Chapter
2.
GWYNEDD---NORTHWEST
WALES
Descriptions
of specific
areas: Anglesey
Llyn Peninsula
Snowdonia
Harlech Dome
Cadair Idris area
Fig. 1. The
geology of
northwest Wales. The solid
areas are
Ordovician volcanic rocks.
The mountains of Snowdonia in northwest Wales are among the wildest scenery in the British Isles. Ice age glaciers sculpted hard Ordovician volcanic rocks into rugged alpine landscapes. The low lying island of Anglesey in the north and the Lleyn peninsula jutting out into the Irish Sea show some of the oldest rocks of the region and form a marked contrast to the mountains of Snowdonia.
The oldest rocks in Wales are Precambrian metamorphic and volcanic igneous rocks nearly nine hundred million years old; they form the bedrock in much of Anglesey and the northern part of the Lleyn peninsula. These Precambrian rocks are a mix of lava erupted from volcanoes and sediments collected on a sinking oceanic plate near the edge of an ancient continent.
About 600 million years ago, a collision with a continent to the south crushed and heated the rocks that recrystallized into metamorphic rocks. Locally, some rocks melted to form granite magma that rose into the metamorphic rocks above to become the granite intrusions of central Anglesey.
The youngest Precambrian rocks pass up into the earliest Cambrian rocks, 570 million years old, an extraordinary formation called the Gwna Melange that contains huge blocks of limestone, sandstone, and igneous rocks in a matrix of schist. This mixed rock may be the remains of undersea slides that mixed shallow water sedimentary rocks, limestone and sandstone, with ocean floor rocks, pillow basalts and chert, in the trench above the sinking plate.
Later horizontal movements sliced the crust along steep to vertical northeast trending faults, and brought rocks of very different types into contact with each other. Therefore, in Anglesey we find almost unaltered rocks next to rocks once deeply buried.
Descriptions of specific
areas:
Anglesey
Llyn Peninsula
Snowdonia
Harlech Dome
Cadair Idris area