478 TRANSACTIONS OF SECTION C. 
either marine denudation or peneplanisation, before the deposition of the 
upper set upon them. With this group any limestones which occur below the 
unconformity appear to be devoid of pipes or swallow-holes contemporaneous 
in origin with the plane of the unconformity. Example: the junction of 
the Carboniferous Limestone with Triassic or Jurassic formations at most 
places, such as Upper Vobster, in the Bristol district, already described by 
many authors. 
In the second group the rocks below the unconformity have not been 
maturely eroded before the deposition of those above, and the junction may 
in consequence be very uneven. Where the underlying rock is limestone the 
unevenness becomes most marked, for there the junction is complicated by 
pipes and swallow-holes contemporaneous with the unconformity and filled 
with material similar to that of the overlying formation, which has been 
deposited in them in situ. An example in which the: unconformity, and 
consequently the piping, has been but slight is afforded by the junction of 
the upper and lower subzones of the Syringothyris-zone of the Carboniferous 
Limestone—i.e. by the mid-Avonian unconformity—at West Williamston, 
in Pembrokeshire.* There the basement-bed of the upper subzone fills pipes, 
up to 8 feet deep, in the upper part of the Caninia-oolite (the top of the 
lower subzone) below, the evidently undisturbed state of both the in-filling 
and the rest of the basement-bed above showing that the pipes have been 
formed before the deposition of the upper subzone. At a short distance 
from West Williamston the pipes in the oolite disappear as we approach 
the area characterised by continuous deposition of the Avonian; but at 
Pendine,’ in the opposite direction, where the unconformity in the middle 
of this formation is greater than at West Williamston, the piping has 
extended for a greater depth into the limestones below. 
A still more advanced stage of solution-erosion is shown by the Car- 
boniferous Limestone at Ifton, in Monmouthshire, near Severn Tunnel 
Junction. The unconformity in this case occurs between the Carboniferous 
Limestone and the Millstone Grit; in the former have been eroded large 
steep-sided cavities, comparable only with swallow-holes, as well as small 
pipes resembling those at West. Williamston, and both swallow-holes and 
pipes have been filled with an original deposit of Millstone Grit. This 
occurrence is the more interesting because the Carboniferous Limestone and 
the Millstone Grit have subsequently been covered up with Trias, but the 
junctions of both with the latter form an even surface, evidently a base- 
levelled plane, below which there are no contemporaneous—i.e. Triassic— 
pipes in the limestone. 
Finally, an extreme case of solution-erosion preceding unconformable 
deposition on limestone is afforded by huge breccia-filled cavities in the 
Carboniferous Limestone of Pembrokeshire.’ These cavities often extend 
from top to bottom of cliffs ranging up to more than 100 feet in height, and 
in some cases continue horizontally for over 100 yards. They are almost 
completely filled with blocks of limestone, of all sizes up to masses weighing 
hundreds of tons, which have fallen from the roof and sides, but also contain 
a little interstitial Triassic material zm situ. The formation of the cavities; 
therefore, preceded the deposition of the Trias, and took place during the 
long period represented by the great unconformity between the latter and 
Carboniferous rocks. In this district, most, if not all, of the succeeding 
younger formation (the Trias), deposited on the upper surface of the lime- 
stone, has been removed, but there is evidence that that surface was a base- 
levelled plane. This belief, however, does not invalidate the conclusion that 
1 ‘Summary of Progress’ for 1906 (Mem. Geol. Surv.), 1907, p. 55. 
2 ‘Summary of Progress’ for 1902 (p. 43), 1904 (p. 44), and 1905 ( (p. 58); and 
‘The Country around Carmarthen’ (in the press), Mem. Geol. Surv. 
3 ‘Summary of Progress’ for 1904 (Alem. Geol. Surv.), 1905, pp. 46-47. 
