Posted by: Rose | May 10, 2009

It’s all your fault…

In the Coast post below, you’ll see a juxtaposition of very obviously different rocks…

Blue Anchor fault

Blue Anchor fault, Blue Anchor Bay, Somerset

The photo shows two Triassic rocks of differing age set against each other; the Red Marls and the Penarth Group of layered shales and limestone. Between the two rock groups is a fault. The Red Marls are the ‘foot wall’ and the grey Penarth Group are the ‘hanging wall’, simply because the grey rock is hanging over the red rock.

By looking at the fault it is possible to determine that the Penarth Group has fallen in relation to the Red Marls. This means that this is a normal fault formed during extensional processes. If the Penarth Group had risen, it would be a thrust fault caused by compression.

As faults go, I think this one is rather lovely. Cream teas are also good.

Due to this exciting day of exploration, my garden now contains a fine and very large specimen of pink gypsum (calcium sulphate), aka ‘Apricot Sorbet rock’. Plasterboard has never been so interesting.

Creamy, jammy scone. Kilve, Somerset.

Creamy, jammy scone. Kilve, Somerset.

Pink gypsum (calcuim sulphate) within the Penarth Group

Pink gypsum within the Penarth Group.



  1. Can I request a bit of clarification.

    If this were a Reverse fault would it be \ rather than / in the photo?

  2. A reverse fault is a compressive fault (a thrust fault is a low angle reverse fault). The hanging wall therefore moves upwards.

    So the fault can be either / or \, the naming is all about the sense of movement between the two rocks.

  3. […] the strain is high and of short duration; the rocks snap apart and create fissures in the ground (a fault). In the Himalayas, two continental plates are converging to create a mountain belt. At depth, […]

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