One of the most fascinating things about geology is its ability to reveal global events from evidence contained within a single outcrop. The cliff exposure at Aust in Gloucestershire, UK, is a spectacularly colourful example of this.
Located beneath the original Severn Bridge, and running alongside the Severn Estuary, the 40m tall rock face records the drowning of an ancient desert by rising sea levels. In the Triassic, the SW of England sat 30ºN of the equator (the position of modern-day northern Africa). As the supercontinent Pangea rifted apart, the land was flooded by Jurassic oceans.
Aust cliff, near the Severn Bridge in south Gloucestershire. The change from red to grey-green in the rock strata records the flooding of an ancient desert by rising Jurassic seas. Photo credit: Charly Stamper
The red beds of the Triassic Mercia Mudstone group (~220 millions years old) form the base of the cliff, a geological archive of a time when this part of the UK was a hot, subterranean landscape. Rounded quartz grains with haematite coatings and accompanying veins of gypsum and celestine testify to an arid, windswept desert scattered with ephemeral lakes and playas.
The sudden change to the grey-green strata of the Blue Anchor formation is immediately evident to the eye. These clay-rich silty rocks contain halite crystals and provide evidence for the gradual ingress of inter-tidal lagoons and brackish lakes conditions.
Further sea level rise is charted by the Westbury formation (~205 million years old). The basal section comprises the famous Westbury Bone Bed, noted for yielding fossil insects, plesiosaur teeth and abundant bivalves; above this lies a layer of Cotham Marble, rich in stromatolites.
The Severn Bridge is easy to get to by bike – follow National Cycle Routes 4 and 41 from Bristol. Access to the cliff is via a steel gate to concrete causeway, at the bottom of the hill from Aust village.
The exposure at Aust is a SSSI; please only collect fossils from cliff debris and be careful of falling blocks close to the rock face.