Longmyndian

▪ geology

      major division of Late Precambrian rocks and time in the southern Shropshire region of England (the Precambrian began about 3.8 billion years ago when the Earth's crust formed and ended 540 million years ago). Named for prominent exposures in the Longmynd Plateau region, Longmyndian rocks consist of steeply angled and even overturned unfossiliferous mudstones, sandstones, conglomerates, and volcanic rocks. Two major subdivisions are recognized: the Western Longmyndian and the underlying Eastern Longmyndian. The Western Longmyndian consists of the Wentnor Series, purple sandstones, conglomerates, and some greenish siltstones and shales; thicknesses of about 4,800 metres (15,700 feet) of Wentnor rocks have been measured. The Eastern Longmyndian is subdivided into the overlying Minton Series and the underlying Stretton Series. The Minton Series, about 1,200 metres in thickness and made up of purple and green shales, sandstones, and conglomerates, is separated from the underlying Stretton Series by an unconformity representing a period of erosion rather than deposition. The Stretton Series, grayish and greenish siltstones, sandstones, shales, and volcanic rocks, is as much as 3,500 metres thick. Rocks underlying the Stretton Series and possibly related to the Longmyndian are known as the Eastern and Western Uriconian, geographically separated from each other but similar in lithology and probably broadly contemporaneous. The Eastern and Western Uriconian consist of lavas, tuffs, and intrusive igneous bodies; they are separated from the overlying Stretton Series by a prominent unconformity. Elsewhere, in the Charnwood Forest and Midlands regions, a sequence of rocks occurs that may favourably be compared to the Stretton Series of the Eastern Longmyndian; three subdivisions have been recognized: the lowermost Blackbrook Series, overlain in turn by the Maplewell Series and the Brand Series. These rocks, collectively known as the Charnian, consist largely of volcanic rocks (most prominent in the Maplewell Series and least in the Brand Series) and of sedimentary conglomerates, sandstones, siltstones, and slates.

      Charnian sedimentary rocks contain impressions of a Precambrian organism known as Charnia; these are especially prominent in the higher levels of the Maplewell Series. Similar if not identical forms are known to occur in Australia. The zoological affinities of Charnia are uncertain; opinions have ranged from including the form in the Coelenterata (corals, hydras, and jellyfish) to the algae.

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