In the discussion on the above paper Professor W. G. Fearnsides said that the " abundance of separated jointed blocks and heavy scree on the south-east side [of Corndon Hill] and comparative absence of litter towards the west required an explanation ". To the writer this appears to be a geomorphological problem directly related to the glacial conditions which prevailed during Pleistocene times, an aspect which was not discussed at the reading of the paper.
The distribution of drift in the Welsh Borderland has much in common with that of the Isle of Man, both local and extra-insular drift being involved. In the words of Lamplugh (Geol. Surv. Mem. Isle of Man, 1903, p. 352), "Their relative distribution is everywhere in keeping with Mr. Kendall's supposition that only the lower layers of the ice-sheet were drift-laden, and that these were diverted by the steeper hill slopes and swept round the flanks of the massif as an undercurrent."
Applying this principle to the Shropshire area, it is clear that hills such as Corndon and the Breidden, to select two examples, must for long have remained as outstanding
nunatakkr
similar to those of the Nordenskiöld Glacier of Spitsbergen at the present day. Their western or ice-ward flanks were directly exposed to the oncoming Welsh ice, although the pure upper ice would have little abrasive power. The eastern or leeward flanks were zones of pluck, a process responsible for the disintegration and removal of the rock-face from top to bottom during the progressive
Summary
A group of more than 100 dykes, mainly porphyrites with some microdiorites, quartz-diorites, and lamprophyres, trends east-north-east from the Solway coast between Creetown and Gatehouse-of-Fleet; the dykes are intruded along the strike of the closely folded Lower Palaeozoic sediments, the belt in which they are emplaced being some eight miles in length and four miles wide. Most of the dykes possess directional textures, formerly described as foliation; these textures are partly of primary origin but mainly due to shearing. The shearing has taken place on vertical surfaces which lie either parallel to the length of the dykes or make small acute angles with their walls.
A feature shown by most of the dyke-rocks is the development of numerous flakes of white mica within their fabrics; in the sheared dykes the micas are oriented along the surfaces of shear. The mica-forming materials may have been derived from an internal source in the zone of shearing, or as emanations from subjacent granite masses.
The relationships between the shear directions, transverse joints, and quartz veins which cross the dykes are analysed, and experimental work due to Reidel is cited in support of the suggested origin of the structures in a belt of shearing. The attitude of the shear surfaces in relation to the ellipsoids of stress and strain is also examined.
The dykes are considered to fill tear (or transcurrent) fractures, on which lateral movement recurred after the emplacement of the igneous bodies. The implications of this mechanism in respect of a wider area are indicated.
The north-eastern area of Dartmoor is traversed by a series of straight valleys which follow fault lines. One fault-system, which crosses the granite from near Lustleigh north-westwards to Sticklepath, is described in detail, and its relationships to structures in a wider area are briefly discussed.
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