Long Excursion to the Lake District

123 LONG EXCURSION TO THE LAKE DISTRICT. AUGUST 7TH TO 17TH, 1920. REPORT BY J. FREDElUCK N. GREEX, B.A., F.G.S., Director o] the Excursion. PLAn: 5. OWI~G to difficulties of accommodation the excursion, originally planned to include visits to the southern part of the Lake District, had to be restricted to places readily accessible from Keswick. While the glacial geology, physiography and minerals were not neglected, the chief object was to examine questions of vulcanicity, sequence and structure. The main controversial points were enumerated by the Director at an evening meeting on August 8th, when he gave a brief account of the points on which his 'views differed from those of earlier investigators. The foundation of his case lav in the identification of the .-, flow breccia'S," as lavas instead of tuffs or agglomerates, since this was the basis of his mapping. In this way he had arrived at a sequence in the volcanics, which had so far proved effective wherever applied. The larger part of this sequence was repeated ten times between Keswick and Coniston, so that the maximum thickness of the Borrowdales was probably only about 3,500 feet. This major folding was accompanied by concertina folding, especially in the softer rocks, in which the folded sheets often tended to horizontality. The Devonian folds were arranged fan-wise about an anticlinorial axis through Scawfell and the south of the Helvellyn mass. Working along these lines he had come to the conclusion, contrary to that of the Survey and the Cambridge School, that the upper and lower junctions of the Borrowdale Series were, as a rule, unfaulted, the volcanics resting conformably on the Skiddaws and being covered with a strong unconformity by the Bala Coniston Limestone series. Identify ing the unconformity in the north and correlating the Watch Hill Beds with the basal Bala, he held that the folds of largest ampli tude were Ordovician. Finally, considering the relations of the intrusions to these structures, he believed that practically all, with the exception of the Shap Granite and certain dykes, were pre-Upper Bala in age. AUGUST 8TH. DERWEXTWATER AXD FALCON CRAG. THE party walked to Castle Head, overlooking Derwentwater. From the top the Director pointed out the main elements of the geology; the Skiddaw Slates; Borrowdale Volcanics; intru sions; drift and alluvium; and drew attention to their effects on the scenery, notably the differences between the sediments PROC. GEOL. Assoc., VOL. XXXII., PART 3, IQ2I. 9 J. F. N. GREEN, and volcanics; the truncated spurs; the drumlins; and the division of the former Derwentwater-Bassenthwaite Lake by the Greta delta. Castle Head, composed of a dolerite, was generally supposed to be a plug, but he regarded it as a sill or thin laccolith, lying nearly horizontally. The surrounding slates had, however, high dips, so that the intrusion, if related to the Borrowdalcs, must be in an area of low sheet-dip. This and other points were illustrated on the published six-inch geological map (Cumberland (4). The dolerite, which had been described by Teall" and others, resembled the ordinary quartz hypcrsthene-dolerites found among the volcanics, but differed in containing a notable amount of biotite. Professor Schwarz suggested that this might be due to absorption of shale, as in South Africa doleritcs contained biotite when intruded among shales; and the Director agreed that the distribution in the Lake District supported this view. The accompanying metamorphism was slight, so that the thickness of dolerite could hardly be great. Proceeding by the lake to the base of Falcon Crag, an andesite lying below the" great purple breccia" was seen. It was un known to Ward, but may not have been exposed in his day. Some time was spent on the breccia and then a rough walk was taken by exposures of the bedded tuff" d " and Ward's" No. I lava" to the line where" 1'\0 2 lava" passed upwards into"f ash." The Director explained that this so-called ash belonged to the commonest type of flow-breccia of the Lake District, found plentifully everywhere, and that he regarded it as the crust of No.2 lava. He demonstrated the peculiar weathered surface in which the enclosed fragments yielded more readily than the matrix, giving a net-like face, the strands of which had a lacy appearance. In this instance the difference between the com ponents is not great, though visible on a freshly broken surface. The party then scrambled to the top of Falcon Crag. The summit consists of amygdaloidal augite-andesite, which shows some flow-brecciation in places and had accordingly been mapped by Ward as an ash, "i." About 150 yards. south, however, it was seen to be highly flow-brecciated with a marked difference between the components, the matrix being much the harder and weathering white. The Director regarded "j," a bedded tuff, as equivalent to his Middle Tuffs, though he had not yet proved it by mapping; and he thought that" l " might be a thin representative of the Harrath Tuffs, so that in Ward's type-section] the whole volcanic sequence would be present with the exception of the rhyolites. These, however, come in along the core of the syncline at Rigghause End, north-east of the foot of Thirlmere. On the way home, which passed by Walla Crag and Rakefoot, • Rdtish Prfrot,rllph". p. 224. tGeology 01the North em Part 01Ihe English Lake District, plate v, LONG EXCURSIO~ TO THE L.\KE DISTRICT. 125 the Keswick stone circle was visited. A rectangular inner enclosure, now usually considered to be the remains of a fenced harrow, aroused much interest. At;Gl:ST 9TII. CARRICK FELL, THE DHY GILLS A~D SILVER GILL. ON arriving by motor at Mosedale, the party saw successively from south to north on the hillside (I) contorted Skiddaw silt stone, (z) the approximate position of the fault between the siltstone and the granophyre, where the rocks became almost indistinguishable, (3) the actual junction of granophyre and gabbro, (4), a small quarry at the junction of gabbro and con tact-altered andesite from which a remarkable strontium-bearing mineral had recently been described byMr. Melmore* as a variety of pectolite. The motors then went on, between crags of gabbro and the remains of an old lake, to Carrick Beck, where the position of the gorge at Linewath across the pre-glacial water shed, by which the lake had been drained,t was pointed out. The Dry Gills were reached on foot. On arrival the Director gave a historical account of this remarkable enclave and explained his view that the Bala shales with their accompanying rhyolite and dolerite had been dropped between two reversed faults, running east and west. The northern of these was plain, being filled by a big ore-bearing vein, easily traceable; the southern, which brings granophyre over Bala shale, was seen in the south fork, where the fault-breccias were readily found. Professor Fearnsidcs noted marked kaolinisation in the shales and drew attention to a hrematitc-staining connected with the fault; and, following this clue, convinced the Director that he had under estimated the hade of the southern fault. Time was then devoted to the collection of fossils, Trinucleus, Ampyx, Orthis, etc., and of minerals, Quantities of kampylite and pyromorphite were found accompanied by wad, barytes, chalcopyrite, malachite, etc. The party then steered across heathery moors at a height of z.ooo feet to the head of Silver Gill, where the gabbro, andesites, Skiddaw Slate, and polygenetic grit (Watch Hill Beds) could be seen near one another. At first sight there seems to be a regular succession downwards from grit through thin dark slate to ande site, all dipping at a high angle west-north-west ; but the latter part of this succession is clearly inverted, participating in the general overfolding which, along the northern strip of volcanics, is usually from north-north-west, but swings round to west north-west along Silver Gill, apparently owing to the obstruction of the Carrick Fell complex. Thus the position of the grit pre sents an extraordinary anomaly. At first sight the obvious 'interpretation is a fault between the slates and volcanics, bringing "Geol, ~fag~, IQ:20, p. 266.. plarr, Geol, Mag., 1895, p. 300; I26 J. F.S. GREEN, up inverted beds lower than the main Skiddaws ; but th e presence of such fossils as Didy mograptus hirundo close to the grits in Ellengill on the west of Grea t Sea Fell, appears to negative this solution. And the mapping of the Surv ey, which the Direct or had verified, shows that the grits of Silver Gill are at the edge of a capping- of grit t o Great Sea FeU, underlain evcrwhere by Upper Skiddaw Slate. Th ere seem to be only tw o possibilities left; firstl y. that the grits ar e pre-Ordovician, pushed over the Skidd aw Slate and volcanics by an enormous thrust, or secondly. that they lie unconformabl y on the Skiddaws and have been folded up with them. Th ere is no known evidence of the former. and the latter view is supported by the composition of the gri ts, which are derivable wholly from the lower palieozoic rocks now exposed in their vicinity, and by th e nature of th e exposed junction with the slates. Th e apparent anomaly in th e sue- NW FIG. IS.-DIAGRAMM ATIC S E CT IO :-I ACROSS SILVE RGILL. cession at Silver Gill can be explained by the foldin g, as in the diagram (fig. IS). A UGUST IOTH.

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