Snowdon Volcanic Group

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.

Snowdon Volcanic Group
Stratigraphic range: Caradoc Ordovician
Lower Rhyolitic Tuff Formation on the slopes of Y Lliwedd
Type	Group
Sub-units	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
Lithology
Primary	ash flow tuffs
Other	basalt, mudstones, siltstones, hyaloclastite
Location
Region	northwest Wales
Country	Wales
Type section
Named for	Snowdon

Outcrops

The 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.

Lithology and stratigraphy

The 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):

• Upper Crafnant Volcanic Formation
• Middle Crafnant Volcanic Formation
• Lower Crafnant Volcanic Formation

The Tal y Fan Volcanic Formation occurs towards the northeast and the Bedded Pyroclastic Formation and Lower Rhyolitic Tuff Formation occur to the southwest.