Osmington Mills 18th September 1999
(First Time For Every Thing!)
George Raggett led the field trip. The object of the field trip was to enhance our understanding of geological processes from direct observation of Coastal exposures of strata from the middle to upper Jurassic age and the Cretaceous succession. The field trip presented us with the chance to observe or deduce structures and sedimentary processes as we encountered them on our day out. In exploring sedimentary processes, we were also on the hunt for invertebrate and trace fossils.
As some of you may have deduced, this was my first field trip with OUGS, in my first year as an OU student. I think I may have over done the "I think I know what I am doing look!" as when everyone saw me armed with my notebook pencil poised, they volunteered me to write up this, only my second ever field trip! Anyway, I apologise in advance foi any gaffs that I make.
The first thing I learned about OUGS Field trips is that Weather does not stop play! Indeed I now firmly believe that there is no such thing as a fair weather OUGS geologist. The weather that Saturday started off atrociously. Driving wind and rain. Never the less there was a good turn out of about twenty or so people, all as keen as mustard, to get amongst the rocks. About 10.30 am we all set off from the Smugglers Inn car park, down to the beach area, locally known as Hannah’s Ledge. Looking west we could see the dark shape of Redcliffe Point. The dark rock at the headland is Oxford Clay, the oldest layer of sediment visible in the area, which was laid down towards the end of the middle Jurassic era, approximately 174 Ma ago. The darkness of the Oxford Clay suggests that when it was deposited, there was not much oxygen. This implies that the Oxford clay sediments were part of a deep offshore basin. The main fossil to persist in the Oxford clay is the Round Oyster. Other lighter clay sediments laid above the Oxford Clay were deposited when the sea was much shallower, indicating a shallow shore environment. Much more oxygen was present.
From the way the land rises and falls, you can distinguish an anticline and uplifted beds. Looking out across the bay towards Portland you can trace an imaginary line which slopes up from the main land in a curve and down in line, with the slope of the landscape that falls away to seaward, that is Isle of Portland fig 1. I was amazed when George explained that where Weymouth Bay is now, was one big hill! The local faulting and the anticlines were caused when the continent of Africa collided with the Continent of Europe. (This created the Alpine Orogeny, which is still growing upwards.) The youngest rocks are the limestone sediments, still present on Portland. Where Weymouth Bay is now, the limestone there was largely eroded away. Sea levels rose and eventually flooded in to form Weymouth Bay. Above the Oxford Clay is a banding of different clay, called Kimmeridge clay. Kimmeridge clay was laid down about 155 Ma ago, in the upper Jurassic era. Present in the Kimmeridge clay are lots of flat oyster fossils. George was soon pointing out lots of fossil Oyster shell fragments to me with my untrained eye.
Gill had a lucky find, when she picked up a fossil echinoid (Sea urchins). For my part, I was a dab hand at finding flint!
After a brief spell looking west in the bay, we turned round and proceeded east along the base of the cliff towards Bran Point. We were shown the strata between the Oxford and Kimmeridge clay sedimentation, which are known as the "Corallian Beds". Some of the sedimentary layers George described as bioturbated. This is a process on the sea floor, where sediments are disturbed by the burrowing activities of organisms. In some places the layers were indistinguishable due to this process. Where there was a distinguishable pattern of sediments, you had clay layers, on top of which was grit and on top of that was limestone. Different sea environments created these rhythms. Deep water followed by shallow water environments. The limestone layers marked shallow sea environments with good aeration (fig 3). George pointed out to me the different layers of sediments within the Corallian beds, starting with Nothe Grits, Trigonia hudlestoni Bed, Nothe clay and Bencliff Grit, forming the lower Corallian Beds and the Osmington Oolite, Tngonia Clavellata Beds, Sandsfoot Clay and Sandsfoot Grit, which form the Upper Corallian Beds. (What could not be seen here was the Ringstead waxy clay or the Ringstead Coral Bed in the upper Corallian Beds, which we saw later in the afternoon). In a layer of carbonate sandstone, known as the Osmington Oolite, ripple marks could be seen in the sandstone, which was further evidence of a shallow, tropical sea environment. Similar rocks are found of the Grand Banks and off the Bahamas. George also pointed out to me in the sandstone large doggers; large rounded boulders of sandstone, where large calcite concentrations made the sandstone less susceptible to erosion than the surrounding sandstone (fig 4). Also on the beach there were traces of oil reservoir sand. This rock is high in organic content. Not suprisingly the rock smelt distinctly of oil!
The original plan was to walk round to Ringstead, but the prevailing weather conditions had kept the sea well up, also the weather showed distinct signs of getting worse rather than better. It was decided that having got wet on the outside, we might as well get wet on the inside as well, so we all retreated back to The Smugglers Inn for lunch. Sure as eggs are eggs, the heavens did their worse, but we did not mind as we had made the comfort of the Inn in the nick of time.
Sufficiently replete, the large majority of the group decided to drive round to Ringstead and tackle the coastline up to Bran Ledge from the other side. George was especially keen to see if there was still evidence to be found of the Ringstead Coral beds after work had been done on sea defences in the area. For the first time that day, the weather decided to co-operate. Now I should mention that the sea defences, that had not so long ago been carried out, were quite controversial. At Ringstead, there are about a dozen homes, which are in danger from coastal erosion. Unfortunately Ringstead is the site of a rare horizontal coral bed that exists nowhere else in Dorset. Much argument was made by Geologists to preserve these beds, but to no avail. The Local authorities shored up the beach at great expense with riverbed gravel. This covered up the waxy clay deposits in which the coral beds are found. It was feared that the coral beds might have been destroyed by this activity. As had been argued prior to the work taking place though, the sea has now started to wash away the gravel and the waxy clay deposit and the coral beds, we found are once more becoming exposed, though they are not as prominent as they were before. Of course, the homes are still threatened by coastal erosion.
As we walked up along the shore, we all got into palaeontologist mode, all hunting for that elusive coral fossil. George, whose eye was much better in than the rest of us eventually found a couple of examples. During our fossil hunt, we found examples of Trigonia Clavellata (a kind of clam), bivalves and even some Nucleolites though at one point Chris became so interested in a wreck, that he failed to realise that he was standing on a large, flat rock, covered in fossils (fig5). Having reached Bran Point from the other side, we turned round and called it a day. It had been a very interesting experience for me. As someone new to Geology, (I hope to be studying S260 this coming year.) I found it fascinating, though the depth of knowledge amongst the group was a little awe-inspiring at times. Having said that, everyone in the group was only too willing to explain everything to me. I can see now why palaeontology is so addictive. Back at the car park, we all showed our appreciation to George, who had put a lot of work into making the day the success it was. Thank you George, for making my first OUGS field trip so enjoyable and interesting.
Peter McLeod.
"Geology of The Country Around Weymouth"
Dr W I Arkell 1st published 1936
"Osmington Mills" Briefing sheet
George Raggett
"The New Penguin Dictionary of Geology"
Philip Kearey 1996