Antarctic ice shelves typically comprise continental meteoric ice, in situ-accumulated meteoric ice, and marine ice accumulated at the shelf base. Using borehole optical televiewer logs from across Larsen C Ice Shelf, Antarctic Peninsula, we identify and report an intermediate ice unit, located between continental and in situ meteoric ice, that is tens of metres thick and formed of layers that progressively increase in dip (by ~60°) with depth. The unit’s stratigraphic position and depth, supported by flowline modelling, indicate formation at the grounding zone. We hypothesise that the unit forms due to changes in the surface slope of feeder glaciers at the grounding zone, resulting in both variable surface accumulation and intense deformation. The top of the unit also marks the depth at which lateral consistency in radar layering is lost from radargrams, which may, to some degree, mark the depth of grounding zone ice across all ice shelves.