Healey Dell - Sedementation. Lower Coal Measures. EarthCache
Healey Dell - Sedementation. Lower Coal Measures.
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Healey Dell is a local nature reserve that lies to the north of Rochdale. Through it runs the River Spodden.
This earthcache takes you to the banks of the River Spodden, from where you can look across to evidence of the Lower Coal Measures, in the form of Broadley Bluff.
From where you stand you can see a cliff above the river. This is Broadley Bluff. The rocks which you see are sedementary rocks.
Sedimentary rock is a type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles (detritus) to settle and accumulate or minerals to precipitate from a solution. Particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, sediment was formed by weathering and erosion in a source area, and then transported to the place of deposition by water, wind, mass movement or glaciers which are called agents of denudation.
Sedimentary rocks are deposited in layers as strata, forming a structure called bedding.
Here you can see sequences of different types of rocks. These were formed during the lower coal measures of the Carboniferous period.
The Coal Measures Group is a term which refers to the coal-bearing succession of rock strata which occurs in Great Britain within the Westphalian Stage of the Carboniferous Period. The Coal Measures Group is preceded (underlain) by the Millstone Grit Group which is of Namurian age.
The Westphalian is a stage in the geological history, and it lasted between 313 and 304 million years ago. It is a subdivision of the Carboniferous system.
At the time that these rocks were laid down, there was a feature known as the Pennine Basin.It was a sedimentary basin which reached from the Southern Uplands of Scotland in the north to the Midlands to the south. This area a vast delta, with rivers laying down sand, rock and clay in layers known as sediment.
Looking across the River Spodden you can see different types of rocks. There are sandstones, mudstones and shales. They all appear as different layers. Where the layers change its is known as a bedding plane.
At the time of the carboniferous, Britain had a hot climate. This area formed part of a large river delta system, the rivers carried large volumes of sand and mud, which were deposited in the delta system. At times, the areas between the river channels formed swamps covered in tropical forest. At other times, the sea rose over the delta, and flooded the forests.
Sand was deposited in the river channels, and over time sandstone was formed. Its is a sedementary rock, this means that the grains of sand that make up the rock were laid down and then compressed over time to make the rock that we see today. Shale is from where mud was deposited in the river floodplains or in the sea.It is thin and friable.
Mudstone (also called mudrock) is a fine grained sedimentary rock whose original constituents were clays or muds.
Looking across the river look upwards and then look down. You will see that the different layers look different. At the top, you can see sandstone layers, then mudstone, with shale at the bottom. As an indication that conditions changed over time, you can also see sandstone further along the river.
There is also a hole evident, this is the entrance to an old mine shaft, indicating that once coal was mined in the area.
THERE IS NO NEED TO CROSS THE RIVER. DO NOT ENTER THE HOLE.
This earthcache, gives you the opportunity to look at different strata, and enables those of all abilities to have this experience without having to get down and dirty.
To log the cache, please contact Hillgorilla with the answers to the following questions.
(1) What does the sandstone look like.
(2) What does the mudstone look like.
(3) What does the shale look like.
(4) Out of the three different types of rock, which have the thicker strata?
(5) As an option, please take a picture of the following:
(i) The different layers.
(ii) You or your GPS at the site.
There is a network of paths at Healey Dell. Take your time and explore this interesting place.
Sources of information - internet search. Local knowledge.
Additional Hints
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