Temple Street Erratic

Erratics are rocks that have been moved by glaciers to end up in an unexpected location. Sometimes they are only moved a short distance, but other times many hundreds of miles (such as Norwegian rocks found in Norkfolk!). Most are small, and below the soil of Radnorshire is a layer of boulder clay that is made largely of transported pebbles, all dumped by the ice. Some, however, can be huge.

The Temple Street Erratic is about 2 m high and 5 m wide, and probably weighs around 100 tons. It is made of a distinctive rock type known as conglomerate: a mass of pebbles and sand cemented together by quartz. It is very likely part of the Caban Conglomerate, which is exposed at Caban Coch dam (and elsewhere) in the Elan Valley, some ten miles northwest.

The Caban Conglomerate was laid down in the very early Silurian Period, some 440 million years ago. There was still an extensive sea at this time, covering most of Wales, but the conglomerate was deposited at this time because the sea level was temporarily much lower. This in turn was due to another ice age, at the end of the Ordovician. This ice age is believed possibly to have been due to the appearance of land plants, sucking carbon dioxide out of the atmosphere, and corresponds to the end-Ordovician Mass Extinction: the second most severe of the great extinction events.

The pebbles within the conglomerate are much older still. Some of them are metamorphic rocks, eroded from a mountain range that has been worn entirely away. A few pieces of granite from these rocks have been dated to 650 million years old, which suggests that some of these were formed around the time of the infamous Snowball Earth: an ancient episode when the planet was almost entirely covered by ice, before the appearance of the first animals.

So... we have pebbles made of rocks formed around the time of the greatest ice age of them all, washed together to form a rock laid down in the end-Ordovician ice age, and finally moved in another ice age within the last 20,000 years.

It's ice ages all the way down.

As an aside, it is possible to see evidence of which way up the rock was originally! Towards the left side, there is a sandy layer between the pebbles, showing that the top was originally to the left or right. On the left side of that sandy layer, there are thin curved laminations that show original ripple structures, and these are truncated towards the left. This can only happen if the layers to the left came after the ones to the right... so yes, the left edge of the boulder was the top, in the original outcrop!