Moor to See

Dartmoor has the largest periglacial landforms in southwest England. Periglacial landforms are those that form from geomorphic processes associated with frost, snow and ice, but glaciers are absent. During the last Ice Age most of the British Isles was covered with an ice sheet from Wales north to near York. The warmer southeast and southwest escaped the ice, providing the opportunity to develop an array of periglacial landforms across a cold windswept tundra right down to sea level.

Dartmoor, the largest upland region of the southwest, is a periglacial landscape. The most impressive landforms lie on the southwestern side. Stone stripes up to half a kilometre long extend down the flanks of Great Staple, Great Mis and Leeden Tors (currently being studied by Plymouth University). These stripes are similar to much larger stripes on the Falkland Islands and would have required permafrost (ground that is always below 0 °C) to form. There are also impressive features at the highest elevations in the north of Dartmoor, such as on the slopes below the hill hosting Yes Tor and High Willhays, the two highest peaks in southern England. Blockslopes and lobes line the flanks of this hill, that could only have moved during seasonal thaw of the soil. In places the blocks have been imbricated en echelon where the effects of frost heave and gravity have combined to align the blocks in an unusual way downhill (see below). Blocks that have been sorted free of the finer soil are common in areas where there are springs on the side of the hill. These supplied the water for ice to heave the blocks during the colder months of the year. In these concentrations of blocks there are pits that have been left behind once the ice underneath has melted (see model above). Such pits are known as thermokarst because they are reminiscent of the collapse pits in karst terrain, formed by the solution of limestone. Thermokarst is fairly common in periglacial block deposits, such as those in the Victorian Highlands of Australia, but isn’t well described (if you know of any modern patterned ground in blocks, let me know!).

When did these block deposits form? Good question. The block slopes are extensive and it is difficult to imagine they formed in a single episode. Dartmoor is a palimpsest and the effects of climate change through time are written and rewritten over the same ancient landscape. There has to be enough time to weather and break the blocks up from the bedrock and then sort them at the surface. Using exposure dating, we know block deposits in Australia formed over the last half a million years or so1. Exposure dating the blocks is problematic. We know from Yanni Gunnell and colleague’s study in 2012 that most of the tors are eroding fairly slowly. Given the age of the landscape, the tors and slopes that produced the blocks are likely to have been exposed over a long period, so the assumption of a single exposure event won’t hold. However, given the relative freshness of the landscape it is fairly safe to assume that these deposits were forming during the last glacial period 20,000 years ago and were probably initiated in one of the previous glacial periods of the last  2 million years.

1. Barrows, T. T., Stone, J. O., and Fifield, L. K. (2004). Exposure ages for Pleistocene periglacial deposits in Australia. Quaternary Science Reviews, 23: 697-708.

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