Cumbrian Green Slate

Cumbrian Green Slate is a sage-green stone quarried from the volcanic rocks of the Lake District. It is not a true slate: it began as volcanic ash rather than mud, and the green comes from chlorite formed when that ash was metamorphosed. Against Welsh slate it is warmer, coarser and thicker, which is why the two never look alike on a roof.

What Is Cumbrian Green Slate?

Cumbrian Green Slate is not, geologically speaking, a true slate in the strictest sense. It is more precisely a phyllite or low-grade metamorphic rock derived from volcanic ash and fine-grained tuffaceous sediments laid down approximately 450-500 million years ago during the Ordovician period. The volcanic origin of the parent material (the Borrowdale Volcanic Group) is what gives the stone its characteristic green colour, derived principally from chlorite minerals formed during metamorphism.

The principal quarrying areas are in the central Lake District, particularly around Elterwater in the Langdale valley (Burlington Slate), Kirkstone Pass, and several other locations across the Cumbrian fells. Burlington Stone at Elterwater is the best-known commercial source; the company has been quarrying and processing Cumbrian green slate for over a century and a half.

The stone splits along cleavage planes developed during metamorphism, producing the flat, rough-faced flags and tiles that are its primary products. The cleavage is less perfectly planar than in high-quality Welsh Slate, giving Cumbrian Green Slate its characteristic slightly irregular, textured surface.

Colour and Character

The colour of Cumbrian Green Slate is its defining quality. The range runs from pale sage green and grey-green through to deeper, warmer olive green, with natural variation between different quarry faces and even within a single piece. The green comes from chlorite (a mineral formed when iron and magnesium silicates were altered during the metamorphic process) and it gives the stone a warmth that distinguishes it from the cooler, more uniform tones of Welsh or Spanish slate.

No two pieces of Cumbrian Green Slate are identical in tone, and this natural variation is considered a quality rather than a defect. The cleft surface enhances the character: rough, textured, and slightly irregular, it catches light and shadow in a way that a sawn finish does not.

Cumbrian Green Slate weathers slowly. External surfaces develop a slightly paler, softer appearance as the surface chlorite oxidises gently, but the characteristic green character is retained indefinitely. Wet stone deepens to a richer green; dry stone lightens towards sage and grey-green.

Applications

Roofing: Cumbrian Green Slate is the traditional roofing material of the Lake District and a significant proportion of Cumbria's historic building stock is roofed in it. The character of Lake District villages (Grasmere, Ambleside, Hawkshead, Coniston) is substantially defined by the green-grey rooflines of this stone. For re-roofing historic buildings in Cumbria, it is the appropriate and often the required conservation specification.

Garden paths and informal paving: Rough-cleft Cumbrian Green Slate makes good garden paths and informal paving. The irregular surface and warm colour suit naturalistic and contemporary garden designs alike. It is not suitable for precision-laid formal paving. Its character is inherently informal and should be used accordingly.

Walling: Dry stone walling and mortared rubble walling in Cumbrian Green Slate is traditional across the Lake District. The stone's moderate weight and irregular cleft faces allow skilled wallers to create the tightly-packed walls typical of the region.

Wall cladding: Split-face Cumbrian Green Slate cladding panels are used in contemporary architecture to bring the character of the Lake District into building exteriors. The warm green colour contrasts effectively with timber, render, glass, and weathering steel.

Steps and copings: The rough cleft surface provides grip, and the green colour reads well against the grass, moss, and stone of Lake District garden settings.

Water features: The green colour and rough texture of Cumbrian Green Slate are particularly effective around water features: the stone's association with the wet, mossy environment of the Lake District fells makes it a natural choice for pond surrounds, waterfalls, and stream edging.

Burlington Stone and Other Producers

Burlington Stone (Burlington Slate) at Elterwater is the most established commercial producer of Cumbrian Green Slate and is the source of much of the stone used in high-quality roofing and architectural applications. The company has supplied stone for major projects including the restoration of Carlisle Cathedral and numerous National Trust properties across the Lake District.

Other smaller quarrying operations exist across the Lake District and wider Cumbrian fells, producing material with varying characteristics. The colour and texture of Cumbrian Green Slate varies between sources, and for conservation work it is worth establishing the original source quarry to ensure appropriate matching.

Care and Maintenance

Cumbrian Green Slate requires minimal maintenance in external use. The cleft surface is naturally non-slip and does not require treatment. Biological growth (moss, lichen) is characteristic of the stone's natural habitat and is typically considered sympathetic rather than problematic in garden and rural settings.

For internal flooring applications, Cumbrian Green Slate should be sealed on installation. The rough cleft surface can trap dirt and moisture, and periodic re-sealing maintains the protective treatment. The inherent variation in thickness of cleft slate flags requires skilled laying on an appropriate bed to ensure a stable, level surface.