Cwm Idwal: A Journey through Geological Time

Written by Daniel Rowen, William and Mary Class of 2014

Today, we head into the mountains for a look into Wales’ past. Cwm Idwal is a valley in the larger Snowdonia National Park that has given key insight into the geological history of Earth.  Cwm Idwal is somewhat of a geological celebrity; Darwin visited here twice (and was actually a geologist first), and some of the names of periods in Earth’s history were born out of the contributions to the field of geology made here in the UK.  The Cambrian Era comes from the Latin name for Wales, and the Ordovician and Silurian Periods refer to Welsh tribes.

Professor Colin Jago, of the School of Ocean Sciences at Bangor University, led us through Cwm Idwal and described the geological history of the area and its impact on the study of geology. Researchers in the 19th century were able to use the area to establish an order for the formation of different types of rocks but without modern radioisotope dating techniques they could not determine their age.

The rocks in the area are volcanic, dating back 450 million years to the Ordovician Period. Rhyolite was formed in violent eruptions at subduction zones that produced viscous lava flows and ash. The ash, ignimbrite, is composed of dense, hot lava and gas that move at high speeds. This has earned it the name nuée ardente, French for “glowing cloud.”  While the rock formations are initially laid horizontally at the surface of the Earth, they are submerged over time and bent by extreme heat and pressure.

On a global scale, landmasses that roughly correspond to North America and Europe were once separated by the Iapetus Ocean; England and Wales were on the “European” side and Scotland and Northern Ireland were on the “North American” side. The ocean finally closed about 400 million years ago.  The resulting collision of continental plates formed the mountains in present day Scandinavia, Scotland, North Wales, and Northern Ireland, as well as the Appalachian Mountains in the Eastern US.  200 million years ago, the North Atlantic started to open and Scotland and Northern Ireland stayed with the “European” side.

Our tour of Cwm Idwal ended in rain, but was enjoyable and informative nonetheless. The beautiful, picturesque landscape provided an opportunity for good photographs. Although most of us lean toward marine biology, Professor Jago’s excellent discussions left us with a healthy appreciation of geology and its importance in the grand scheme of things.

About David Malmquist

David Malmquist is the Director of Communications at the Virginia Institute of Marine Science, College of William and Mary.
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