South Elmsall Quarry: The layers of rocks at South Elmsall Quarry are Magnesian Limestones of the Cadeby Formation. These were deposited during the latter part of the Permian period, about 255 million years ago. The rocks are predominately carbonates of calcium and magnesium, which forms the mineral dolomite. This is a natural compound, not a mixture.
The beds were deposited in England after a long period of weathering and erosion which left no trace of rocks from the early Permian period. At this time in Earth’s history, studies tell us that the distribution of the continents looked very different from today’s. All of the major land masses were part of a giant single continent, called Gondwanaland, which stretched from the South Pole, where there was an ice cap similar in size to today’s, across the Equator and well into the northern hemisphere. There was no evidence of a northern ice cap at this time. That part of the continent which would later become South Elmsall Quarry was near the northern edge of this large continental land mass, where a long period of erosion had worn away the uplands caused by the folding of the Carboniferous rocks, and left a gently sloping desert landscape, ready to be partially flooded by a shallow sea. This ancient, and now long-gone “sea”, is called the “Zechstein Sea” and lay in the subtropical latitudes of the time.
The layers of rocks deposited by the Zechstein Sea are geologically very unusual. They are mainly evaporite rocks. These are layers of rocks made up of minerals formed by deposition from evaporating seawater, and include dolomite, gypsum, anhydrite, halite (or rock salt) and very rare and valuable potassium and magnesium chloride minerals like sylvite (KCl). At South Elmsall, however, only the older, dolomitic, parts of these layers are exposed, and they represent evidence of the conditions close to the western edge of the Zechstein Sea, which extended eastwards into Germany. The younger evaporite beds are found in boreholes further east, and are buried now under later layers of rocks. The area of South Elmsall seems to have been close to the western coast of the Zechstein Sea, where limestone was being deposited, containing fossil shellfish indicating a marine environment. No marine deposits are found west of Leeds as this was a desert land area at the time. Gentle wave action in the sea, which kept turning over tiny shell fragments, allowed them to be coated with calcium carbonate to form tiny 2mm diameter spherical grains called ooliths. The size of the ooliths are an indication of the amount of wave energy at the time of their formation. Larger waves can roll larger, (and therefore heavier) ooliths, until they become too heavy to move. Layers of rock containing these ooliths can be seen in the South Elmsall Quarry, and indicate very shallow water.
Also living in this sea were small branching organisms called bryozoan, which began to form domed mounds, or reefs, about a metre or so across. The water depth was probably no more than 5 metres, and almost without tides. These reefs then died out, but left a suitable platform for mats of photosynthetic blue-green algae to develop, trapping layers of sediment, and slowly building up bigger domes over the tops of closely spaced bryozoan reefs. Reefs of this type can be seen today in Shark Bay, Western Australia.
The reason these bigger, ‘algal’ (cyanobacteria), reefs called ‘stromatolite’ (layered) reefs survived is probably because the water had become too saline and did not allow gastropods, fish and other animals, to survive there and graze them away. We are fortunate at South Elmsall in that the reefs are revealed by the differential weathering of the layers of sediment trapped by the algae (cyanobacteria) and bryozoans. These are the curving structures easily visible in the eastern face of the quarry.
After deposition, these Permian rocks have been uplifted and gently tilted to the east. Today they extend deep below the North Sea and into Germany. These thick salt beds helped to form many of the oil traps for North Sea oil and gas fields.
Magnesian Limestone was quarried for use as a road aggregate and local building stone, and can be seen in local houses and walls. Dolomite is also used as a lining for refractory furnaces.
EARTH SCIENCE PRINCIPLES
In this area it is possible to demonstrate the following Earth Science principles.
- The Principle of Uniformitarianism: The biological, physical and chemical processes we see today, operated in much the same way in the past. “The present is the key to the past”.
- The Principle of Original Horizontality: Bedding planes represent the original horizontal at the time of deposition of sedimentary rocks. Their current angle shows the accumulated amount of distortion caused by earth movements since deposition. An exception to this principle is the underwater scree slopes at this locality which were deposited at a steep angle.
- The Principle of Lateral Continuity of Beds: This states that sedimentary layers extend in three dimensions and might therefore be found elsewhere.
- The Principle of Superposition: In a bedded sequence of strata, the oldest layers were deposited first, and are found below the younger layers, which were deposited later.
- The Principle of Cross-Cutting Relationships: Structures, like faults and joints, which cut through rocks must be later, and therefore, younger than the structures they cross cut. They must also be older than the ones that cut across them.
To log this cache, please send me the answers to the following questions:
1) Explain in your own words how the Bryozoan reef feels to the touch. How does this differ from the texture of the Stromatolite reef and the base of the reef?
2)What is the width of the Bryozoan reef?
3) Due to the underlying Magnesian Limestone, the soil around the quarry is very rich in Lime. This helps to generate a characteristic type of Limestone grassland vegetation which is noted for its colourful flowers. Please fill the blanks below to show some of the plants can be found in the quarry:
• Greater ********
• M*** T******
• Wild *******
• ***** Grass
• K***** V****
Although not compulsory, we would love to see a picture of you/your GPS at this location.
Please send your answers through my profile to the set questions before logging your find. All answers will receive a reply.
- Many thanks for allowing the Earthcache to be placed to the Countryside Ranger, Countryside Environment and Street Scene - Wakefield Council