The River Exe is Devon’s largest river rising near Simonsbath on Exmoor and following a course of over 50 miles to the sea at Exmouth. During its course it goes from the north coast to the south coast of Devon, as well as going through Devon’s county town, Exeter. The River has historically been an important resource and boundary. In 50 AD the Romans turned the site that would later become Exeter into a stronghold because it was the lowest point at which the Exe could be crossed. From there the River Exe provided Exeter with an ideal transport link and enabled the growth of the cities port, as well as that of Topsham. According to a legend the Countess of Devon had a disagreement with the port at Exeter so built a weir across the river. This led to the construction of the Exeter Canal in 1566, making it England’s oldest shipping canal. The navigation led from Turf Lock up to Exeter Quay, where it rejoined the river. The River Exe also helped with the development of Tiverton, providing water power to fuel its booming wool industry.
Along the shore of the river there are many important habitats, not to mention the mudflats themselves. There are several local nature reserves and the entire river is a Site of Special Scientific Interest (SSSI). It is especially important to the birdlife, with species including, Avocet, Curlew, Teal, Brent Geese, Oystercatchers, Dunlin, Redshank, Grey Plover, and Godwits. The river is also important for recreational activities, including canoeing, sailing and wakeboarding. On Exmoor the River Exe also provides water for Wimbleball Reservoir, supply Exeter and parts of east Devon.
The river runs over a number of different geologies. In the upper catchment the geology is predominantly Devonian Siltstones and Sandstones (408 – 360 million years old). The middle courses are over a mixture of Carboniferous Mudstones and low grade Shales (360-299 million years old) and Permian Breccias, Sandstones and Mudstones (299 – 252 million years old). The River Exe Estuary runs entirely through these Permian Breccias and Sandstones. Along the estuary there are plenty of good exposure of the geology. The most impressive of these are along the coast, including the cliffs of Exmouth and Dawlish.
The location of this EarthCache, on the estuarial cliffs near Lympstone, is the best exposure of Permian Breccia lithology in east Devon. Breccia is a term used in geology for a sedimentary rock comprised of angular fragments of rock (over 2mm in diameter) that are cemented together by a matrix. This matrix can either be comprised of the same or different types of components to that of the larger fragments. Breccia is very similar to conglomerates, such as Puddingstone. The main difference between the two is the shape of the particles. Breccia is made up of angular fragments, where as conglomerates are made up of rounded fragments, that have commonly been rounded by water action. Breccias can have different properties to each other, for example their resistance to weathering can and do vary.
Within the Exe valley area the various different types of Breccia, including Heavitree Breccia (also known as Wonford, Exminster and Whipton Breccias), Alphington Breccia, Sampford Peverell Red Breccio-conglomerate and Newton St Cyres Breccia are all grouped under the Exe Breccia group. These Breccias have a range of properties, from the soft Alphington Breccia to the Sampford Peverell Red Breccio-conglomerate, which as its name suggests contains inclusions of both Breccia and Conglomerate characteristics. The Exe Breccias have historically been extensively used as cheap local building materials. Heavitree Breccia is widely used in Exeter and are commonly used for both walls and buildings. The Sampford Peverell Red Breccio-conglomerate was commonly used for the bridges and the banks of the Grand Western Canal that ran between Taunton and Tiverton.
At the posted coordinates the lower two thirds of the cliff consists of Exe Breccias, which is a coarse red Breccia of a silty sand matrix (material around inclusions) with imbedded sandstone, slate and quartz. Within this there, particularly near the foot of the cliff, there are a few mud rich bands. The topmost part of the cliff is composed of well rounded pebbles in a sandy matrix thought to be a raised beach/river terrace of Pleistocene age (2.6 million years ago to 11.7 thousand years ago).
At high tide the water covers the lower parts of the cliffs, which subjects the Exe Breccias to water erosion. The main process of erosion on this cliff face will be from hydraulic action, which is where the force of the water causes air to be trapped in cracks and crevice’s, particularly along weaker areas or faults, and the pressure slowly weakens the cliffs over time. In particularly rough weather the pebbles and cobbles on the beach will be picked up and either strike or rub against the cliffs causing erosion through abrasion. The hydraulic action in particular can lead to interesting features of erosion, such as small hollows, erosion of weaker layers and caves.
To log this EarthCache please with email me or message me through my profile the answers to these following questions
1) Estimate the height of the rock face.
2) Describe the inclusions in the Breccia (size/shape).
3) Look at the exposure of Breccia (Hint it does not go to the to of the cliff). Describe the difference in appearance/feel of the Exe Breccia between the top of the exposure to the bottom.
4) Based on your previous answer why do you think this is?
5) The ‘toe’ of the Breccia formation extends from the base of the cliff into the estuary. Estimate its length from the base of the cliff.
6) At the southern end of the cliff face there is a small feature about 0.5 meters in width. What type of erosion do you think caused it and how deep is it?