Page:VCH Northamptonshire 1.djvu/48

 A HISTORY OF NORTHAMPTONSHIRE TYPICAL SECTION OF THE GREAT OOLITE SERIES Description of the Beds Approximate maximum thickness I. 2. CoRNBRASH : White, ruddy, or blue limestone, very fossiliferous, with /Ammonites macrocephalus, A. discus, etc. Forest Marble Series : Variegated clays, hard flaggy blue-hearted limestones, shales, and oyster-beds, etc. Great Oolite Clay : Blue and purple clay, with wood and car- bonaceous matter, and ironstone Great Oolite Limestone Series : Hard shelly limestones in courses, with marly or oyster-bed partings. Ammonites gracilis Upper Estuarine Series : Very variable {a) Green, grey, or blue clay, or marl, with vertical plant-markings and carbonaceous matter (f) Blue, dark brown, or nearly white clay, with vertical plant- markings, or carbonaceous matter ; or possibly an oyster- bed. Ironstone at base ft. in. 15 O 8 o 12 O 25 O 30 90 o The Upper Estuarine Series The scale of the accompanying map does not permit of the divisions of the Inferior and Great Oolite series being separately and respectively represented on it, hence the connection between the two sets of Estuarine beds is not seen. Speaking generally, in the valley of the Nene the two Estuarine series come together, and in the valley of the Welland they are separated by the Lincolnshire Oolite. The Upper Estuarine beds are even more variable than the lower in thickness and character over large areas. They also undergo rapid changes in the same area. In some places from eight to ten very distinct beds might be chronicled, but a three-fold division (see Typical Section) answers for most purposes. The occurrence of carbonaceous matter, large pieces of wood in some places vertical plant markings in others, and limestones, as well as both marine and fresh or brackish water mollusca — Modiola, Ostrea, Cyrena and JJnio, etc. — point to such variable conditions as could be best secured in the estuary of a large river, hence the name given to them by Prof Judd. Probably the finest section ever exposed within the county was at Roade Cutting, on the L. & N.-W. Railway, but a very good one could recently be seen in one of the ironstone workings near to Finedon. In the eastern parts of the county a nodular fossiliferous ironstone, about one foot thick, occurs at the base ; and even in the western, midland and other parts, where the two Estuarines come together, in almost all cases a ferruginous band marks the junction. The commonly irregular junction, with its ferruginous band, is supposed to indicate an unconformity 18
 * {b) Hard blue-hearted limestone, marl, hard shale, or oyster-bed (6 ft.)
 * Water-bearing.