The northern parts of Buckbarrow, blending into the slopes of Seatallan, are overlain by drift deposits, much peat being in evidence around Glade How. The summit area shows outcropping of the Birker Fell Formation, an undivided sequence of generally plagioclase-phyric andesite lavas with subordinate sills. At the bottom of the slope are examples of granophyric microgranite, representative of the Ennerdale Intrusion. Dykes of basalt and dolerite run through both formations.