THE SUCCESSION AND STRUCTURE OF THE BORROWDALE VOLCANIC ROCKS SOUTH-EAST OF ULLSWATER

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Abstract
The lower 8500 feet of the Borrowdale Volcanic succession is exposed in the Ullswater region and is divided in this paper into four groups. The lowest, the Ullswater Group, faulted against the underlying Skiddaw Slates, consists of about 2700 feet of basalt and basic andesite, followed by 1500 feet of flow-brecciated andesite. Above this, the variable Birk Fell Group (approximately 1300 feet thick) consists of rhyolite, felsitic tuff (including streaky welded tuff), and andesitic tuff. The succeeding Place Fell Group (2500 + feet) is formed of andesitic and felsitic tuff with flows of basic andesite and basalt. There is some evidence that the andesitie and basic rocks were derived from the south-west and the felsitic rocks from the east. The highest beds mapped are the Angle Tarn Andesites, probably to be correlated with the Wrengill Andesites of other parts of the Lake District. The principal structural feature is a broad north-easterly trending asymmetrical syncline, which is continuous with that of Scafell and Helvellyn. Cleavage is moderately developed and also has a north-easterly trend (average 054°), with a near-vertical dip. Of several well-developed fault systems, the most important has a north-westerly trend (average 305° and consists of dextral wrench-faults with large strike and dip-slip components. It is suggested that there was an early phase of wrench-faulting and a later phase of normal faulting along the same lines. Another important group of faults are northerly (354°) high-angle dip-slip faults, terminated by the north-westerly faults and possibly contemporaneous with the suggested later phase of movement of these faults. There are joint systems with the same trends as these two groups of faults, and also a strong north-north-westerly (324°) group of joints interpreted as tension structures of the main folding. Most of the mineral veins seem to be related to the later normal faulting and may be post-Carboniferous. The Skiddaw Slate–Borrowdale Volcanic junction is everywhere faulted. This is interpreted as a thrust and probably reflects the disharmonic relations between the two groups.

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