Limestone Unconfonnities, etc. 515 



surface, the plane of the unconformity, and have evidently been base- 

 levelled, by either marine denudation or peneplanization, before the 

 deposition of the upper set upon them. With this group any lime- 

 stones 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 Si/rinffothi/r is- 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 characterized 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 

 Carboniferous Limestone at If ton, 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 unconform- 

 able deposition on limestone is afforded by huge breccia-filled cavities 

 in the Carboniferous Limestone of Pembrokeshire.^ These cavities 



' Summary of Progress for 1906, Mem. Geol. Surv., 1907, p. 55. 

 - Summary of Progress for 1902 (p. 43), 1904 (p. 44), and 1905 (p. 55) ; and 

 " The Country around Carmarthen " (in the press), Mems. Geol. Surv. 

 ^ Summary of Progress for 1904, Mem. Geol. Surv., 1905, pp. 46-7. 



