CHAP. XLIX 
SUBSIDENCES OF THE PLATEAUX 
45 
subside, partly by ruptures of the crust producing faults and partly by a 
downward movement of a more general kind. In course of time, these 
disturbances turned the drainage into the hollow now traversed by the 
Bann. Denudation would necessarily accompany them, and the surface of 
the country would be continually eroded and lowered. 
Lough Neagh has been carefully sounded by the Admiralty, and its chart 
affords much suggestive material for the consideration of the geologist. 1 
From the soundings there given it has long been known that the lake 
deepens towards its northern end, and attains a maximum depth of 102 feet. 
But it is not until we trace on the chart a series of contour-lines for 
successive depths, as shown by the soundings, that we realize the remarkable 
form of the lake bottom. We then discover that below a depth of oO feet 
a well-defined channel extends for rather more than half the length oi the 
lake. This channel begins to be distinctly perceptible between Kilt ugh 
Point and Langford Lodge. It first runs in a northerly course on the west 
side of the centre of the Lough, but when it comes into a line with Saltera 
Castle on the western shore, it wheels round so as to conform to the curve 
of the Antrim coast-line, which it follows northward until, about two miles 
from the exit of the lake, its outline ceases to be traceable on the gently 
shelving bottom. Its total length is thus about 12 miles. 
There can hardly be any doubt that this channel is a former bed of the 
River Bann. It occupies exactly the position which that stream would take 
if the lake were drained, and its depth and breadth correspond to those of 
the valley-bottom of the present river. If this conclusion be accepted, some 
important conclusions may be further deduced from it. 
1. The presence of a former course of the Bann on the bottom of Lough 
Neagh proves the lake to be much younger than the Ice Age. The thick 
boulder-clays and Glacial gravels which so encumber the country around and 
descend under the lake, would assuredly have filled up the river-channel had 
it existed at the time of their deposition. The channel has obviously been 
cut out of these drifts since the Glacial period. When the erosion took 
place, the present Lough Neagh could not have existed, but the Bann 
followed a continuous course across the plain which the lake now covers. 
The river probably maintained its place for a long period, so as to be able to 
excavate so wide and deep a bed in the drifts, if, indeed, it did not to some 
extent slowly carve its bed out of the underlying basalts. It must be 
remembered that sediment is being continually poured into Lough Neagh, 
and that some of the silt must have accumulated in the submerged river- 
course, thus lessening its depth and width. That the channel should still 
be so marked may be used as an argument for the comparatively late date 
of the subsidence. 
2. The submerged river-course is a clear proof of subsidence. The 
present Lough Neagh cannot be looked upon as a glacial lake formed by 
rock-erosion or by irregular deposition of drift. Its floor must have been a 
land surface when the Bann cut out its bed upon it. The whole area has 
1 Lough Neagh surveyed and sounded by Lieut. Thomas Graves, R.N. 
