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A generalised section through The Lower Greensand of Bedfordshire is informally divided into the Red Sands and Silver Sands (Upper Woburn Sands) and the Brown Sands (Lower Woburn Sands) (ref sections Formation & Sand Features). The sands are up to 120m thick in total and fill a 25-30km wide NE-SW trending trough. Click here to see a diagram.

stone lane quarrySection of Stone Lane Quarry


The formation also includes, in the upper horizons, Silty Beds (see Sand Features) up to 10m thick, in which the uppermost ironstone horizon contains the Shenley Limestone.

 

Shenley Limestone - General
Shenley Limestone is a very rare and small formation that developed in a geographically small area around Shenley Hill in Leighton Buzzard.

It appears that eroded remnants of the Silty Beds formed low mounds on the sea floor.
A gritty ironstone capped these mounds and thin lenses of limestone formed between them – the Shenley Limestone.

shenley limestoneShenley Limestone sample, encased within sandstone


It is a pale fawn brown phosphatic limestone with scattered polished goethite balls.
The formation is noted for its rich and varied fauna of brachiopods and rare ammonites.
Nine Acre Pit was designated by Natural England as a Site of Special Scientific Interest (SSSI) for its exposures of Shenley Limestone. However, it has been recently noted that this formation does not actually outcrop on this site but in the neighbouring quarry, Mundays Hill.

 

The Shenley Limestone - Composition
Is only found in the Heath & Reach area, in the uppermost levels of the ironstone horizon within the Silty Beds.

Is a bioclastic limestone filling cavities in the topmost ironstone. It is a highly unusual micrite, full of poorly sorted clasts (pebbles and sand) and many fossils, largely cementing/attaching animals such as bryozoa, brachiopods or serpulids, with abundant echinoid spines and rare (possibly reworked) ammonites.

Is a marine deposit, a very near-shore accumulation resulting from still water deposition (micrite) with high energy currents or waves adding pebbles and debris.

 
 
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