During the last two weeks of May, Dr. Alley and were in the UK. Part of the experience involved complaining about the inadequate width of UK roads, but there was also some undeniably beautiful geology. One such location was Helvellyn, in the Lake District of England. This is one of the “top walks” in the UK, but note that the term “walk” has a very loose definition in British English. Really, this is a hike, with a climb of nearly 3000 ft in about 4 mi. If you’re a fan of glacially carved, open landscapes, it deserves the hype.
The exposed rocks of the Helvellyn Range are part of the Borrowdale Volcanic Group. The volcanics here transitioned from primarily intermediate lava flows (andesite) to more silica-rich magma that resulted in pyroclastic flows and ignimbrite deposits. This all occurred around 450 million years ago (Ordovician) — around the same time as the Taconic Orogeny in North America (most notably in New England).
That’s the raw material for the terrain, but the carving is much more recent. Glaciers from the last glacial advance have gouged out the sides of these mountains into deep u-shaped troughs with steep sides and wide bottoms. Below Helvellyn sits a deep bowl called a cirque. This is where a large mountain glacier once originated, digging out a hole from which it later advanced.
If you go up into the cirque today, that depression has been filled with a small lake called a tarn. Tarns sits at the headwaters of watersheds, and the outlets are often some of the cleaner water you can find (although they may be turbid from lots of sediment). This tarn, though, had a large number of sheep around it. So I’d be a little more suspicious.
Actually, the sheep are a more important aspect the the geology than you might think. Looking down from Helvellyn, you can see the “Striding Edge”. This is an arête, a sharp ridge that, thousands of years ago, was the boundary between two parallel glaciers. The glaciers would have been flowing away from you in the image below.
Finally, when walking along the northwest side of the mountain, you go by the Greenside Mine. In the 19th century, veins along a normal fault through the mountain were mined for the mineral galena (PbS), which also contains impurities of silver.