Great Oolite Group
marl: a calcareous mudstone.
Image: Marlstone in hand specimen - £1 coin for scale (Ⓒ Dexter Brown)
* Ma is an abbreviation for million years
Previously known as the Marly beds (Woodward, 1894) these are limestones with subordinate interbedded marls.
The Hampen Formation are characteristically grey to brown, thinly bedded, fine to very fine-grained, well-sorted, ooidal grainstone to packstone, commonly slightly sandy or silty, with small-scale cross-bedding.
Thickness of Formation
The vertical thickness of this formation ranges from 4 to 11 metres.
The Hampen Formation was deposited on a shallow shelf fringing the shore of the London Landmass to the east. Three principal facies can be recognised:
a nearshore lagoonal region, dominated by clastic sediments, with faunal and floral evidence of periodic brackish and freshwater influence;
a shallow marine, more offshore region dominated by oyster reefs, where clastic and carbonate sediments intermix;
a deeper, more fully marine area further offshore, where the carbonate sediments and their marine fauna are identical to those of the overlying White Limestone Formation.
The Hampen formation tends to form hollows between the more resistant, harder units of the Taynton Limestone Formation below and the White Limestone Formation above.
Vertically the Hampen formation has a limestone and marl, resting sharply on the upper ooidal grainstone of the Taynton Limestone Formation. The upper boundary is more diffuse with the White Limestone Formation. Laterally the Hampen Formation passes into a very shallow marine-paralic in the Rutland Formation. Westwards it passes into the upper parts of the Fuller's Earth Formation.
North Cotswold Hills.
The type section and reference sections are all located in neighbouring Gloucestershire.