cliff-top storm deposits

gloup

Quoy Geo

 

 

 

Kilns of Brin Novan

Significance: the Kilns of Brin-Novan (HY 382349) is an excellent example of an interconnected series of caves, arches and blow-holes which illustrate the sequential development of these features.

The cliffs at Sacquoy Head are 10-15 m high and developed in the flat-lying Rousay Flags (Wilson et al.  1935). Beds in the Devonian sandstone are 0.5-1 m thick but show more closely-spaced unloading joints in the upper 2 m. The rectilinear shape of the coastline and of the Kilns of Brin Novan reflects the exploitation by wave action of crossing joint sets, with the 60-240 set dominant.

The northernmost Kiln is the oldest and largest. The Kiln is an enlarged gloup or blowhole, measuring some 30 x15 m, and connected to the sea through an arch. The rock surface above the feature is washed by waves for a further 30 m inland but it is unclear whether or not these waves move entirely upwards through the gloup or are supplemented by waves washing over the cliff edge. The potential force of water moving upwards in the gloup in shown by the 1.8 m long block of fresh sandstone to the rear and the scar of its original location on the subjacent rock face. The gloup shows a large, 7 m high cavity in the rear rock wall. The adjacent cliff edge to the N is being eroded back towards the northern arm of the gloup. This face shows pervasive fracturing and sagging and is therefore susceptible to wave attack and collapse.

The middle gloup is more shaft-like in form and tapers downward, with stepped, joint-bounded sides,  to a 2 m wide hole at its base. The rear wall of the shaft shows clear scars of recent block removal and of weakening of the rock wall by joint exploitation by waves. The connection with the sea seems to be via the northern gloup.

The southern gloup is a shaft, with vertical sides in places. This seems to be a relatively  recent feature as there has been erosion of the weathered storm beach to seaward. The lack of recent scars from joint-block removal is puzzling, however, although the upper surface is beginning to open out by removal of the uppermost, well-jointed rocks.

If a space-time transformation is used, whereby the northern gloup is taken as the oldest and the southern the youngest, then the Kilns provide a sequence of development. All stages of evolution are structurally controlled by the joint system and the dominant process is hydraulic action and the removal of joint blocks. An initial cave is enlarged and joint intersections in the cave roof are attacked. Blocks here are relatively easy to move as they are small and the weight of overlying rock is less. Collapse creates a gloup and this appears to open out from the surface down. Waves emerging from the gloup are able to remove the closely-jointed near-surface rocks and any overlying deposits. Enlargement of the gloup may lead to more widespread collapse and the formation of arches and large caves, prior to the eventual creation of a geo. A key question lies in the rate of cliff retreat and gloup formation and the presence of overlying storm deposits at Kilns of Brin Novan offers the prospect of dating (Hall, 1993).