The site is about 5km east of Stoke-on-Trent within Park Hall Country Park. This is as a National Nature Reserve managed by Stoke-on-Trent City Council. Permission for this EarthCache has been granted by the Countryside Team Coordinator, City of Stoke-on-Trent. All paths have public access and there is a visitor centre.
Background geology
Park Hall Country Park is based on a sandstone ridge comprising red sandstones and conglomerates (pebble beds) of the Lower Triassic Sherwood Sandstone Group, laid down between 250 million and 215 million years ago, when Britain lay much closer to the equator, at about the same latitude as the Sahara Desert today. The environment of this time was that of a hot, arid desert and the pebble beds were deposited by flash floods and intermittent rivers, each pebble bed possibly being laid down in one event over a short period of weeks. After flash floods, great thicknesses of sands and gravels were laid down as horizontal bars on the beds of wide river channels, with finer materials settling out in quieter waters. Evidence of these beds can be seen in the quarry faces. At the edge of sand or gravel bars, the moving sediments avalanche down the slopes. This “cross-bedding” slopes to the north at Park Hall indicating a river flowing from the south. An examination of the pebbles at Park Hall also indicates a flow from the south. Most of the pebbles are fairly smooth and well rounded, indicating a long transport by water.
Walk to the viewpoint at N53 00.135 W002 06.307 from where you can look down into Hulme Quarry. As you can see from the notice board this is an important geological site. Try to imagine this place about 230 million years ago; it was very hot and the air was dry. You could have heard strange noises from behind the nearby rocks and you would have known there were small dinosaurs watching you. If you stayed here long enough you could witness a flash flood when a huge amount of water would rush through depositing the pebbles you can see, both in the quarry face opposite and on all the paths around you. Have a look at some of the pebbles. They are made of a variety of different rock types and don't come from this area. Some come from S.W. England and some from N.W France. The red colour of the rocks is due to the presence of haematite, an iron oxide mineral, found mainly in fine clays, which sticks to the quartz sand grains, and is typically formed in hot arid conditions. In patches a buff/grey colour can be seen. One possible cause for this is the reduction of the iron oxide due to the presence of organic materials trapped in the sediments, which use up some of the oxygen in the decay process. The rivers flowing in this area were enormous. If you could get close enough you would see that in some of the beds the pebbles get larger upwards but they also vary sideways. Pebbles getting progresively larger indicates that the river depositing them increased in energy and the fact that the beds vary laterally means that there were braided streams (looking like plaits) over the whole area. Towards the top, the pebbles give way to sandstones which indicate a much lower flow of water. In fact some of these may be wind-blown deposits as there is some evidence for sand dune formation.
These deposits have been exposed by quarrying and they offer an opportunity to view the sequence of sandstones and pebble beds; that is why the area has been designated a Site of Special Scientific Interest (SSSI) leading to Park Hall Country Park receiving National Nature Reserve status. As well as the geological interest, the quarry is a nesting site for kestrels, little owls and sand martins.
Further information is available in the Visitor Centre - for opening hours telephone 01782 331889.
What to do:
See how many different types of rock you can find in the pebbles around you. Some may even contain fossils.
A photo from here would be good but it's optional.
From here make your way to N53 00.327 W002 06.670. You are now standing in an old quarry, Sand and gravel from here were used for aggregate. The rocks here are similar to those you saw towards the top of Hulme Quarry. Here you can examine the sandstone more closely. You can also see a band of different material about 0.5m from the ground, marked out by vegetation growth. This rock is made of smaller grains than the rest of the sandstone and may represent deposition in a temporary lake. Small grains indicate low energy deposition unlike the high energy environment of the pebbles seen earlier.
What to do:
Look very carefully at the rock that forms the main part of the face and describe what you see.
How do you think the sand grains are held together?
Now make your way down to N53 00.298 W002 06.710 and look at the rocks to the left of the path as you are facing downhill. Although the bank is rather overgrown, try to find several pieces of rock. They are a confusing mixture of several rock types and are nothing like the red sandstones and pebble beds you have seen at the previous sites. Here there are grey, black mudstones, bits of limestone and some pieces which look like coal. In fact this is a spoil heap from the coal mining that was taking place in the valley below. These are from the older Carboniferous rocks which occur below the red Triassic sandstones seen previously. Here you are standing at the edge of the coal field.
Please email your answers to me, JurassicEdie