Threlkeld Knotts (c. 500 m above sea level) in the English Lake District has hitherto been considered to be a glacially-modified intrusion of microgranite. However, its surface features are incompatible with glacial modification; neither can these nor the subsurface structures revealed by ground-penetrating radar (GPR) be explained by post-glacial subaerial processes acting on a glacially-modified microgranite intrusion. Here we re-interpret Threlkeld Knotts as a very large post-glacial landslide involving the microgranite, with an estimated volume of about 4 × 107 m3. This interpretation is tested against published and recent information on the geology of the site, the glacial geomorphic history of the area and newly-acquired GPR data. More than 60 large post-Last Glacial Maximum (LGM) rock–slope failures have significantly modified the glaciated landscape of the Lake District; this is one of the largest. Recognition of this major landslide deposit in such a well-studied environment highlights the need to continuously re-examine landscapes in the light of increasing knowledge of geomorphic processes and with available technology in currently active or de-glaciating environments.