Did you know that Lake Vyrnwy covers and is surrounded by rocks up to seven times older than the famous Tyrannosaurus rex dinosaur, who lived and died some 66 million years ago? That means that the bedrock that surrounds Vyrnwy was formed between 455 and 431 million years ago, during the Ordovician and Silurian periods.
The rocks are mostly composed of petrified sand and mud, and so they are sandstones and mudstones. They were originally deposited in a deep sea environment. Sand and mud derived from the continent were fed into the sea, from which they were remobilized and transferred towards the deep waters. The sediments slid down the marine slope forming a turbidity current, a mixture of sand, mud and entrained water that travels stuck to the basin floor because of its high density in comparison to the surrounding waters. A turbidity flow has many similarities with snow avalanches. Avalanches are made of a mixture of snow particles (comparable with the sand and mud in a turbidity current) and entrained air (comparable to the water), that due to their higher density with respect to the atmospheric air travel at high velocities stuck to the mountain slope (i.e. marine slope) towards the valleys (i.e. lowermost part of the marine basin, where turbidity currents deposit their sediment load). Once the dip of the slope decreased, so did the velocity of the sediment mixture, and as a consequence, the ability of the water to transport coarse grains was increasingly compromised. That is why the sand was usually deposited first, within seconds, while the mud remained suspended in the water several hours or even days because of its smaller size, after which they covered the sands with a homogeneous bed. This process, repeated over time millions of years, has produced a characteristic alternation of sandstone and mudstone layers known altogether as turbidites. Each sandstone-mudstone pair was derived from a single turbidity current, and thus reminds us that geological processes are not always that slow, and can generate visible features within a few hours.
Intercalated within these sediments is a characteristic interval that was deposited as a result of a catastrophic event. Four hundred and fifty-five million years ago, an explosive volcano blew up expelling an ash-cloud that generated a pyroclastic flow composed of fine-grained volcanic fragments with entrained hot air that allowed their transport at fast velocities. This flow was likely similar to the one that buried the Roman city of Pompeii in AD 79, destroying everything on its way. The flow entered the sea, where it was deposited interbedded in between the Ordovician turbidites. A close look at these rocks, known as tuff and characterised by a massive appearance and light-grey colour, can be obtained on the road midway from Llanwddyn and the Lake Vyrnwy dam.
The rocks were deformed during the collision of the ancient continent of Laurentia (North America) with the Avalonia microcontinent (Wales and England, among other areas such as SW Portugal and Spain), which led to the Caledonian Orogeny. The collision started in the late Silurian (some 420 million years ago) and finished in the mid-Devonian (390 million years). As a result, the rocks that are found today at lake Vyrnwy were folded forming an anticline, a concave down structure that causes the rock strata to dip towards the northwest in the area. As a consequence of their inclination, the rocks in the valley get progressively younger from the Lake Vyrnwy dam towards the west and northwestern bank. Unfortunately, the majestic forests that surround the lake don’t allow an easy recognition of the deformed rocks, though occasional outcrops can be seen along the road and on the higher mountain areas, where the vegetation cover grows scarcer.
After that, no record of the complex geological times that followed is preserved in the Lake Vyrnwy environment. At least, not until very ‘recent’ times. During the period known as Quaternary, which includes the last 2.6 million years, numerous glaciers covered the Welsh landscape. Frozen water moving very slowly for thousands of years eroded the landscape away, leaving evidences of their cold presence in the Vyrnwy area. One of them is the valley itself that hosts the Lake Vyrnwy, which was carved in the hard Ordovician and Silurian bedrock by the advancing glacial ice. Upon its melting during the last 100,000 years, the sediments contained in the ice were concentrated and deposited on the lower part of the valley, forming a thin veneer of sediments known as glacial till. These deposits, characterised by rock clasts of different lithologies, shapes and sizes embedded in a fine-grained matrix, cover the lowermost part of the valley in the northeastern bank, where they can be usually recognized by the greener meadows that grow on top of them.
Now that you know a bit more of the geology of Lake Vyrnwy, what about a walk enjoying the wonders of the four-hundred-million-year-old beautiful environment that surrounds Lake Vyrnwy Hotel & Spa? Don’t get too lost in time!
Manuel I. de Paz , PhD student University of Oviedo, for LVH&S