Page:VCH Essex 1.djvu/36

 A HISTORY OF ESSEX To the geologist the richly fossiliferous strata of the Red Crag at Walton-on-the-Naze, and the Pleistocene valley -deposits with their many notable remains of mammalia and mollusca, have attracted a large share of attention. Our knowledge of Essex geology, due chiefly to the labours of Prestwich, Searles V. Wood, jun., Prof. W. Boyd Dawkins, and Mr. W. Whitaker, has been augmented by the workers of the Essex Field Club and of the Geologists' Association, and notably by Mr. T. V. Holmes. 1 The following is a Table of the Formations met with in Essex, the names in italics referring to those not exposed at the surface : Period Formation Character of the strata Approximate thickness in feet Alluvium Silt, peat, clay 2O to 50 Recent to Blown Sand Fine sand about 1 5 Neolithic Marine Sand and Shingle. Sand with shells and pebbles of flint, quartz, etc about 10 Brickearth Loam i o to 30 Valley Gravel Sub-angular flint gravel. . 10 to 50 Pleistocene Boulder Clay Chalky clay, with flints and Palaeolithic erratics 2O to 70 and Glacial Loam Loam 10 to 15 Glacial Glacial Gravel and Sand Sub-angular gravel with flints, quart/.itc, etc 20 to 75 Older Plateau Gravel. . . Pebbles of flint and quartz. 5 to 12 Red Crag. Red shelly sand and gravel, Pliocene laminated clay, phosphatic nodules and ironstone. 5 to 21 Bagshot Beds Sand and pebble beds. 20 to 40 London Clay Brown and blue clay with 400 Eocene Blackheath or Oldhaven Beds. Sand with shells and flint i o to 50 Woolwich and Reading Beds. Mottled clay, sand, and flint 25 to 80 Thanet Beds Grey sand and sandy clay. 10 to 85 Upper Chalk Chalk with flints .... 400 to 450 Middle Chalk Bedded chalk with few flints. 200 Upper Blocky chalk with curved Cretaceous Upper Greenland jointing, and marl Green sand and brown loam. 120 7O Gault 5O to I7O S'tlurion ^ (or older) unknown 1 See Bibliography in Whitaker's 'Geology of the London Basin,' Mem. Geol. Survey, vol. i. (1872) p. 393 ; and Geology of London, vol. i. (i 889) p. 87 ; also ' List of Works on the Geology, etc., of Essex,' by W. Whitaker and W. H. Dalton, Essex Nat., vol. iii. (1889) pp. 61-84. 2