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Nestled between the foothills of the Berwyns to the south, the Arenig range to its north, and with the majestic, rugged peaks of the Aran range dominating its south-westerly aspect, Llyn Tegid is the largest natural expanse of water in inland Wales.
Local legends abound in praise of this spectacular stretch of water, relating to its origin, to the creatures that are believed to live within it, and to the characters who lived at its margins. As impressive as its links with Arthurian legends might be, what we are concerned with is its geological history and features.
Formed at the end of the last ice age, Llyn Tegid lies in a natural fault-line known as the Bala Fault, flanked by 300 to 500 metre hills with loftier peaks of Aran and Arenig in the background. This fault-line follows the southern shore and is concealed underwater, just a few metres from the water's edge, where the bed of the lake suddenly plunges from a few metres to a depth of over 43 metres in places.
Back in the ice-age, the land around the Arenig range, to the north west of the lake, lay under several hundred feet of ice, which flowed out from the peak of Arenig, radiating into the surrounding plains and valleys. As the glaciers scoured through this valley, along the fragile margins of the fault-line, one edge of the fault was eroded away, resulting in the steep, underwater cliffs alluded to above. As the glaciers melted, they deposited a debris of rock and soil, known as moraine, at the south-western end of the valley, trapping behind it the waters of the melting glacier and thereby forming a lake substantially larger than that which we know today.
Further deposits of moraine were established where geographical features, such as rocky promontories along the valley sides, impeded the flow of the glacier. Over the following millennia, the waters of the River Dee have eroded the bank of moraine, resulting in the lowering of the lake to current levels. Exploration of this valley in a region between one and four miles west of the lake reveals curious earth banks separating flat areas of land in the valley bottom into various levels. These banks are believed to have been caused by the catastrophic, erosive force of the waters of the lake when rivers flowing westward out of the lake breached the deposit of moraine. At such a time, the massive flow of water out of the valley would have washed vast quantities of rock and soil further down the valley as they scoured their way through the moraine, leaving the banks that we see today.
Nowadays, the lake level is such that the River Dee flows through the lake and exits it at its northern extremity, just to the south east of the town of Bala, a region that can be explored by geocachers who follow the TUFTY series of traditional caches.
To log this earthcache you need to visit the lake and determine answers to the questions listed below from two different locations.
At location 1, use the information panels near the Lake Warden's Office to establish the following information. Your answers need to tally with the information as it is given on the information panel. There is plenty of information available on the internet, but much of it is conflicting, and inaccurate!
1. How many thousands of years ago was Llyn Tegid formed?
2. What are the overall dimensions of the lake?
3. Take a walk along the footpath on the raised earthwork that stretches from the public car-park on the foreshore along the north eastern end of the lake and estimate the height of the footpath above the water-level at the foreshore. (Note that in periods of excessively heavy winter rains, the level of the lake can be such that it reaches almost to the top of this earthwork).
At location 2, you will witness the current erosive forces of the lake as its water levels during excessive winter storms wash away soil and stone from around the tree-roots on the foreshore. Go to the coordinates listed in the waypoints above, and answer the following question....
4. What material can be seen to be forming the lake bed at this point?
5. Say why you think this material is here.
Please don't log your answers, but mail them to me via the e-mail link in my profile page. Your photos, whilst in no way obligatory, would be most welcome, as would your logged observations of this magnificent, geological feature.
Many thanks to the Snowdonia National Park and the Lake Warden at Llyn Tegid for granting permission for this earthcache and for offering their assistance.
Thanks also to the local office of the Environment Agency for the information that they have offered regarding lake water levels.
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