THE LAKE DISTRICT : GEOLOGY 131 



Ordovician. 



The Skiddaw Slates are the oldest rocks in the district, and are comprised 

 of shales, mudstones, siltstones, sandstones and grits. Owing to folding and 

 faulting the true sequence is unknown, but the bulk of the arenaceous material — 

 variously classed as Skiddaw Grits, Watch Hill Grit, and Loweswater Flags — 

 is now believed to occupy a low position, argillaceous material forming the rest 

 of the sequence with the exception of the Latterbarrow Sandstone of West 

 Cumberland, which is there the highest member of the series. The most 

 important fossils are graptohtes, and on these Dr. Elles has divided the series 

 into the following zones : — 



(1) Bryograptus kjerulfi (4) Didymograptus hirundo 



(2) Dichograptus (5) Didymograptus bifidus 



(3) Didymograptus extensus 



The Borrowdale Volcanic Series, some 10,000 feet in thickness, and made up 

 of lavas, tuffs and agglomerates, succeeds the Skiddaw Slates, the junction in 

 some places being a passage, in others an unconformity, but in many cases it 

 is obscured by faulting. The lavas are mainly andesites, but rhyolites occur, 

 though it may be noted that some of the rocks classed as rhyolites are really 

 intrusive. The fragmentary rocks vary from exceedingly fine-grained tuffs 

 to coarse agglomerates. When cleaved and not too coarse, these provide 

 roofing slates — the Green Slates of Borrowdale and Coniston. 



Marr and Harker, and Green, have published general sequences considered 

 to be applicable to the whole region, but detailed work by Mitchell, Hartley, 

 and the Geological Survey, has shown that great variations occur from place 

 to place. A main grouping into (1) lower lavas, predominantly andesitic, 

 (2) tuffs, (3) upper lavas, mainly andesites below and rhyolites above, may 

 be applied if it be remembered that tuffs may be associated with the lava group 

 (as, for example, the Mottled Tuffs at the base of, and Frostwick Tuffs within, 

 the lower group) and lavas with the tuff group (as, for instance, the Wrengill 

 Andesite). Even this grouping fails to the west of Wastwater, where, apart 

 from a basal tuff, fragmentary rocks are scarcely represented. 



No fossils have been found in these volcanic rocks, but their general 

 equivalence to the Llandeilo of Wales is indicated by their position between 

 the Skiddaw Slates and the Coniston Limestone Series. 



The Coniston Limestone Series, which represents the Caradocian rocks of 

 other parts of England, consists chiefly of calcareous and ashy sediments 

 rather than good limestones. The lowest or Stile End Beds (50-250 feet), 

 with a conglomerate at the base, rest unconformably on the Borrowdale 

 Volcanic Series. They are followed by the Stockdale Rhyolite (0-450 feet) 

 and the Applethwaite Beds (100-400 feet), a conglomerate at the base of the 

 latter marking a second unconformity. Fossils are abundant, particularly 

 brachiopods and trilobites. 



The Ashgill Series, about 100 feet thick, conformably succeeds the Coniston 

 Limestone Series. At or near the base is a white limestone, about 12 feet 

 thick, often referred to as the Staurocephalus Limestone from its characteristic 

 trilobite. The rest of the series consists of shales ; some of these are ashy and 

 mark the last phase of vulcanism during the deposition of the older Palaeozoic 

 rocks of the Lake District. 



