An unplanned, but unique, experiment has given an in situ measurement of the strength of deforming subglacial till under the central region of a major valley glacier. We report on both planned and unplanned borehole investigations of the subglacial shear zone of Columbia Glacier, southeast Alaska. Basal samples, coring and down-hole water samples show that the fiord-filling lower reach of the glacier is underlain by a thin, ~7-cm, veneer of rock debris. Fluidized debris intruded at least a meter up the borehole. At a higher site, 13 km from the terminus and above the fiord, probing, samples, and the bending of a drill stem, which was stuck in the basal zone for 5 days, showed that the basal till layer was ~65 cm thick. Horizontal velocity of the till decreased monotonically downward from the ice/till interface. Till at the interface moved with the ice velocity. Plastic deformation of the drill stem gave an estimation of the strength of the basal till, which is normally described as a viscoplastic material. If the till is assumed to be either perfectly plastic or Newtonian viscous, then the strengths are as follows; the plastic yield strength of the till was 5.5¿103 Pa (0.055 bar) with an upper bound of 1.3¿104 Pa (0.13 bar), while the nominal viscosity was of the order of 2¿108 Pa s (2¿109 poise), with an upper bound of 5¿108 Pas. In neither case is the till ''strength'' enough to supply the bulk basal shear stress to resist the glacier flow. ¿American Geophysical Union 1993