Geology of Snowdonia National Park
The bedrock geology of Snowdonia is largely formed from a sequence of sedimentary and igneous rocks originating during the early Palaeozoic (the Cambrian, Ordovician and Silurian periods, lasting between 539 and 419 million years ago).
The Cambrian and Ordovician rocks accumulated largely on the Avalon terrane, a piece of the Earth's crust which was involved in the Caledonian Orogeny roughly 490–390 million years ago, a long drawn-out continental collision which resulted in their being intensely faulted and folded and subjected to low-grade metamorphism.
During the Cambrian and Ordovician, sandstones and mudstones were laid down within the Welsh Basin, a relatively shallow marginal area of a broader and deeper ocean known as Iapetus, which lay to the north.
Variations in water depth over this time, related both to crustal extension and to the growth and decline of a south polar ice-sheet, gave rise to changes in conditions of deposition and hence a varying rock sequence.
Volcanic activity began in the Ordovician period as ocean crust was subducted to the northwest of Snowdonia and continued into the early Silurian until continental collision had ceased.
[3] Cardigan Bay developed as a depositional basin during the Mesozoic and some of those sediments are recorded within Snowdonia's boundaries, albeit concealed at depth.
The early Ordovician 'Nant Ffrancon' label for example has enjoyed both formation and group status at different times whereas it is currently classed as a subgroup.
The oldest exposed rocks at the core of the dome are the interbedded sandstones and siltstones of the Dolwen Formation, dating from the Comley epoch.
[citation needed] Silurian sedimentary rocks, of both Llandovery and Wenlock age, are found along the eastern and southern margins of the park.
Bodies of intrusive igneous rocks of Silurian age wrap around the margins of the Harlech Dome and are frequently exposed within the northern mountains.
However, the sinking of a borehole at Mochras on the coast south of Harlech revealed a previously unknown succession of Mesozoic and Cenozoic rock strata, hidden by a thick cover of superficial deposits.
It is unconformably overlain by Middle Oligocene to Lower Miocene age strata (Palaeogene and Neogene periods), all deposited within a half-graben whose eastern edge is the Mochras fault.
There are deposits of alluvium — river laid sand, silt and gravel — along the floors of most of the larger valleys, though these are typically narrow and discontinuous in their extent.
The largest is that which originates on the steep northwest slopes of Graig Goch and which has resulted in a mass of debris up to 30m high covering the otherwise flat floor of this trough and behind which is the lake.
[14] There are further smaller examples which form Bwlch Cyfyng southwest of Abergynolwyn and on the southeastern slopes of Mynydd Gwerngraig east of Cadair Idris.
A further mass of slipped material is recorded beneath the peak of Pen yr Helgi Du at the head of Cwm Eigiau.
Though not strictly within the national park, offshore and usually submerged beneath the surface waters of Cardigan Bay are several ridges of cobbles and pebbles traditionally associated with legends such as those around Cantre'r Gwaelod but recognised today as medial moraines of glacial origin.
In the Snowdon area there are occurrences of copper, lead and zinc mineralisation above Glaslyn, on the eastern flanks of Lliwedd and east of Yr Aran.
