88 
THE AVONIAN OF THE AVON GORGE 
the massive grey AS'emmw/G-Limestone ceases abruptly where it 
abuts against a contorted mass of red rocks, and where the 
height of the section drops suddenly. 
^ ^ The massive limestones which compose this portion 
of the sequence all belong to the Upper Seminula- 
Zone (82), whereas the red rocks against which they end belong 
to the upper part of D2, and the S2 limestone-mass is sharply 
separated from the subjacent D2 beds by a reversed fault 
whose plane hades southward with the dip. 
The under surface of the limestone is strongly slickensided, 
and the contortion in the soft D2 rocks, due to the overthrust 
of the massive S2 limestone, is finely shown, although it extends 
only a short distance north of the fault. The vertical displace- 
ment produced by the fault amounts to about 1,100 feet, cal- 
culated as follows : — The lowest portion of the limestone-mass is 
composed of ‘ Seminula-OoMte,^ a rock which we shall see again 
in the middle of the Great Quarry, and there is a perfectly con- 
tinuous sequence from the ‘ Seminula-Oolite ’ of the Great Quarry 
up to the D2 beds immediately below the fault. Hence, 
knowing the average dip, and the horizontal distance from the 
‘ Semmula-Oolite ’ of the Great Quarry to the ‘ Seminula-Oolite ’ 
just above the fault, the upthrow can be estimated. 
Having noticed the crumpling of the soft D2 beds by the 
overthrust of the great limestone-mass and also the smoothing 
of the under surface of the limestone, it is worth while to 
retrace our steps a few yards in order to study the effect of 
the overthrust upon the limestone-mass itself* 
Above the northern end of the station-yard, the surfaces of 
the beds are sharply bent and smoothed, thus indicating the 
immense force behind the overthrust and marking how the 
limestone-mass was held back by friction at the fault plane, 
while the upper beds were sheared over the lower. 
We must now return to the bottom of Bridge Valley Road 
and continue our walk downstream along the towing path. 
From this point to the bottom of the Carboniferous Limestone 
Series, a distance of a little more than I mile, we continuously 
