Essex Rock & Mineral Society Field Trip
Elsenham Quarry, Essex
Report of the Society’s field visit on 1st March 2008

The Society’s visit to Elsenham Quarry on 1st March was a success with 20 members and guests attending. Thanks are due to Brett Aggregates Ltd. for allowing the Society to visit, which was our first visit to this quarry. Some 20 members and guests attended and an enjoyable day was had in good weather despite strong winds. I apologise to those members who asked to come on the trip but were told it was fully booked. No maximum number was originally given for this trip because, from previous experience of visits to Essex sites, this was thought to be unnecessary. A limit of 20 people was considered appropriate because of the difficulty of moving around and managing a party in a working quarry.

London Clay forms the bedrock which is exposed in the floor of the quarry and in the drainage trenches. Laid down in a subtropical sea about 50 million years ago it contains fragments of fossil wood but no other fossils have been found here. Immediately on top of the London Clay is a metre or two of iron-stained sand and above this are several metres of pale grey sand. At the top of the sequence is a capping of several metres of glacial till, or boulder clay, which is typically present across the whole of north-west Essex and often referred to as the Boulder Clay plateau. It is an unsorted clayey residue laid down by the Anglian ice sheet about 450,000 years ago and contains numerous glacial erratics including some Jurassic fossils brought here from the Midlands by the ice sheet.

Up to the late 1970s it was thought that the various layers of sand between the London Clay and the till were glacial outwash, laid down perhaps in front of the Anglian ice sheet. Since then these sands have undergone a radical reinterpretation. The iron-stained sand on top of the London Clay is now thought to be Red Crag, absent of any fossil shells because they have been completely dissolved by percolating ground water. At the base of this sand, and lying directly on the London Clay, is a basal pebble bed which is known to be the western equivalent of the Red Crag nodule bed at Walton-on-the-Naze. Here at Elsenham it contains typical phosphatic nodules, pieces of hard sandstone called ‘boxstones’, and occasional worn fragments of whalebone but without the high polish of the material from The Naze. One piece of whalebone had a piece of boxstone attached which proves that these bones and the boxstones are the same age. Other fossils were very rare and extremely worn, and all of them derived from older formations such as the London Clay.

The light grey sand above the Red Crag sand is now interpreted as Chillesford Sand which forms part of the Norwich Crag formation. It contains seams of flint pebbles and was probably laid down in an estuary about 1 million years ago but the exact age is controversial.

There are numerous sedimentary structures, particularly cross bedding, which provide evidence of tidal currents. The lack of any trace fossils (burrows or other markings made by marine creatures) indicate that the sand was deposited rapidly. Some years ago, when the quarry was working pits further west, a further layer of sand was present above the Chillesford Sand which was interpreted as part of the Kesgrave Formation which was deposited by the River Thames when it flowed across this area, far to the north of its present course, at least 600,000 years ago.

The record of fossils found on the day looks impressive but is very misleading when presented as a list. Apart from fragments of pyritised wood from the London Clay, fossils were only obtained from the Red Crag nodule bed and from the Anglian till and these two deposits only contain fossils derived from elsewhere and are therefore always worn and are often only fragments. Typical of this was a crab Zanthopsis leachi from the Red Crag nodule bed (derived from the London Clay) that was so worn that it was only just recognisable as such and could easily have been dismissed as just a phosphatic nodule. However, it was the only record of a crab from this quarry so it is worth recording. Other notable fossils from the nodule bed and derived from the London Clay included pincers of the lobster Hoploparia gammaroides, a crinoid stem, and a few extremely worn shark and ray teeth including Aetobatus sp., Myliobatis sp. (?), Phyllodus sp. and Otodus obliquus.

A small, worn tooth of Carcharodon megalodon* also turned up which was a lucky find by
Rick Johnson.

Small tooth? (in situ) The pebbles are approximately 15 to 20mm in diameter.

Rick's prefered method for finding sharks' teeth. Hands in pockets, kick water over pebbles to scour away shingle, then pick up teeth.

The nodule bed also produced some interesting rocks such as bunter quartzite cobbles, vein quartz, welded tuff (almost certainly originally from North Wales), miscellaneous igneous rocks and a small fragment of silicified fossil wood. This indicates that a very early ancestor of the Thames was bringing these rocks into the Red Crag Sea from the west. A piece of puddingstone was also found that may also have come from the nodule bed. The Anglian till at the top of the quarry produced some typical Jurassic fossils such as devil’s toenails (Gryphaea) and belemnites but one interesting find was a fine brachiopod derived from the Cornbrash.

Mention must also be made of a fine Jurassic septarian nodule that was discovered at the base of the till at Elsenham Quarry in November 2007 (pictured on the cover of the January newsletter). The nodule had been moved next to the car park and we were able to examine it now that it had been washed clean by the rain.

The surface was covered with scratches made when it was lodged at the base of the ice sheet but of particular interest were the impressions of two ammonites, typical of the Kimmeridge Clay of the Midlands, which is clearly where the nodule originated.

*Those members that know about shark taxonomy will notice that I have not used the genus Carcharocles which now seems to be the new genus for this beast. However, it seems to me that there are two schools of thought on this. The debate can be read on the internet (see http://www.elasmo-research.org/education/evolution/carcharodon_vs_carcharocles.htm)

Gerald Lucy