Arkell.1956.Jurassic..

i8 THE BRITISH ISLES
seldom a little more (Portsdown boring 1294 m.). Where normally
developed, they show a marked tendency to cyclic lithology ; the tripartite
sequence, clay, sand, limestone is repeated many times. Yorkshire
and Scotland differ by having a deltaic development of most of the Middle
Jurassic, but otherwise are essentially similar. The sand/sandstone
member of the various cycles is always more or less diachronic, sometimes
markedly so. The highest Upper Jurassic stages, the Upper Kimeridgian,
Portlandian and Purbeckian, are present only in the south of England.
They are perfectly conformable with one another and the Purbeckian
passes up imperceptibly into the Wealden. In Lincolnshire and Yorkshire
marine Neocomian rests transgressively on Kimeridge Clay
(Swinnerton, 1935), and farther south, in places where the Wealden lake
beds are absent, marine Aptian or even Albian rests transgressively or (as
in Dorset) unconformably on various formations of the Upper Jurassic.
Volcanic rocks are absent from the British Jurassic. Conglomerates
are few, thin and altogether inconspicuous, except in the Lower
Kimeridgian of East Scotland, close to an Old Red Sandstone shoreline
(Bailey & Weir, 1932; Waterston, 1951). Internal unconformities are
seldom so marked that they can be detected without mapping, but very
gentle ones do exist, especially in the Bajocian of the Cotswolds. Disconformities
are frequent. Besides the Middle Jurassic deltaic beds in
Scotland and Yorkshire there are beds of partly 'estuarine' facies in the
Bathonian of the Midlands and a thin brackish or freshwater bed with
Viviparus in the Bathonian of Oxfordshire. The Purbeckian is largely
brackish and partly lacustrine. Liassic shorelines exist in South Wales
and round the Mendips and neighbouring hills, which were an archipelago
in the Lower Jurassic sea.
Summaries and revisions of many areas have been published since
1933. The principal ones are as follows:—
General lie of the Jurassic of England: Lees & Taitt, 1946; Kent,
1947, 1949; White, 1949.
Dorset: Kellaway & Wilson, 1941; Chatwin, 1948; Arkell, 1947,
1951a, 1951&.
Cotswolds: Gardiner & others, 1934; Richardson, 1933; Cox, 1941,
1950; Kellaway & Welch, 1948; Channon, 1950.
Oxford district: Arkell, Richardson & Pringle, 1933; Richardson &
others, 1946; Arkell, 19396, 1943, 1944, 1947a.
East Midlands: Richardson, 1938-40; Hollingworth & others, 1944;
Hollingworth & Taylor, 1946, 1951; Edmunds & Oakley,
1947; Kent, 1947; Wilson, 1948; Whitehead & others, 1952.
Lincolnshire: Richardson, 1938-40; Kent, 1941; Swinnerton & Kent,
1949; Wilson, 1948a; Evans, 1952.
Yorkshire: Wilson, Hemingway & Black, 1934; Rastall &
Hemingway, 1940-49; Smithson, 1934, 1942; Wilson, 1936, 1938,
1948a; Arkell, 1945; Sylvester-Bradley, 1953.
Scotland: Macgregor, 1934; Waterston, 1951.
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