Monday, 2 March 2009
Geological Anomalies Part 2
Northamptonshire is blessed with a spine of fine limestones running from South West to North East through the county and on up into Lincolnshire. Along the margins of the Welland valley there are very localised deposits in the bottom beds of the Lower Lincolnshire Limestones where the sandy stone can be split into thin plates suitable for producing a roofing slate. The fine layers running at a slight angle to the main bedding contain mica flakes and some shelly deposits and are strongly calcite-cemented to produce a strong, brittle, durable stone. These plates are separated above ground by a simple process of de-lamination by frost action. Depths of these old workings vary, this one is under about 30 feet of overlying limestones and is accessd via a vertical shaft. The roof of the mine is a bed of some 18 inches, supported at random intervals with walls of waste stone and the arisings from the "sand" excavation. The slate bearing rock conveniently rests on a partially cemented sandy Limestone which can be excavated by hand with a very sharp pick, allowing the material we want to be under-mined. This involves lying on your side and burrowing away until a natural fault in the bed is reached, this may be up to 12 feet horizontally so the bed is propped with stacks of waste stone. The limit of progress is identified by the sound of cracking stones as the small props take the weight - time to get out from under the "log", pull away the props and instigate a fall. In the bigger picture of this country's geology the slate producing limestone of North East Northamptonshire is a tiny anomaly, yet it has had a massive effect on the appearance of the built environment in this neck of the woods. MORE EXCAVATION !