The Armboth Dyke makes a good half day geology excursion in a scenic but quiet part of the UK Lake District. Parking is on the west shore of Thirlmere, in a pay-and-display car park accessed by the narrow road that winds around that side of the lake (Grid reference NY 305 172). The car park is in an excellent setting, with direct access to the wooded lake shore, and would be a great place for the non-geologically minded to wait while you venture onto the adjacent hill. It is probably worth noting at this point that the dyke itself is mostly exposed on rather featureless rolling moorland at around 400m above sea level (Fig. 1), and might not be a good place to visit in thick mist, unless you are very confident with a map and compass. If you happen to be in the business of teaching students to make geological maps, this site makes a great practise day, without too many problems of recognising weathered rocks in the field.
So, assuming you have decent weather, leave the car park and take the path uphill from the west side of the minor road, a few metres to the north of the car park. The path is steep-ish, and can be slippery if wet, so take care especially descending. As you climb steadily up the hillside, keep an eye on the rocks in the path – look out for a distinctive pinkish igneous rock with a fine-grained ground mass and larger quartz and feldspar crystals a few millimetres across (Fig. 2). The path crosses the dyke a few hundred metres before you arrive on the open hillside, so if you reach this point and haven’t spotted it, you might need to backtrack.
The dyke itself is exposed for around 2km in a roughly north-south direction, and is 5m to 10m wide. It is intruded into the Borrowdale Volcanic Group, which is mostly grey-green tuffs and agglomerates here – this is the unit that makes up most of the high ground in the Lake District. Helpfully, the pinkish dyke (with the quartz and feldspar crystals weathering out) is fairly easy to distinguish from the country rock (Fig. 3). In many places, the dyke can be seen as lines of pink boulders running across otherwise grassy terrain. By trying to follow the line of these exposures, it becomes obvious that the dyke is not a single continuous feature, but a series of disconnected segments. The usual interpretation is that the dyke is offset by several strike-slip faults. As the dyke is vertical, only a strike slip component of movement would produce any offset – normal or reverse faulting would not produce any offset. Try sketching this if it isn’t obvious.
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Mark Wilkinson (UK)