Clue to what is beneath
The path here is wet and slippy at any time of the year – take care. Why is it so wet ? Use the map to find the rock here.
The rock beneath here is called the Avon Group. This mainly consists of beds of limestone (evidence of marine conditions) and beds of mudstone. This muddy area possibly overlies some mudstone beds, which not only stop rain water draining away, but also forces water that has flowed down through the limestone to come out to the surface.
Which rocks let water through
Sandstone is made up of grains of sand and there are often gaps between the grains where water can get through.
Limestone also lets water through as it often cracks and it will even dissolve as water goes through the cracks, making the cracks even bigger.
Mudstone is a very fine grained rock and the grains are so close together that water cannot get through.
Granite is made of interlocking crystals, so it does not let water through.
The tiny particles of clay minerals have electrically charged surfaces, which binds the particles together and will also trap water molecules. This electrical trapping causes muds to bind all sorts of minerals, which in sandstones would just wash through. This in turn can make soils derived from mudstone rocks a rich source of minerals supporting plant growth.
Towards Task Site 7
As you leave Task Site 6 you should get an ‘Environment Change’. Check out what is happening.
The high fencing and gate is to enclose the deer park. From now on you may spot deer amongst the trees. Did deer exist on the planet around 340 million years ago? You can find out when mammals first appear in the fossil record via the information you can access by touching the clock. Interact with the time scale (list of geological periods) and the white arrow at the top of this scale.
Task Site 6 Question
What kind of rock do you think is most likely to be stopping the water from sinking through at this location?