Columbia subject headings:
Harvard subject headings:
Geology, Stratigraphic--Ordovician
Geology--Wales--Snowdonia Region.
Volcanism--Wales--Snowdonia Region.
Princeton subject headings:
Geology, Stratigraphic--Ordovocian.
Geology--Wales--Snowdonia.
Volcanic ash, tuff, etc.--Wales--Snowdonia.