GEOLOGY Period Formation Character of Strata Approximate thickness in feet Recent Alluvium Valley Gravels .... River-mud, etc Gravels of existing rivers . . I-IO 5-20 Pleistocene River Drift Clay-with-flints . . Brickearth Glacial Drift . . . J Older river-gravel and sand Reddish clay (on Chalk only) . Loam and sandy clay Gravel and sand Boulder-clay with chalk and erratics 5-10 1-20 5-3° 10-20 3-100 Eocene Reading Beds .... Plastic clay, loam, and sand ? io shown Upper Cretaceous Upper Chalk .... Middle Chalk . . . J Lower Chalk . . . -j Upper Greensand . . . Upper Gault .... Lower Gault .... Soft white chalk with layers of flints Chalk Rock — very hard, cream- coloured chalk Hard white chalk with few flints Melbourn Rock — hard, nodular chalk Grey and white chalk . . . White blocky and hard grey chalk Totternhoe Stone — hard, sandy chalk Chalk Marl — grey, marly chalk Chloride Marl or Cambridge Greensand — glauconitic marl Micaceous and glauconitic sand Variously-coloured clay, part sandy, and clayey sand . . Light and dark grey marly and sandy clay ioo shown 2-15 200-2 I O 8-io 2-6 60-80 20 70-80 ?I0 0-20 25-30 150-280 Lower Cretaceous Lower Greensand . . VVoburn Sands and Potton Beds — brown ferruginous sand- stone, dark clay, and light- coloured sands, rarely green. 230-280 Upper Jurassic- Kimeridge Clay . . . Ampthill Clay .... Oxford Clay .... Kellaways Rock . . . Dark-coloured clay and shale . Black clay with bands of lime- stone Greenish grey and brown clay Calcareous grit, shale, and sand- stone 10 40-60 300-400 10-50 Middle Jurassic Cornbrash Great Oolite Clay. . . Great Oolite Limestone . Upper Estuarine Series . Inferior Oolite .... Tough grey limestone and clay Variegated clays, partly calca- reous Limestone, marl, and clay . Variegated sandy clays and lime- stones Northampton Sands — brown sandstone and ironstone 2-15 5-10 25-30 15-30 ?I2 Lower Jurassic Upper Lias Blue clay and shale . . . 66 seen
