UB Aust Cliff University of Bristol
EARTH
SCIENCES

 
Aust cliff can be found directly under the eastern end of the 'old severn bridge'.   Access is via the M4 junction 21, proceed along the A403 southwards and take the B4461 until it meets the estuary coast (vehicles can be left on the roadside).  Go through the gate and walk along the concrete causeway, running along the base of the cliff towards the pier.
The succession comprises Mesozoic strata, which reveals a change from Triassic terrestrial deposition through fluctuating Rhaetic transgressions to the fully marine of the Lias.  It is interesting to note how the rocks can be classified using simply their colour.

The photograph below shows a clear section of the cliff.  The prominant Keuper marls are overlain by Tea Green marls and then black shales and the thin limestones of the Westbury beds.  Above these are the Cotham beds and with overlying flaggy limestones at the base of the Blue Lias.  The whole cliff is cut by eight normal faults that downthrow sections to the north and south.

 
 
Fossils are abundant in many of the rocks especially within the Rhaetic Bone bed.  This bed occurs in patches at the base of the Westbury beds, and consists of a conglomerate in a calcite cemented sandy matrix, together with an abundance of teeth, scales and bones.  The suite of fossils previously found includes; coprolites of aquatic reptiles, fish remains of dipnoans and ganoids, scales especially Gyrolepis, ichthyosaur vertebrae, and even bones from the dinosaurAvalonia. Hypothesis for the formation of this bone bed include mass mortality due to salinity changes, storm deposits on coastal flats, or simply concentration due to a decreased sedimentation rate.

Both the Westbury and Cotham beds reveal cycles of sedimentation with sandy or shelly limestones resting on an erosional surface.

The Westbury beds as a whole are composed of shelly limestones with a low species diversity common of brackish water assembleges.

The faunas of the Cotham beds suggest a lacustrine affinity with liverwort, ostracods and some arthropods.

Liassic sediments herald the marine intrusion, with faunas including corals, ammonites and echinoids.

 
 

Bibliography:

R. Savage, R. J. G. (1977) Geological excursions in the Bristol district, pg. 111-118,

Aust Cliff section- photograph modified from original from Kellaway, G. A. & Welch F. B. A., 1993. Geology of the Bristol districtpg. 133. HMSO, London.


 
 
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