MYNYDD PRESELI DOLERITE
The rocks that make up the famous prehistoric
structure of Stonehenge
in Britain are often referred to as ÒStonehenge BluestoneÓ. They are
Ordovician-aged porphyritic dolerites that have been subjected to low-grade
metamorphism during the Paleozoic assembly of Pangaea.
Here's a sample of spotted dolerite from the
Preseli Hills of Wales It consists of (mostly) albitized plagioclase
feldspar (whitish & whitish-gray), pyroxene (blackish), and the low-grade
metamorphic minerals chlorite, epidote, and actinolite (all greenish).
Unit & Age: Mynydd Preseli Dolerite Suite, Llanvirnian Stage,
early Middle Ordovician
Locality:
Preseli Hills, North Pembrokeshire, southwestern Wales, southwestern Britain.
Mynydd Preseli Dolerite (6.7 cm across at its widest), the rock type that
makes up the prehistoric structure at Stonehenge.