Devon County Geological Sites
Site reference no. SX64SE3 Name: Thurlstone Sand
District: South Hams Parish: South Milton and South Huish
National grid ref: 554190 OS sheets: 1:50k 202 1:25k 1362 1:10k SX64SE GS 1:50k 355/6 GridRef x y Lat Long SX676416 267600 41600 50.259503 -3.8588023
Locality description (address): Thurlstone Sands, coast 2.5km WSW of South Milton.
Nature of site: Coastal cliffs, dunes and foreshore.
Geological / geomorphological features: Outliers of probable Permian breccio-conglomerate unconformable on Lower Devonian Meadfoot Beds. At the northern end of Thurlestone Sand the cliffs are formed of coarse, reddish to brownish purple breccio-conglomerate composed of cobble and some boulder sized clasts up to 0.6m in a pebble and gravel matrix. The larger clasts show some degree of rounding and include quartz,,slate, yellow and green bleached slate, and schist resembling that of the Start Complex. The deposit is coarsely bedded and shows some fining upwards in the beds which dip about 150 south. This is probably a depositional dip as the Meadfoot Beds immediately below the unconformity show contemporaneous flexure and toppling of the cleavage in the direction of dip. Imbrication indicates transport from the east or northeast. The Meadfoot Beds are silty banded slate with bedding and cleavage dipping steeply north. They are red and purple stained for some distance beneath the unconformity. A steep E-W fault throws down 2m to the north. Toppling and collapse of the Meadfoot Beds forming the cliff north of the unconformity has caused coastal erosion and the closure of a cliff top road. Protective measures have included the use of gabions. The conglomerates are being deeply eroded along joints forming caves. At the southern end of the beach breccio-conglomerates crop out on the foreshore. At low tide they can be can be seen to lie in a small palaeovalley in the Meadfoot Beds which widens as it extends seawards with Thurlstone Rock, a sea stack in the form of an arch, at the western extremity. The sand dunes which lie behind the beach have barred two small valleys. These are now mainly silted up. At the southern end of the beach [67624140], if the position of the tide and the overlying sand permits, a peaty clay deposit is exposed resting on Meadfoot Beds. This deposit contains abundant disorientated fragments of wood, including a tree stump is lying on its side. It is the seaward end of the deposits filling the southern valley which has been overridden by the advancing sand dunes with rising sea level. A similar deposit at North Hallsands is about 1700 years old.
Reasons for registration as a Regionally Important Geological / Geomorphological Site: This section of coast displays a range of excellent features. The Permian unconformable on Meadfoot beds is the most significant. With Holocene barred valleys and deposits, sand dunes and coastal erosion features also of considerable interest
Site sensitivity: Generally none. The cliffs at the northern end of the beach are vulnerable to wave attack and their value as a County Geological Site could be affected if further coastal protection work becomes necessary.
Safety: Generally safe
Interest groups: Schools. Years 5-11: Years 12-16: Years 17-18. University. Undergraduate - Research Professional geologists - Amateur geological groups - General public.
Access and Parking: Access possible except at high-water spring tides. Walk down to beach access and turn right. A paying car park (no coaches) lies to the left where the road from South Milton turns north to Thurlstone. The National Trust paying car park at the southern end of the beach is an alternative access point.
Date of assessment (V = visited) : V 29th September 1995 R T Taylor
Site owner : Public access
References: HOBSON, D.M., The Plymouth Area. Geologists’ Association Guide No. 38. The Geologists’ Association. USSHER, W.A.E. 1902. The Geology of the country around Kingsbridge and Salcombe. Memoirs of the Geological Survey, England and Wales. London, HMSO.