Tedbury Camp - Jurassic sea bed EarthCache

Stand on a Jurassic sea bed where 170 million years of Earth's history is missing!

From the parking place, walk to the south west though the kissing gate and follow the East Mendip Way. This is part of a long-distance footpath that crosses the Mendip Hills from Frome to the Bristol Channel. Pass over a narrow metal bridge (N51º14.413 W002º21.708) on the right close to the railway line and turn left immediately, still on the East Mendip Way, along the side of Fordbury Bottom stream.
At N51º14.345 W002º21.791, take the path to the right and climb the steps on to the flat quarry floor. You have just climbed up the back face of a former quarry.

The rocks that you standing on at the top of the steps and which form the flat surface you can see in front of you are grey Carboniferous limestone. The yellowish coloured rocks ahead of you are the much younger Jurassic limestones. This site is amazing because you are standing on a flat Jurassic sea bed and 170 million years of Earth's history is missing between the two rock types! The flat, natural surface is called an unconformity. The quarrymen wanted the grey Carboniferous limestone as road aggregate and so cleared the yellowish Jurassic limestone away. However, better rock was available in other quarries so activity here was abandoned.

Geological Background
Sediments are weathered and eroded, transported and deposited, often with animal and plant remains in them. Over time they become cemented and compacted to form sedimentary rocks. Rocks are then uplifted by plate tectonic movements and the process (the rock cycle) starts again.

1. Deposition of the older Carboniferous limestone: The oldest rocks at Tedbury Camp are the grey sedimentary Carboniferous limestones deposited about 340 million years ago. These rocks are made up of fine grained calcium carbonate and the skeletons of marine animals, like brachiopods, corals and algae. The presence of these fossilised animals indicate that the sea was warm, of normal salinity and well oxygenated, suggesting that the area was close to the Equator at this time. After the animals died, their shells became buried in the fine grained calcite “mud” (i.e. calcite, not clay mud) to form the limestone. The sea here must have been deep enough so that waves did not re-erode the fine grained calcite.

2. Folding and Faulting: Around 290 million years ago, these Carboniferous rocks were uplifted, folded and faulted by the movement together of two large tectonic plates. The Mendip region lay to the north of the area of the main mountain building, and probably reached an original altitude of about 1500m. If you push a tea towel along a flat surface from one direction, it folds in an asymmetrical way. This is what happened to the rocks in this area. The pressure was basically from south to north with a shortening of the land by about 20km. At Tedbury, we are on the northern side of the Beacon Hill pericline.

3. Weathering and Erosion (and mass extinction): During a long period of geological time the folded Carboniferous rocks in this area were subjected to weathering, erosion, and then further deposition and re-erosion. The whole story is told in the series of EarthCaches Mendip Hills 1 - 4.
As the sea advanced and retreated around the Mendip islands during Early and Middle Jurassic times, the sea floor was probably buried many times by sediment and then re-exposed by erosion. Whenever conditions were appropriate, the sea floor would have been colonised by rock boring and encrusting animals. Such surfaces are called ‘rockgrounds’ and at Tedbury Camp it consists of very hard Carboniferous Limestone that was deposited more than 170 million years before being overlain by the Jurassic limestone. The rock-borers included worms and bivalves, whilst the encrusters were mostly oysters and serpulid worms. At the present day the most diverse communities of boring and encrusting organisms occur on calcareous rocks in shallow tropical seas. By around 175 million years ago the area had been eroded to an almost level rock platform.

Mass extinction: A mass extinction is defined as a period in time when the loss (extinction) of fossil species from the record, equals or exceed 75%. Two of these mass extinctions occurred during the 170 million year “gap” in the depositional record at Tedbury Camp. As a result, the fossils found before and after this period form very different collections of species.

4. Deposition of the younger tropical limestone: The Jurassic limestones here are called oolitic because they contain tiny spherical grains called ooids or ooliths. These form in highly agitated, strongly evaporating sea water of depth about 15 - 20m. A tiny shell fragment is rolled backwards and forwards in the current and gradually becomes coated with layers of calcium carbonate. When the spherical grain is too heavy to be supported by the currents it sinks to the bottom and builds up great ooid banks. These can be seen off the Bahamas today. The limestones here also contain shelly fragments of marine bivalves, belemnites and ammonites, many species of which had not evolved when the grey limestones were deposited. A simple ecology of the time can be pieced together from the fossils likely to be found ; bottom living (benthonic) filter feeders, such as bivalves and marine worms, along with grazing animals like gastropods, living on algae and plankton. Predators like ammonites and belemnites lived in the waters above. Evidence from further afield also suggests large marine reptiles such as plesiosaurs and ichthyosaurs were the top predators in these Jurassic seas. After the animals had died their shells became buried with the ooids to form the limestone you see here.

5. Folding and Faulting: Much later these beds too were uplifted but there is very little evidence of folding or faulting at this site. Further south the effects of this plate tectonic folding were more severe, creating the Alpine ranges of fold mountains across southern Europe.
No doubt there were many thousands of metres of later rocks deposited here but there is no evidence of rocks younger than the yellowish Jurassic limestones at this site. Those that were deposited have been lost by ancient periods of weathering and erosion. This site today is about 100 metres above sea level and is undergoing modern day processes of weathering, soil formation and erosion. Modern rivers, like the Mells River, have cut through the overlying Jurassic limestones in places to reveal the folded Carboniferous beds beneath.

6. Today: Quarrying (biological weathering?) has recently exposed the quarry faces. In time, if human action does not intervene, the weathered material from this area will find its way into the Mells River and be transported to the River Avon, and eventually become part of the modern day sediments in the Severn Estuary. The rock cycle continues!

What to do:
The following diagram may help you to find all the features to be seen here:-

1. At N51º14.359 W002º21.850
(a) Measure or estimate the dip angle of these beds of Carboniferous limestone. In which direction are they dipping?
(b) The rock was deposited horizontally. Why is it dipping now?
(c) Find a piece of unweathered rock and look at it carefully
i. what is its colour?
ii. what is the grain size - smaller or bigger than sand size?
iii. can you see any fossils whole or pieces of broken shell?

Note: If you are collecting small pieces of rock, label this one Carboniferous Limestone, Clifton Down Limestone. A serious collector would also note the location and date.

2. At N51º14.334 W002º21.847
Search the top of the Carboniferous limestone surface carefully here. You may be able to see the borings of marine creatures which lived in the Jurassic sea and bored into the sea bed (the Carboniferous limestone). If you can find the borings (there is a diagram in the notes above), estimate their average length.

3. At N51º14.353 – W002º21.884
(a) This is the Jurassic limestone; find a piece of unweathered rock and look at it carefully
i. what is its colour?
ii. what is the grain size - smaller or bigger than sand size? Can you recognise the ooids?
(b) identify the two hardgrounds A and B (marked on the diagram). This is where the deposition of sediment on to the sea bed slowed. Calcium carbonate in the sea water precipitated out and cemented the top of the sediment beneath. Describe what you see at these two hardgrounds.
(c) look around at the Jurassic limestone. How many types of fossils can you find - whole or pieces of broken shell?
(d) find some examples of the mineral calcite. It is white in colour and can form well-shaped crystals. It often replaces the original shells of the fossils or occurs in joints in the limestone. Look at the face around N51º14.335 W002º21.904, you will notice numerous near-vertical faults and shatter zones. You may find crystals associated with some of these. Describe these minerals.

Note: If you are collecting small pieces of rock, label this one Middle Jurassic Inferior Oolite, limestone. A serious collector would also note the location and date.

4. Now walk to N51º14.344 W002º21.897 and face north. If you look down at the unconformity surface and in front of you and behind you, you will see a line which is nearly north-south. This is a fault line and the two sides of it have moved in different directions horizontally. Go to N51º14.333 W002º21.901 and look down. What can you see here that is not repeated on the other side of the fault?

5. Go to N51º14.337 W002º21.881. Here there is a prominent linear feature running approximately SW-NE. Can you suggest what it is? Is the surface between the two sides bored?

You can either return to the car park by the way you came or take an alternative route. Take the path at N51º14.357– W002º21.882. At N51º14.365 W002º21.994 turn right off the track on to a path which leads down to the River Mells. At the bottom, turn right. (If you turned left here you would pass the remains of Fussells ironworks on your left and, eventually, you would arrive in Mells.)
Continue to N51º14.384 W002º21.882 where you will be standing beneath acave in the Carboniferous limestone.
At N51º14.380 W002º21.868 a slump in the limestone on your right can be seen.
At N51º14.380 W002º21.852an amazing bedding surface in the Carboniferous limestone can be seen.
Continue along this path until you reach the bridge over the Fordbury Bottom where you were earlier.

Please email your answers to me, JurassicEdie