. Bulletin of the British Museum (Natural History), Geology. BASE OF THE SILURIAN IN THE LAKE DISTRICT 55 CO l_ T3 u a> c CO â 01 â 1 â o o> 3 rr V CO - <o. 008m a o c o c -Q) o CO < brown-weathering black shales Jm. n r grey shales continuous exposure into Browgill Beds (in type development) atavus Zone acuminatus Zone ? persculptus Zone not to scale: thickness of units given in metres Fig. 2 Eastern Lake District: beds about the Ordovician-Silurian boundary on Browgill, NY 4974 0587. Rickards (1978) attempted a general interpretation of the environment of deposition of the basal
. Bulletin of the British Museum (Natural History), Geology. BASE OF THE SILURIAN IN THE LAKE DISTRICT 55 CO l_ T3 u a> c CO â 01 â 1 â o o> 3 rr V CO - <o. 008m a o c o c -Q) o CO < brown-weathering black shales Jm. n r grey shales continuous exposure into Browgill Beds (in type development) atavus Zone acuminatus Zone ? persculptus Zone not to scale: thickness of units given in metres Fig. 2 Eastern Lake District: beds about the Ordovician-Silurian boundary on Browgill, NY 4974 0587. Rickards (1978) attempted a general interpretation of the environment of deposition of the basal Silurian strata, envisaging a west- or northwest-facing fault scarp, according to Hutt (1974) active during deposition of the early Llandovery, against which were deposited deeper offshore, black shales and upon which and behind which were deposited the Basal Beds and their equivalents. By upper atavus Zone times the scarp feature was further submerged and covered in black shale deposition. Associated with these features were a series of ridges and hollows striking ENE/WSW, that is roughly the same as the fault scarp strike. The hollows received a greater thickness of black shale in a more highly anaerobic environment (Rickards 1964). The ridge and hollow system persisted in the Howgill Fells region, and possibly in the main Lake District outcrop, until late in the Llandovery. Thus the onset of the Silurian in the Lake District is marked by condensed deposition of shelly limestone, and possible non-sequences, in the eastern, presumed shoreward or shallower region; and by relatively thick, black shale deposition in the western Lake District. The post- glacial marine transgression is recorded in the gradual spread of black shale deposition over the whole region, the last area to succumb being the eastern Lake District area of Skelgill which is interpreted as being on the crest of an old scarp structure, itself certainly operative as far back as the Caradoc. It seems likely tha
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