68 PHYSICAL DISTURBANCES IN THE SOM. AND GLOS. COALFIELD. 
level structure, stood out in strange contrast with the Coal measures 
which lay beneath in a great basin, the sides of which rose at a high 
angle, so that the overlying rocks lay unconformably on the upturned 
edges of the older rocks. 
Mr. McMurtrie then dealt with the Mendip Range, with its central 
mass of Old Red Sandstone, from which the Mountain Limestone and 
Millstone grit dipped north and south at a high angle, with indica- 
tions of folding over on its northern flank. The adjacent Coal 
measures had been completely inverted for a distance of four or five 
miles between Nettlebridge and Mells, and dipped at varying angles 
towards the hills, instead of from them, while resting on the crest 
of this fold were three remarkable outliers of Mountain Limestone, 
which thus lie above the coal strata, instead of far beneath them, 
and at a distance of from 600 to 1900 yards from the parent rock 
on the Mendips, of which they once formed a part. The effect of 
this inversion had been very different on the various groups of coal 
measures. The massive sandstones of the Pennant and New Rock 
series had been folded over bodily, and were almost normally regular, 
while the shales of the Vobster series had been contorted and broken 
up in a way difficult to describe. Conterminous with this great 
physical movement there existed four miles away, in the interior of 
the coalfield, at Radstock and Writhlington, a great overlap fault 
by which an upper slice of the Radstock Coal measures had been 
thrust bodily forward a distance of 350 yards on the three lower 
seams, and from 250 down to 140 yards on the three U23per seams 
of that group. 
Mr. McMurtrie then pointed out that all these great earth move- 
ments he had described were only separate links in one greater 
physical disturbance, to account for which various theories had been 
suggested at different times. The view now generally accepted was 
that all these phenomena had been primarily caused by some enormous 
lateral pressure, probably the result of a contraction of the earth’s 
erust, which had operated from south to north, as had happened 
under like conditions in the Belgian coalfield. The strata on the 
Mendips had thus been raised to a great elevation, probably accom- 
panied by a certain amount of folding over. The adjacent Coal 
measures had been inverted as described, while the upper Coal 
measures of Radstock had been thrust bodily forward on the plane 
of the overlap fault, but with a narrower lap on the three upper 
veins, which rather suggested that the elevation of the Mendips had 
begun towards the close of the coal measure period, and before these 
upper seams had been deposited. Enormous denudation must have 
followed the upheaval of the Mendips, the strata so removed having 
been estimated by Sir Andrew Ramsay at from 4000 to 6000 feet, 
but judging from the thickness of strata from the upper Coal measures 
down to the Old Red Sandstone, which had then disappeared, it had led 
to the conclusion that the denudation could not have fallen short of 
13,000 feet. Unless, therefore, the elevation and denudation were 
coincident, which seemed unlikely, the summit of the Mendips might 
once have been an ice-field, from which glaciers descended to the seas, 
