Most of the rock of the Elan Valley was formed 745 - 450 million years ago (the Ordovician and Silurian periods). This took place under the Atlantic Ocean.
Throughout this time-period fine-grained silts and muds settled on the ocean floor in layers. Marine organisms (Trilobites, Brachiopods and Graptolites) became entrapped in the layers of mud.
The layers of mud and silt became compacted together under the weight of the sea to make the sedimentary rocks, mudstones and siltstones and the commonest rocks in the Elan Valley, slates and shales.
Another type of rock found in the Elan Valley is conglomerate. This was formed by pebbles and fragments of rock which were deposited on the sea bed during submarine avalanches and became fused together under the pressure of the sea.
All of these rock types can be seen clearly in Caban Coch Quarry (Grid ref. SN 924646)
Around 400 million years ago the land masses of North America and Europe moved towards each other squashing the bed of the Atlantic Ocean, this is called Continental Drift.
The movement of the continents slowly buckle the rock layers under the sea into huge fold mountains. As this occurs faulting takes place (the rocks crack and slide against each other) resulting in a range of jagged mountains like the Alps and the Himalayas.
The jagged new mountains became exposed to weathering by rain and ice-ages. This has occurred over the last 400 million years and is still taking place today.
The last ice-age (70,000 to 12,000 years ago) engulfed the Elan and surrounding area with a glacial ice-sheet. The movement of ice smoothed the mountains to give a plateau (an upland with a near level summit).
One major effect of the ice-age was the glacial drainage diversion of the River Ystwyth (Grid ref. SN 854759).
The River Ystwyth once flowed into a pre-glacial lake which had its outflow into the River Elan.
During the ice-age the lake froze. As snow filled the valley it compressed and turned into ice, forming a glacier. This ice cut a channel to the west.