 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" () and "Great Whinscale" ().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").