Essex Rock & Mineral Society Field Trip
Althorne, Essex, April 17th 2005
Our meeting point was the station car park at Althorne,
the leaders Rick Johnson and Jeff Saward had brought along their collection
of material from the tidal foreshore exposure of London Clay at Butts
Cliff.
After an introductory talk about what we could find and where we should
be looking at this site, we set off on a moderate walk along the sea
wall to Butts Cliff. One could easily walk along this path and never
notice the location and end up in Burnham.
Just
one of the trays with a selection of sharks' teeth and vertebrae recovered
by Rick and Jeff from this site. This represent many years work. One
is lucky to find a few specimens on any tide.
This box contains 10 different species of fossil crabs found at this location. Before preparation these were small phosphatic nodules perhaps with a hint of the crab showing. The shape of the nodule is the first thing one notices when collecting. One just needs to get their eye in. (I came back with a small nodule containing two crabs, one juvenile, the other adult of the species Glyphithyreus wetherelli). The faunal rage is similar to that found at the Isle of Sheppey but when one considers the limited foreshore exposure at Butts Cliff (approx. 60 m.) the species count of this collection is staggering.
Members scouring the pebbles and gravels in search of sharks' teeth and crab nodules. Kneeling and searching the gravels is the most profitable technique for finding the smaller fossils. One needs a kneeling board of sorts. The other method of standing and viewing the deposits tends to lead to the recovery of bigger fossils and less dirty trousers.
The tooth (centre left of picture) is from the shark species Weltonia burnhamensis (I can't remember what was said but it closely resembles the specimen photograph in the excellent had out prepared by our leaders), and was found by Rick Johnson one of our co-leaders on this trip. The specimen is about 2cm in length. To the bottom right of the picture is the tip of a largish pair of tweezers. Take note: finds like these are found amongst the gravel accumulations along the foreshore. Rick explained that they are eroded from the foreshore exposure of the London Clay and transported by long shore drift, down river. He speculates that the wake of the many boats that pass by, help to pushing the specimens gentle up the beach. Further down river the sediments re-enter the fluvial system. So what is being collected are specimens in transit, so to speak.
Our other leader, Jeff Saward, specializes in collecting Crabs and Lobsters. Jeff explained that to find these one has to look for the phosphatic nodules. Generally those which contain crabs have a distinctive oval shape and generally the only indication that they may contain a fossil is a partially exposed edge. One has to painstakingly carve away at the nodule to reveal the specimen to its best, and this is arduous task using a scalpel to carve away the phosphatic matrix. It can take many hours work but some of the specimens that Jeff has prepared are stunning.
In the exposed cliff section one can find a line of septarian nodules, or more commonly called cement stones. These were once collected for the making of cement in olden times, this greatly increased the rate of cliff erosion. However, they now accumulate at the base of the section and assist in slowing the erosion natural helping to maintain a balance. However, many other sections of the river bank are protected by a sea wall to protect farm land from flooding.
There are two other nodule bands that can be seen exposed on the foreshore on a good low tide. One of these may be a continuation of the nodule bed seen exposed in the cliff.
It was said that one member collected a baryte rose from one the nodules on the day (I did not see this).
This view is of the London Clay and nodule band in the cliff section. Note the apparent angle of dip from right to left that is indicated by the position of the two cement stones. Each nodule was approximately 30cm in length. Extending a line between the two, and just right of centre of the two nodules can be seen the faint impression of where a third was. The cavity wall of this was encrusted with selenite. Additionally, selenite specimens can be found scattered on the foreshore close by. I personally believe that the formation of the selenite is later than the formation of the nodules and is part of the weathering process as the cement stones near exposure through erosion.
It is not unusual to come across small tiny worked shards of flint. These are typical of those said to date from Mesolithic times. Rick and Jeff asked us to look out for bone material as they have concluded that the London Clay was cut by an old river channel dating from those times. At low tide, and beneath any modern silting, there is a distinct change of the sediments which earmark the location. From here they have recovered some bone material including a Bear claw and Wolf teeth. All this puts another slant on this location dating back to the Mesolithic. Rick & Jeff are trying to collect as much of this material for expert identification to complete a faunal list for this locality. A task not made easy by the inclusion of relatively modern bone from farm animals and the like that is often found on the foreshore.
For a couple of us, we found it very pleasant to sit on the riverbank and watch the boats go by whilst having an picnic.
Additionally, as this Essex exposure of London Clay can account for: a third of all known lobsters, half the known number of crabs and most of the known species of sharks' recorded for the London Clay; one should give serious thought to paying this site a visit or two each year.
For those of us that need to know, the exposure of London Clay at Butts Cliff, Althorne has been assigned to the Ypresian of the Eocene. Division 'D' of the London Clay or Nanno Planktonic (NP) zone 12.
Below is a list of some of the species that have been found at this site and compiled by Jeff and Rick. Please bear in mind that they have been working this site since the late eighties. Fossils are not that easy to find but it is a most pleasant location on a warm sunny day.
|
Crabs 01 Basinotopus lamarcki (Desmarest 1817) Lobsters 01 Homarus morrisi (Quayle 1987) Rays 01 Aetobatis irregularis Agassiz |
|
Sharks 01 Abdounia beaugei (Arambourg,
1935) |
Lastly: Always the Lapidary, I did notice a few banded pebbles, not many but enough to rouse my curiosity. Perhaps I'll have a poke about looking for these and leave the fossils to the others in future, I enjoyed the day immensely.
All text (with the exception of the fossil list) and photographs © R Coleman, May 2005