Page:VCH Northamptonshire 1.djvu/44

 A HISTORY OF NORTHAMPTONSHIRE The Upper Lias varies very little in thickness throughout the county, and the zones are well marked. The map will show where the Upper Lias is exposed, and as the clays are largely used for brickmaking, the neighbourhood of any town or large village situated on it, or on the Northampton sand, will generally have one or more exposures. The Lower Oolites The Lower Oolites consist of ironstones, sandstones, clays, marls and limestones ; and since sandy strata, and even some limestones, indicate comparative nearness to the source of the material, they are seldom as persistent in thickness and superficial extent as argillaceous beds. In addition, oolitic limestones probably indicate warm, agitated, and therefore shallow water during the time of their formation. There was so much coquetting of the land with fresh and salt water about this period that we can scarcely speak of the series of Lower Oolitic rocks as we did of the Lias, but will make such references to the physical conditions then existing as seem called for, in the appropriate place. The beds can be usually identified without the aid of characteristic fossils. The Inferior Oolite Inferior is a term used to designate position only. In our small scale map the whole series is represented under one colour. TYPICAL SECTION OF INFERIOR OOLITE SERIES Name of Series Description of the Beds Approximate maximum thickness in feet Lincoln- shire Oolite shelly oolitic ragstones, and marly beds. Many fossils sandy limestones (or sands only) 80 5 Northamp- ton Sand 3. Lower Estuarine Beds : Mostly white or bluish sands with vertical plant markings 4. Variable Beds : Calcareous and slaty with much com- minuted shell when distinct, but may incline to 3 or 5 grey carbonate of iron, calcareous beds, and more rarely pyritous beds 15 30 The Northampton Sand The Ironstone Beds constitute a valuable source of iron, and are consequently worked at many places. The red ore consists largely of a hydrated peroxide of iron (Brown Hsematite or Limonite) ; it may present 14
 * I. Lincolnshire Limestone : Cream-coloured freestones,
 * 2. CoLLYWESTON Slate : Fissile, calcareous sandstones, or
 * 5. Ironstone Beds : Rich red ore, ruddy sandstone, green or
 * Water-bearing beds.