Gullies on Worms Head Causeway

Gully with seaweed in Carboniferous Limestone wave-cut platform

The Causeway between the Worms Head and the Rhossili headland is a wave cut rocky platform covered twice a day by the tide. It is composed of Carboniferous Limestone and represents the remnants of an anticline in which the rock layers were at one time folded upwards into a hill-like structure. The top of this structure has been worn away over time leaving a flattened feature with upstanding layers of rock clearly visible – older rocks remain in the middle and progressively younger rocks spread out sequentially from the central anticlinal axis.

The axis cuts across the causeway in a northwest to southeast direction. This central part of the anticline is composed of strata belonging to the Black Rock Limestone Group of the Carboniferous Limestone. The bedrock under the Coast Guard Lookout on the tip of the headland and the path down to the causeway are also Black Rock Limestone Group.

If you cut across the causeway at low tide in a direct east to west direction, with the Worms Head in the distance kept on your right hand side, you cross the axis of the anticline with its Black Rock Limestone and eventually encounter the younger strata of the Gully Oolite and then the High Tor Limestone sub-groups of the Carboniferous Limestone.

Extreme caution is required when venturing out onto this area because it is pock-marked by hundreds of potentially treacherous rock pools and gullies. The pools are spread out randomly and come in all sizes and shapes, from large irregular ones to small rounded ones; some shallow, some deep; full of seaweed that often conceals their presence and is slippery underfoot.

The gullies are all deep but by varying degrees – from about half a metre to two metres. They have an almost geometric arrangement following courses between the strata in the direction of the anti-clinal axis (leading towards the Worms Head itself) and across the strata at almost right angles leading to the sea on the outer westernmost edge of the causeway. Water from the larger rock pools lying on higher surfaces of the exposed limestone drains down in cascades and falls to the gully cuts that carry streams of saltwater to the sea. See the earlier video clip Out on Worms Head Causway 1.

Click here for earlier posts about the rocks, seaweed, and seashore creatures to be found on the Worms Head Causeway.


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One thought on “Gullies on Worms Head Causeway

  1. Pingback: Out on Worms Head Causeway 1 | Jessica's Nature Blog

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