Climbing and Cleavage - the geological history of the Llanberis Slate Quarries

So after just two trips to the Llanberis slate quarries of North Wales I am now a fully converted Slate Head. The sharp crimps, invisible micro-edges and frictionless slopers create a completely different climbing experience to any other rock type; with precision, balance and feet-up-by-your-face flexibility being much more important than pure strength and power. Routes are varied, balancing their way up apparently slick vertical slabs, bridging up awkward technical grooves, jamming thin micro-wire protected seams and even powering through overhanging series of roofs - keeping things interesting.

The Dinorwic Quarry, Australia area

But what is it that makes this rock behave the way it does, creating the contrast between those sharp edges and featureless, smooth faces? The slate is old and has experienced numerous events of geological upheaval to get to how it is today.

Originally fine particles fell out of suspension to form a fine mud at the bottom of an ocean basin, the Welsh Basin, slowly building up, resulting in thick deposits over millions of years. This was back in the Cambrian Period (named after Wales, Cymru) around 540 - 490 million years ago (the slates further south around Blenau Festiniog were deposited a few hundred million years later in the Ordovician). At this time the microcontinent of Avalonia (what was to become Wales and England) was on the southern edge of a large ocean, the Iapetus.

The Welsh Basin was a back arc depositional basin, an area of extension and subsidence behind a volcanic arc, created due to the subduction of the Iapetus Ocean beneath Avalonia.

Deposition in the Welsh Basin in a back-arc setting

With the closure of the Iapetus, the Northern and Southern halves of the British Isles were pushed together in the Caledonian Orogeny through the Ordovician to Devonian times (ending around 390 million years ago). This impact formed the mountain belts of northern Wales, Scotland and the north of England (which would then have been much larger than today), compressing, folding and faulting the rocks of the Welsh Basin, subjecting them to high directional pressures. This directional pressure (or stress) caused the low grade metamorphism of the mudrocks, leading to realignment and recrystallization of platy (flat) clay minerals (micas) and forming the slates. It is this alignment of minerals which causes slates to cleave along planes of weakness in one direction. This cleavage is what made the slates perfect for use as roofing tiles and creates the sharp edges and smooth faces we see today on the rock.

Slate formation: (1) Deposition of fine platy-clay minerals in a low energy marine setting; (2) Compression during mountain building event realigns the platy minerals perpendicular to the direction of applied pressure; (3) Planes of weakness are created which the slate cleaves along 

Fast forward nearly 400 million years and the Llanberis slate quarries were mined for hundreds of years, getting deeper and deeper into the earth and creating the stepped layers of exposed rock walls we can now explore.