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.
The edges of the dyke are locally exposed, for example at grid reference NY 300 164 to the south of where the path brings you onto the moor (below the arrow in Fig. 1). Look for a series of small cliffs, where some searching in the heather will reveal a slightly odd feature. The margins of the dyke are not really chilled against the country rock and, as the groundmass of the dyke is very fine grained, this would be difficult. Instead, there is a conspicuous lack of the usual quartz and feldspar crystals in the edges of the dyke. Why these crystals are excluded from the edges of the dyke really isn’t obvious. The larger crystals must have formed during relatively slow cooling, at depths rather greater than the depth of final intrusion, which was sufficiently shallow (and cold) to cause rapid cooling and the formation of the fine-grained ground mass. Therefore, the larger crystals formed before the intrusion was emplaced and would be expected to be present throughout.
The dyke is Devonian in age (from the BGS Geology of Britain Viewer, (http://mapapps.bgs.ac.uk/geologyofbritain/home.html), so is related to the closure of the Iapetus Ocean. The land-masses that we now know as England and Scotland collided to produce the Caledonian Mountains – an alpine-scale mountain belt that stretched from Norway and well into the USA (recall that the Atlantic Ocean is a relative new-comer on this time scale, not opening this far north until the Palaeocene). Similar porphyritic igneous rocks can be seen elsewhere in the Lakes, for example, the Threlkeld microgranite only a few kilometres further north, and the Carrock Fell complex to the north of the main A66 Keswick to Penrith Road, both of which are worth a visit. All the intrusions are the result of melting associated with crustal thickening and subduction of the oceanic plate in the collision.
For more information and sketch maps of the Armboth area see: Lakeland Rocks and Landscape: a Field Guide by the Cumberland Geological Society, Ellenbank Press.
Mark Wilkinson (UK)