The Armboth Dyke, Lake District
The Armboth Dyke makes a great half day geology tour in a scenic however quiet a part of the UK Lake District. Parking is on the west shore of Thirlmere, in a pay-and-display automotive park accessed by the slim highway that winds round that facet of the lake (Grid reference NY 305 172). The automotive park is in a wonderful setting, with direct entry to the wooded lake shore, and could be a terrific place for the non-geologically minded to attend when you enterprise onto the adjoining hill. It’s in all probability price noting at this level that the dyke itself is generally uncovered on slightly featureless rolling moorland at round 400m above sea degree (Fig. 1), and may not be a great place to go to in thick mist, until you’re very assured with a map and compass. In the event you occur to be within the enterprise of instructing college students to make geological maps, this website makes a terrific practise day, with out too many issues of recognising weathered rocks within the area.
So, assuming you may have respectable climate, depart the automotive park and take the trail uphill from the west facet of the minor highway, a couple of metres to the north of the automotive park. The trail is steep-ish, and may be slippery if moist, so take care particularly descending. As you climb steadily up the hillside, keep watch over the rocks within the path – look out for a particular pinkish igneous rock with a fine-grained floor mass and bigger quartz and feldspar crystals a couple of millimetres throughout (Fig. 2). The trail crosses the dyke a couple of hundred metres earlier than you arrive on the open hillside, so in the event you attain this level and haven’t noticed it, you may have to backtrack.
The dyke itself is uncovered for round 2km in a roughly north-south path, and is 5m to 10m large. It’s intruded into the Borrowdale Volcanic Group, which is generally grey-green tuffs and agglomerates right here – that is the unit that makes up a lot of the excessive floor within the Lake District. Helpfully, the pinkish dyke (with the quartz and feldspar crystals weathering out) is pretty simple to differentiate from the nation rock (Fig. three). In lots of locations, the dyke may be seen as traces of pink boulders operating throughout in any other case grassy terrain. By making an attempt to observe the road of those exposures, it turns into apparent that the dyke is just not a single steady characteristic, however a collection of disconnected segments. The standard interpretation is that the dyke is offset by a number of strike-slip faults. Because the dyke is vertical, solely a strike slip element of motion would produce any offset – regular or reverse faulting wouldn’t produce any offset. Attempt sketching this if it isn’t apparent.
The perimeters of the dyke are domestically uncovered, for instance at grid reference NY 300 164 to the south of the place the trail brings you onto the moor (under the arrow in Fig. 1). Search for a collection of small cliffs, the place some looking within the heather will reveal a barely odd characteristic. The margins of the dyke aren’t actually chilled in opposition to the nation rock and, because the groundmass of the dyke may be very wonderful grained, this could be troublesome. As an alternative, there’s a conspicuous lack of the same old quartz and feldspar crystals within the edges of the dyke. Why these crystals are excluded from the perimeters of the dyke actually isn’t apparent. The bigger crystals will need to have shaped throughout comparatively gradual cooling, at depths slightly better than the depth of ultimate intrusion, which was sufficiently shallow (and chilly) to trigger speedy cooling and the formation of the fine-grained floor mass. Subsequently, the bigger crystals shaped earlier than the intrusion was emplaced and could be anticipated to be current all through.
The dyke is Devonian in age (from the BGS Geology of Britain Viewer, (http://mapapps.bgs.ac.uk/geologyofbritain/home.html), so is said to the closure of the Iapetus Ocean. The land-masses that we now know as England and Scotland collided to provide the Caledonian Mountains – an alpine-scale mountain belt that stretched from Norway and nicely into the USA (recall that the Atlantic Ocean is a relative new-comer on this time scale, not opening this far north till the Palaeocene). Comparable porphyritic igneous rocks may be seen elsewhere within the Lakes, for instance, the Threlkeld microgranite just a few kilometres additional north, and the Carrock Fell complicated to the north of the primary A66 Keswick to Penrith Highway, each of that are price a go to. All of the intrusions are the results of melting related to crustal thickening and subduction of the oceanic plate within the collision.
Additional studying
For extra data and sketch maps of the Armboth space see: Lakeland Rocks and Panorama: a Area Information by the Cumberland Geological Society, Ellenbank Press.
Mark Wilkinson (UK)
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