Cwm Cau – Cader Idris – South Snowdonia
Cwm Cau is part of the Cader Idris massif, lying to the south of the highest summit, Pen y Gader. It is best approached from the Minfordd carpark and the steep Public Footpath. The cache site lies at 480m.
The Earthcache
At the bottom of the cwm, the Welsh word for a corrie or cirque, lies Llyn Cau. Most lakes in the bottom of cwms are shallow, perhaps 2 metres deep at most. Llyn Cau is 50 metres deep. This unusual depth, at a relatively high altitude, attracts scuba divers and the local mountain rescue team once had to rescue a diver, suffering from the “bends” (not a medical condition normally found at 1,600 feet above sea level). One theory is that Llyn Cau was hollowed to its unusual depth by a rotating ball of rock and ice, trapped under a slowly sliding glacier during the last ice age. Geologists call this period of time the “Younger Dryas” and it occurred about 12,500 years ago. What held all of this back was a rock step, harder than the surrounding strata, the top part of which is now a “roche moutonée” at the south of the entrance to the cwm. This is now a popular site for A-level geography field trips.
Roche Moutonée is French for “sheep rock”. They are shaped by heavy glaciers flowing over them because the rock is too hard for the ice to cut through. The upstream sides are ground smooth by the ice and rocks in the glacier and the downstream sides are steep and rough from the ice falling down them. You will normally find roche moutonée on the sides and bottoms of “U” shaped, glacier carved valleys. Roche moutonée are often marked with striae, caused by pebbles trapped between the bedrock and the glacier. Striae run in the direction that the ice flowed in, whereas newer grooves, caused by water erosion, tend to run straight down to the floor (gravity!).
Other evidence of ancient glaciers that you might spot on the way up to the Earthcache site are erratics. These boulders were left behind by the receding glaciers at the end of the last ice age. Nowadays some of the bigger ones make great things to sit on and rest on the way up to the Earthcache.
Keen geologists can carry on up the Minffordd Path, via Mynydd Pencoed, to the top of Cader Idris (Pen y Gader). Just before you reach the summit, you will find some brilliant pillow lava. Parts of these, and their associated jasper, can be found 6 miles away and almost 3,000 feet lower on the beach at Fairbourne. They were transported there during the last ice age, by glaciers.
Please send the answers to these questions to me via the Geocaching website…
Which direction did the glacier flow over the roche moutonée? (NE, S, NE etc.?)
What is a striation? Can you upload a photo of one?
Please feel free to log your find before you send me your answers but logs without answers will eventually be deleted.
Bibliography
“Snowdonia Rocky Rambles” by Bryan Lynas – ISBN 1-85058-469-9 – Various geological walks around the mountains of North Wales. Walk 6 covers Cwm Cau & Cader Idris.
“The Grey King” by Susan Cooper – 4th in “The Dark is Rising” series – fantasy fiction set locally for children 11+