Bridport Sandstone – Youngest Lower Jurassic (lead authors JT and WGT)

Series	Stage	Ma	Traditional		South Dorset		South Somerset	Use
Middle Jurassic	Aalenian	174.1	Inferior Oolite Group
Lower Jurassic	Toarcian	182.7	Upper Lias	Lias Group				Building Stone
							Ham Hill Limestone Member
						Bridport Sand Formation
					Downcliff Clay Member			Brick Clay
			Middle Lias		Beacon Limestone Formation
	Pliensbachian

The youngest formation of the Lower Jurassic Lias Group is the Bridport Sand Formation (Toarcian, 183-174 million years).  It extends from the Dorset coast to the central Cotswolds, capped by the Inferior Oolite Group .

The magnificent, but dangerous, cliffs at West Bay (Bridport Harbour) comprise a sequence of micaceous, quartzose, silty, very fine-grained sand and sandstone with alternations of friable sands and calcareous sandstones.  The sequence is bioturbated (burrowed) throughout but this is best seen in weathered calcareous layers.  Belemnites and the occasional bivalve, as well as the burrows, indicate deposition in marine conditions. 

Blue-hearted calcareous layers (less porous & permeable than the intervening sand layers). Photo: WGT

The hard concretions have been used in industrial buildings in West Bay. The weathered skin shows the bioturbation and, when this flakes off, the “blue” interior is revealed. Photo: JT

Outcrop of Bridport Sand Formation, East Cliff, West Bay - showing hard calcareous layers. Photo: WGT

The sands were originally porous and permeable throughout but, during burial, shelly fossils were dissolved and the calcium carbonate precipitated in layers, creating hard, dense, sandstones. These have been used locally as building material.  The orange-yellow-buff colour is superficial, due to oxidisation of fine-grained iron minerals (e.g. pyrite).  The internal colour is blue-grey, indicating the iron minerals remain in a “reduced” state. (see image left)

Bioturbation (crustacean burrows). Photo:WGT

Text GT & JT   Mar 2017