Between a Rock and a Hard Place

Science snap (7): Thrusting under our noses

As Earth Science researchers, we are extremely fortunate that fieldwork often necessitates trips to exotic and far-flung places. But sometimes we are guilty of ignoring the riches right on our doorstep.

In Bristol (UK), perhaps our greatest geological asset is the Avon Gorge. At the end of the Last Glacial Maximum, torrents of icy meltwater scoured out a 2.5km long gouge through a series of Devonian and Carboniferous limestones and sandstones. The bottom of the 90m deep gorge is now filled with the River Avon and the sheer cliffs of the north side are home to fossil corals, rare plants and challenging climbing routes; they also expose an excellent thrust fault.

This particular example lies at the intersection between Bridge Valley Road and the Portway, just underneath the Clifton Suspension Bridge (see here for map). Compressional forces associated with the formation of the supercontinent Pangea (~290 Ma) caused the the older Clifton Down Limestone to be thrust over the younger Upper Cromhill Sandstone. Friction along the overhanging fault plane deformed the younger sediments, and the resulting instability of the rock face has caused major issues for the adjacent roads.

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Thrust fault in the north side of the Avon Gorge where the older grey Clifton Down Limestone (right) has been thrust over the younger red Upper Cromhall Sandstone (left); the intensity and friction of the thrusting is manifest in the deformation of the younger sediments. The fault outcrops at the intersection between Bridge Valley Road the Portway (A4) and is conveniently located adjacent to set of traffic lights and a cycle path – look out for it next time you’re stuck on a red light or peddling past.

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Charly completed a PhD in experimental petrology. She used to make pretend volcanoes; now she works in renewable energy. Charly tweets at @C_Stamper.

3 Comments

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    Wow! What a fantastic outcrop! Thank you for sharing that for us folks who might not get over there to that spot any time soon.

    (I used to work on Lower Carboniferous limestone stratigraphy in the eastern US, and knew a little about the Avon Gorge section. Can you recommend a website that has photos and info on the Gorge? Can someone walk through there easily to see the main geologic structures and features?)

    Thanks, guys!

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      Hi Mike,

      Walking (or cycling) along the north side of the Gorge, it is definitely possible to see the main features. There are several former quarries and popular climbing spots where you can get right up to the rock face.

      This website is good for general information http://www.avongorge.org.uk/index.php

      There is also a paper that is really useful but pretty hard to get hold of: The Geology and the Evolution of the Avon Gorge: Proceedings of the Bristol Naturalists’ Society 47, 1989, Pages 45-64. If you are really interested I can scan you a copy!

      Charly

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        Thanks a lot Charly. I will check out your link and other info. I’ll let you know later about the scan – I appreciate the offer.

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