To the south of the fault on Clough Head are rocks of the Birker Fell Andesite Formation. These rocks are among the earliest of the volcanic rocks of the Borrowdale Volcanic Group, and are part of a thick succession of andesite sheets which now outcrop in a wide band around the western and northern sides of the Lake District. These sheets were formed by successive eruptions of mobile andesitic lava from shallow-sided volcanoes. The composition of the erupting magma varied from time to time, and the geological map shows an area of basaltic andesite along the crags on the western side of the fell, and areas of dacite higher on the fell. Individual lava flows may be separated by beds of volcaniclastic sandstone, that is, sedimentary deposits formed from the erosion of the volcanic rocks and deposited by streams, floods and possibly wind.: The geological map shows a number of areas of volcaniclastic sandstone on the crags on the west side, as well as a larger area surrounding the summit, into which an andesite sill was intruded.