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The Lower Greensand Formation outcrops from Hunstanton in Norfolk, through Cambridgeshire, Bedfordshire, Buckinghamshire and southwards to the Isle of Wight. Its sediments comprise a set of sands, sandstones, silts, clays and ironstones that change in character across the region. They were deposited in the early Cretaceous from c. 115 million years ago a period known as the Lower Cretaceous. The sands within it are up to 120m thick in total and fill a 25-30km wide trough trending North East –South West. Click here to see a diagram.

The quarries of Heath & Reach, Bedfordshire are among the best places in England to view and interpret the features preserved in the Lower Greensand. Stone Lane Quarry shows us three units of Lower Greensand which rest on a phosphate pebble base.

profileSand Profile, showing the three types of sand quarrySection of Stone Lane Quarry from which the section analysis is drawn

The phosphate pebble bed found at the base of the Lower Greensand from Great Brickhill (Bucks) to Potton (Beds) and Upware (Cambs) is evidence of an early pulse of the sea-level rise that eventually flooded this region. These reworked phosphatised marine fossils were one of the ‘coprolite’ beds exploited for fertiliser production during the late 1800s to early 1900s.

The lowest, Brown Sands, were laid down over the phosphate beds in the seaward end of an estuary. They are strongly iron-stained and full of the burrows of shrimp and other animals, which stick out of the sand face like tiny drain pipes, preserved by an inner coating of hard ironstone. The Brown Sands contain Seams of Fuller’s Earth (a bentonite clay derived from re-worked ash) which are evidence of volcanic eruptions in north-west Europe.

Above the Brown Sands are the Silver Sands, showing “cross-stratification” in beds that may be over 1m thick. These were large dunes forming a sand bar at the mouth of the estuary.

Above these is the Silty Beds, a unit containing many different thin, flat beds of silts, sands, clays and ironstone. The sand layers were the lower flats, while the muds were deposited in the back lagoons.

fullers earth specimenFullers Earth Specimen (Nine Acres quarry) cross stratificationCross stratification, Mundays hill quarry

Throughout the Woburn Sands there are signs that currents and tides moved the sands back and forth. These movements built up many thin layers of sand to form dunes on the seafloor; We see the pattern of those layers when cliffs and quarries display sections through the dunes. All the sands are cemented together by iron oxide (rust!). Sometimes there’s enough iron oxide to bind the sands into hard brown sandstone.

As geological time progressed, all of Britain came to lay at the bottom of a warm tropical ocean.

The Gault Clay tells us something of this, but about 95 million years of the story are missing, eroded by the ice that deposited a thick layer of till on top of the Gault.

With the further passage of time and geological activity, sea levels declined and dry land appeared, from time to time to be covered by the glaciers of several ice ages. Glacial erosion produces ‘till’ as the glaciers ground their way across the surface. The latest glacial period ended about ten thousand years ago. Till is a mix of materials scraped from rocks by the glacier. The till here is almost Gault-grey, but includes pebbles of flint and Chalk, as well as fossils such as Gryphaea from the Jurassic Oxford Clay. The fossils are in good condition, so they did not travel far under the glacier.

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