Birker Fell

On Birker Fell, looking to Green Crag

Birker Fell, also known as Birker Moor, is an upland wilderness area in the western portion of the Lake District, in Cumberland. It is not a single high peak, but a broad, undulating area, approximately 2.3 square miles square, with numerous crags and prominences scattered across its area. The highest point of the fell is at Green Crag (1,604 feet), at SD200983.

On the south-east side of the fell the land drops down to Dunnerdale, which marks the border with Lancashire, while Ulpha Fell stands to the south-west. Harter Fell is to the north-east, and Eskdale bounds the moorland to the north-west.

One small hamlet, Birkerthwaite (SD179982), sits in a shallow valley near the centre of the fell, but other than this the only human habitations are scattered hill farms. A road crosses the fell between Eskdale Green and Ulpha, in Dunnerdale.

Landscape

Apart from Green Crag, other notable high points on the fell include:

• Cook Crag (1,539 feet, SD200987);
• White How (1,457 feet, SD204974);
• Great Worm Crag (1,401 feet, SD194969);
• Kepple Crag (1,076 feet, SD198999);
• Great Crag (1,060 feet, SD187978);
• Rough Crag (1,047 feet, SD162977);
• Water Crag (1,001 feet, SD154975);
• Brantrake Crags (850 feet, SD148982).

Between these points is a plethora of smaller crags and knolls, separated by shallow valleys and bogs, giving the high fell an uneven and chaotic appearance.

Water plays an important role in defining the character of Birker Fell. Between the crags flow many small streams, known as becks or gills in the local terminology. Many of the becks rise in one of the numerous bogs which occur in the area, the largest of which are White Moss, Sike Moss, Tewitt Moss and Foxbield Moss.

At the western edge of the fell lies Devoke Water, which claims the title of largest tarn in the Lake District. It lies at an altitude of 770 feet and is approximately a thousand yards long (east-west) and 450 yards wide.

The southern border of Birker Fell against Ulpha Fell traces a line between Devoke Water and the valley of Crosby Gill, a large stream that drops down the southern flanks of the fell to the village of Ulpha, on the Cumberland side of Dunnerdale. The northern edge of the fell is marked by a steep range of crags which drop sharply to the floor of Eskdale. These crags are cut by the cascades of Stanley Force and Birker Force, two of the most spectacular waterfalls in the Lake District.

Geology

Geologically, Birker Fell is of importance as the type locality of the Birker Fell Formation of the Borrowdale Volcanic Group. The Birker Fell Formation forms the lowest, and hence oldest, portion of this famous sequence of andesite and dacite volcanic rocks. The Birker Fell Andesite occurs throughout much of the western Lake District. The rocks of the Borrowdale Volcanic Group are thought to have formed in the early Ordovician by island arc volcanic activity, due to subduction during the closing of the Iapetus Ocean.

The central and northern areas of Birker Fell are dominated by composite andesite lava flows. Within the andesitic lava flows, the Great Whinscale Dacite lava flow and the associated, underlying Little Stand Tuff form a marker band that runs northeast-southwest across the fell, but are best seen at Silver How (SD195983) and Great Whinscale (SD197989). A small area of basaltic material occurs approximately 1 km north of Birkerthwaite, composed of plagioclase- and pyroxene-phyric andesite-basalt lavas (the Birkby Fell Member), and tuff and lapilli-tuffs (the Devoke Water Member). The main outcrop of the Devoke Water Member occurs to the south-west of Devoke Water itself, on Ulpha Fell. The south and south-eastern portion of the fell is composed of a sequence of tuff beds with highly variable composition: from rhyolitic through to basaltic. These overlie the Birker Fell Formation and form the next section of the sequence.

In the north-western corner of Birker Fell, extensive faulting has juxtaposed the Borrowdale Volcanic Group rocks against granites of the late Ordovician Eskdale Intrusion.

References

• Akhurst, M.C.; Chadwick, R.A.; Holliday, D.W.; McCormac, M.; McMillan, A.A.; Millward, D.; & Young, B. (1997). Geology of the west Cumbria district. Memoirs of the British Geological Survey, sheets 28, 37, 47 (England and Wales). British Geological Survey. Keyworth, Nottingham. pp. 138
• British Geological Survey (1996). Ambleside. England and Wales Sheet 38. Solid Geology. 1:50 000. British Geological Survey. Keyworth, Nottingham. ISBN 0-7518-2889-0
• Moseley, F. (Ed.) (1990). The Lake District (2nd ed.). Geologists' Association Guide No. 2. The Geologists' Association. pp. 213 ISBN 0-900717-73-4.