravel, as its
stratification is perfectly regular; it cannot be in the London clay,
but such a cavity may probably exist in the chalk. When the well
was bored at Colchester Water Works, the tool, after passing through
a layer of flints, sank suddenly, and the water rushed up with great
force. "This shows," says Mr. Fisher, "that the spring is connected
with an extensive and free reservoir. There are two other wells, at
distances of a mile and a mile and a-half, in a straight line east of
this one, which are evidently connected with the same reservoir; for
on Mondays, when the pumps are not at work at the water works,
the water stands higher than usual in those wells." Mr. Fisher's con-
clusion, therefore, is "that the motion of the water in this subterranean
reservoir, caused by the draught of water at the wells, disturbed the
equilibrium of the roofing of the chasm at a point where it was barely
stable, and caused the subsidence in question."
Fig. 3.—The Lexden Subsidence (Mr. F. Rutley), "Geological Magazine," 1865.
A, Gravel; B, London Clay; C, Dotted lines to show successive stages of erosion; D, Point at
which the debris would stand highest.
The depth at which the abundant supply of water was obtained at
Colchester Water Works is given by Mr. Fisher as 294 feet. He also
points out that the chalk appears to be lying nearly horizontal in the
neighbourhood of Colchester. Mr. Rutley's comment on Mr. Fisher's
explanation is: "If the cavity at this depth were large, why did not
a larger area subside ?—and if small, surely it would not cause a roof
of from 200 feet to 300 feet to sink." His own view is, "that erosion
has taken place in the low-level gravel at a small depth below the pit,
say twenty feet or a little more (allowing an inequality in the thick-
ness of the bed of gravel, which at the well a mile distant, was found
to be about twelve feet), and that when this erosive action had suffi-
ciently undermined, the roof fell in. The erosion would have been
either from water from the river Clone, or from rain having percolated
the gravel, and run down an inclined surface of the clay at its junction
with the gravel." He adds a diagram to illustrate his view. (Fig 3 )
B 2