The Caledonian Horn Head Slide is a spectacular ductile shear zone transecting Neoproterozoic Appin Group Dalradian metasediments in Donegal (NW Ireland). Two conflicting stratigraphic interpretations exist for the inverted succession exposed in the hanging wall of the structure. These are based on correlation with two quite separate exposed pelite formations elsewhere. The two formations are lithologically indistinct and unfossiliferous. Here we document the novel use of assayed and logged spectral gamma-ray measurements in comparing the contentious pelite in the hanging wall of the Horn Head Slide to the two possible correlative pelite formations from a wide area of their unequivocal outcrop. The data from the contentious pelite show a clear statistical and stratigraphical affinity with one candidate unit only, thus providing the stratigraphical definition necessary for refining previous cross-sections. A new model, based on our spectral gamma-ray correlation, is proposed to account for the northwestwards directed emplacement of the Lower Falcarragh Pelite Formation along the slide. This model requires pre-Caledonian normal faulting as a precursor to the ensuing compressional event in which stratigraphically younger rocks were thrust over older, a common instance in fold and thrust belt geometry. Our work suggests that spectral gamma-ray measurements may provide a rapid, field-based method for differentiating unfossiliferous pelite or mudstone units at outcrop and in geophysical well-logs, even in structurally complex areas.