| 词条 | Snowdon Volcanic Group |
| 释义 |
| name = Snowdon Volcanic Group | image =File:On the ascent of Y Lliwedd - geograph.org.uk - 887475.jpg | caption =Lower Rhyolitic Tuff Formation on the slopes of Y Lliwedd | type = Group | age = Caradoc Ordovician | prilithology = ash flow tuffs | otherlithology = basalt, mudstones, siltstones, hyaloclastite | namedfor = Snowdon | namedby = | region = northwest Wales | country = Wales | coordinates = | unitof = | subunits = Upper Crafnant Volcanic Formation, Middle Crafnant Volcanic Formation, Lower Crafnant Volcanic Formation, Tal y Fan Volcanic Formation, Bedded Pyroclastic Formation, Lower Rhyolitic Tuff Formation | underlies =Cadnant Shales Formation etc | overlies =Cwm Eigiau Formation | thickness = up to 1800 m | extent = | area = | map = | map_caption = }} The Snowdon Volcanic Group is an Ordovician lithostratigraphic group (a sequence of rock strata) in Snowdonia, north-west Wales. The name is derived from Snowdon, the highest peak in Wales where it outcrops. This assemblage of rocks has also been referred to as the Snowdon Volcanic Series. OutcropsThe rocks occur across the Snowdon massif and to its south, southeast and west, around the eastern flanks of the Carneddau and within the Idwal syncline.[1] Lithology and stratigraphyThe Group consists of up to 1800 m thickness of ash flow tuffs with sandstones, mudstones and siltstones and important basalt, hyaloclastites and breccias erupted or sedimented during the Caradocian Epoch of the Ordovician Period. The Group includes (in descending order, i.e. oldest last):
The Tal y Fan Volcanic Formation occurs towards the northeast and the Bedded Pyroclastic Formation and Lower Rhyolitic Tuff Formation occur to the southwest.[2] References1. ^British Geological Survey 1:50,000 scale geological map (England & Wales) sheets 106 Bangor and 119 Snowdon 2. ^http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=SV (BGS on-line lexicon of rock units) 3 : Ordovician System of Europe|Upper Ordovician Series|Geology of Wales |
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