11. Rock of Ages
200 years ago, when the Middleton Estate was being transformed into a water park, the science of geology was in its infancy. No-one knew where rocks came from and how long they had existed. Then 19th century geologists realised that the rocks of North Wales held the secrets of Earth’s geological history. This is reflected in the names of Silurian, Ordovician and Cambrian to describe geological time zones: the Silures and Ordovices were Welsh tribes, and Cambria is the Roman name for Wales.
1. The oldest rocks (at the car park end of the Broadwalk) are Precambrian, Coedana Granite from Anglesey. They are igneous rocks very close to the end of the Precambrian period. They are six to seven hundred million years old. Because they formed from molten magma deep in the earth`s crust there are no fossils. The earliest fossils of this age in Wales come from near Carmarthen.
2. The next rocks are Cambrian, slates from Penrhyn. They are metamorphic rocks i.e. fine-grained rocks that formed from mud on a deep ocean floor and were then deformed under high pressure and temperature. They rarely contain any fossils. They are 520 million years old.
3. Ordovician rocks are 465 million years old. This was a time of major volcanic activity in Wales. The rocks are Rhyolite, dark green in colour, from Haverfordwest.
4. Silurian rocks are 430million years old. The examples are from the Aberystwyth area. They are dark grey siltstones, or Aberystwyth Grits, deposited during storms in deep water. There are few fossils. Rocks of a similar age in the Llandovery area contain shell-like fossils and were deposited in shallow water off shore.
5. The Devonian rocks are 390 million years old. They are red and grey sandstones from the Old Red Sandstone at Brecon. These are continental deposits and contain some of the earliest land plants. At this stage Wales was on the southern margin of a giant continent straddling the equator.
6. The Lower Carboniferous rocks are 335 million years old. They are limestones – rocks produced in warm tropical seas often incorporating reefs and lots of shells later fossilised. Land would have been across Mid Wales and the rocks here come from north and south of the landmass.
7. The Upper Carboniferous rocks are from Abercarn in Gwent. They are 302 million years old. They are coarse sandstone with abundant plant debris (black marks) swept in by rivers from the vegetation on the flood plains and from the swamps that produced the coal.