Portland Admiralty Roach is a kind of

stone In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition, and the way in which it is formed. Rocks ...

from the

Isle of Portland An isle is an island, land surrounded by water. The term is very common in British English British English (BrE, en-GB, or BE) is, according to Lexico, Oxford Dictionaries, "English language, English as used in Great Britain, as distinct fr ...

used to construct " The Cobb", the well-known

seawall A seawall (or sea wall) is a form of coastal defense constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast. The purpose of a seawall is to protect areas of human habitation, conservation ...

Lyme Regis Lyme Regis is a town in west Dorset, England, west of Dorchester and east of Exeter. Sometimes dubbed the "Pearl of Dorset", it lies by the English Channel at the Dorset–Devon border. It has noted fossils in cliffs and beaches on the Heri ...

Dorset Dorset ( ; archaically: Dorsetshire , ) is a county in South West England on the English Channel coast. The ceremonial county comprises the unitary authority areas of Bournemouth, Christchurch and Poole and Dorset. Covering an area of , ...

. The stone is rich in

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s (known as shells to local masons) often consisting of almost fifty percent fossilized remains with an average size of approximately 35 mm. The stone, which was quarried by Albion Stone, has a maximum bed height of 2 metres. The stone is renowned for being highly frost resistant and being able to withstand the chemical effects of

seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appr ...

. The stone is an open textured oolitic limestone from the

Portlandian The Portland Group (defining the Portlandian) is a Late Jurassic ( Tithonian) lithostratigraphic group (a sequence of rock strata) in South East England. The name is derived from the Isle of Portland in Dorset where the strata are exposed and ...

formation formed during the Tithonian age, (152.1 ± 4 Ma to 145.0 ± 4 Ma (million years ago)) of the Late

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

epoch. It is formed from ooliths in a

micrite Micrite is a limestone constituent formed of calcareous particles ranging in diameter up to four μm formed by the recrystallization of lime mud. Flügel, Erik, ''Microfacies of Carbonate Rocks: Analysis, Interpretation and Application,'' Springe ...

(fine grained calcium carbonate) matrix. There are many voids in the stone, caused by percolating rainwater dissolving all or parts of its many shell fossils, mainly bivalves and

gastropods The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. The ...

See also