Thornton Force.
Study the rocks behind the waterfall. There are two different types. In the picture on the left the people are standing on Lower Ordovician slates.
- At what angle are these rocks dipping?
- Overlying the slates are Carboniferous limestones. What is the dip of these beds?
- If Carboniferous rocks are overlying Ordovician rocks and there is a no evidence of faulting what must have happened to the intervening Devonian rocks?
As the rocks above the erosion surface are dipping at a different angle to the rocks below we call this an unconformity. If you look closely just above the junction between the two rocks types you can see pebbles and boulders at the base of the limestone. These are all that remains of an early Carboniferous beach. Therefore the upper surface of the sandstones and slates represents a very ancient wave cut platform about 350 million years old.
Key observations that should be made at this locality are highlighted on the field sketch below
