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.
Within the Lower Greensand deposits of till, clay and the various horizons of sand (ref sections Formation & Sand Features), a variety of fossils are to be found.
The Gault Clay tells us something of the period when all of Britain lay at the bottom of a warm tropical ocean, 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. The till, which itself is almost Gault-grey in colour, contains:
A mix of materials scraped from rocks by the glacier, but includes pebbles of flint and Chalk;
Worn, reworked fossils from the Gault, where it has been previously eroded and carried forward by glacial movement.
Fossils such as Gryphaea from the Jurassic Oxford Clay. The fossils are in good condition, so they did not travel far under the glacier.
The Gault Clay
The Gault Clay which caps the layers of sand was deposited at the bottom of a tropical ocean. There are lots of tiny marine fossils in this clay, including ammonites, belemnites (little squid) and bivalve shells. The clay horizons have been analysed for palynoflora, which proved to be particularly rich in brackish species and terrestrially-derived pteridophyte spores, with some freshwater algae. This evidence, together with the sedimentary structures, indicates a near-shore estuary mouth location.
||Belemnite (Little Squid)
The Silty Beds
The Silty Beds which appear only in the Heath & Reach area include pale sands laid down in fast currents as well as fine-grained organic sediments rich in fossils.
|Silty beds - silts, sands and clays laid in tidal flats
The Red Sands
The base of the Red Sands shows trace fossils including Planolites, Teichnichus, Skolithus, Ophiomorpha, Macronichus and echinoid burrows. Sedimentation was too rapid later on to allow organisms time to burrow. In Stone Lane Quarry is the Cirripede Bed which lies under the Gault in the southwest corner of the quarry. The bright red sediments contain many fossils, including barnacles, oysters, fish teeth, icthyosaur bones and corals.
The Silver Sands
The Silver Sands contain abundant fossils of wood (Picture 4.2.h), largely from cycads, tree ferns and pteridophytes. The wood is preserved in several different ways:
charcoal (indicating forest fires), lignite (often in pyrite coated nodules, testifying to rapid deposition), limonite or haematite cemented nodules, and tree trunks.
|Fossil wood lodged within silver sand (Stone Lane Quarry)
Evidence of the newly evolved angiosperms is a very rare occurrence in the Aptian fossil record, which makes the search in these sands particularly exciting!
The Brown Sands
They are beige in colour, fine-grained with quartz grains (pale), many mica flakes (silvery and shiny) and occasional black rock fragments. You may also see hard ironstone horizons (iron pan), and many fossil burrows dug by worms, shrimps and other animals living in the estuary. They commonly include limonite cemented wood fragments and other iron nodules and iron pan horizons.
|Stone Lane Quarry
||Fullers Earth Specimen
The Brown Sands are also notable for abundant trace fossils including Taenidium, Siphonites, Planolites, Rhizocorallium and Teichnichus. Within the Brown Sands lies a layer of Fullers Earth. Fullers Earth formed from layers of ash vented by Cretaceous volcanoes, probably in northwest Europe, but possibly west of the UK under what is now the Atlantic. Buried at depth, the ash changed chemically over time to become Smectite clay. The slopes of the Greensand Ridge are among the few places in Britain where Fullers’ Earth is found.
The Phosphate Pebble Base
The phosphate pebble bed found at the base of the Lower Greensand from Great Brickhill (Bucks) to Potton (Beds) and Upware (Cambs) is made up of reworked phosphatised marine fossils (Picture 4.4.e). These were one of the ‘coprolite’ beds exploited for fertiliser production during the late 1800s to early 1900s.
|Coprolite - cm scale. By kind permission B. Horne. Copirignt B. Horne
‘Coprolite’ comes from the Greek word kapros and lithos, meaning ‘dung’ and ‘stone’. Real dinosaur dung dating from the Jurassic period is found in Britain, but the coprolites of Bedfordshire, Cambridgeshire, Hertfordshire and Buckinghamshire are nodules of phosphate-rich Cretaceous sediment, often containing fragments of fossils.
|Coprolite nodules (Bedford Museum)
Rising sea levels due to global warming in the Lower Cretaceous period created a seaway running southwest from the Wash across Bedfordshire and on toward the Isle of Wight. The currents and tides washed ammonites and other fossils out of the Jurassic clays. These worn, rounded fragments were deposited into areas with high levels of phosphate from dead shellfish and other animals. The nutrient-rich sediments coated these derived fossils (derived from other sediments) to form concretions while calcium phosphate slowly replaced the calcium carbonate of the fossils. Burrows of animals living in the seafloor filled with sediment: the casts are known as trace fossils. Both trace and derived fossils are known as coprolites when they are mined from the Gault Clay and the Woburn Sands.
To read more about the Coprolite industry click here.