| The Geology of Black Ven and Lyme Regis |
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The Stratigraphy of the Rocks of Lyme Regis |
Stratigraphy The rocks of Lyme Regis come from two distinct geological periods, the Jurassic and the Cretaceous. The Jurassic rocks of Lyme are from the Lower Jurassic system known as the Lower Lias. These rocks are sub-divided into distinct named units, the Blue Lias, the Shales-with-Beef, Black Ven Marls and Belemnite Marls. There is a time gap (unconformity) between the underlying Lower Jurassic rocks and the overlying rocks of the Mid to Upper Cretaceous period. This means that the rocks of the Middle Jurassic, Upper Jurassic and Lower Cretaceous were eroded before the Mid to Upper Cretaceous rocks were deposited. On the coast the Cretaceous rocks are represented by the Upper Greensand. Farther inland the Upper Cretaceous sequence passes conformably from the Upper Greensand into the Lower Chalk. Blue Lias The base of the Blue Lias crops out to the west of Lyme Regis in Pinhay Bay, and gently drops to below sea level before the Lyme Regis frontage is reached. Much of the Lyme sea frontage is underlain by Blue Lias Limestones, and the top of the sequence (Grey Ledge) crops out at beach level below the sea wall that protects Langmoor Gardens. The Blue Lias then rises to the east, so that at East Cliff, Grey Ledge crops out above the sea wall. The Blue Lias is made up of an interbedded series of limestones and mudstones, which were laid down in marine conditions. They are very fossiliferous; the most notable fossils are the ammonites, which are numerous, and very large examples can be seen in the wave cut platform bewteen Pinhay Bay and Monmouth Beach.. The limestone bands are more competent (harder and stronger) than the intervening shale and mudstone layers and therefore form more pronounced layers in the cliff line. The most prominent limestone bands were given names by the quarry-men who once worked the limestone ledges for cement and stucco. Shales with Beef The Shales-with-Beef are so named because of the impersistent calcite bands that are found within them. These bands have a cone-in-cone structure (Whitten and Brooks, 1972) that makes them resemble a beef steak. The shales are thin and papery and are easily crumbled in the hand (friable). Geotechnically the are relatively weak and consequently slopes that consist of Shales-with-Beef are unstable at high angles. The Shales-with-Beef are capped by the Birchi Nodule Bed, a fossiliferous and persistent nodular limestone that can be traced from Ware Cliffs to Black Ven and Charmouth. Another persistent band within the Shales with Beef is Table Ledge. This indurated marl used to mark the top of the Blue Lias, but has recently been included with the Shales-with-Beef in a revision to the stratigraphic nomenclature (Gallois, 1996). The thickness of this ledge varies from 0.3 m to 1 m, and is found five metres above Grey Ledge. Black Ven Marls The 43 metres (House, 1993), of the Black Ven Marls are very well exposed in landslide scar at the rear of Black Ven, and form one of the landslide benches. They consist of dark grey shales with impersistent limestone bands, some of which are fossiliferous. There is also evidence for shallow water conditions (Coinstone bed) and a non-sequence (time gap) between the Black Ven Marls and the Belemnite Marls (House, 1993). Belemnite Marls The Belemnite Marls are so called because of the many belemnites that have been preserved in them. They consist of mudstones that are bluish-grey in colour, with alternating darker and lighter bands. The lighter bands have a higher calcite content than the rest of the unit and may indicate a shallower environment of deposition. The greater degree of cementation means that the Belemnite Marls are slightly stronger than the underlying Black Ven Marls, and so form a steeper cliff (House, 1993). At the top of the Belemnite Marls is the laterally persistent limestone "Belemnite Stone". Below Stonebarrow and Golden Cap the sequence passes conformably into the Green Ammonite Beds. At Black Ven and Lyme Regis the overstep of the Cretaceous unconformity means that the top of the Belemnite Marls is removed, with possibly all of the unit removed from the sequence west of the River Lim. Upper Greensand The
Upper Greensand is a formation that unconformably overlies the Lower
Jurassic Lias limestones and shales. It is exposed at the top of Golden
Cap, Stonebarrow and Black Ven. It also crops out on Timber Hill below
the golf course at Lyme Regis and above Coram Avenue on the western
side of the town. The lower part of the formation is made up of sands
with Cowstones. The Cowstones are large "cobbles" or concretions
of sand cemented by calcite, and were used in the construction of the
original Cobb (Fowles, 1982). Above the Cowstones is the Foxmould sand.
This is a poorly cemented sand that, when saturated with water, liquefies
very easily. It also acts as a groundwater reservoir and because of
this, when it is encountered below the water table in construction works,
it is known by the locals as flow-sand. The Foxmould weathers to a greyish
yellow colour (House, 1993) and, because it has less glauconite than
other "greensands", when freshly exposed it has a golden appearance.
DAVIES, G. M., 1956. The Dorset Coast, a Geological Guide. A & C Black, London. FOWLES, J., 1982. A Short History of Lyme Regis. Dovecote Press, Wimborne. GALLOIS, R. W., 1996. Provisional Geological Map of the Lyme Regis Foreshore. Unpublished. HOUSE, M. R., 1993. Geology of the Dorset Coast (2nd Edition). Geologists' Association, London. WHITTEN,
D. G. A. AND BROOKS, J. R. V., 1972. The Penguin Dictionary of Geology.
Penguin Books, London. |
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