Hydrogeologic information in the form of textural properties, field- and laboratory-determined hydraulic conductivity values, vertical hydraulic gradients from piezometer nests, and major-ion and isotopic contents ( 18O, 2H, 3H, 13C, 14C), was obtained from four sites located in southwestern Ontario on thick deposits of clayey till and glaciolacustrine clay. At each site the piezometers were installed from the water-table zone, situated 1–4 m below the surface, to a maximum depth between 20 and 35 m below the surface. Values of hydraulic conductivity determined in the laboratory by consolidometer and triaxial methods are in the order of 10 −8–10 −7 cm/s and are similar to values calculated from rates of water-level rise in piezometers. Mean values of average vertical groundwater velocity, calculated with the measured values of hydraulic conductivity, porosity and hydraulic gradient, using the Darcy equation for a saturated non-fractured porous medium, range from 0.13 to 0.26 cm/yr. Tritiated pore waters only occur within 3–6 m of ground surface, thereby indicating that groundwaters below this depth recharged prior to at least 1952. 18O in the pore waters exhibit a distinctive regular shift with depth from water-table values of −9 to −10‰ (SMOW), characteristic of present-day precipitation, to values between −14 and −17‰ at depths of 20–40 m, which are characteristic of much cooler waters. Corrected 14C ages of the groundwater are greater than 8000 yr. B.P., thereby suggesting that the groundwaters at depth entered these deposits during or after the formation of these deposits approximately 11,000 to 14,000 yr. ago. 18O, 2H and Cl − concentration profiles were simulated with a one-dimensional model for transport by advection and diffusion in a saturated porous medium. Reasonable agreement between the model and the field profiles was obtained with values of effective diffusion coefficients of 3.0 · 10 −6 cm 2/s for 18O, 2H and Cl − and with groundwater velocities between 0.03 to 0.05 cm/yr. Based on these results, it is concluded that the pore water in these deposits is a mixture of late Pleistocene and modern waters and that the distribution of 18C, 2H and Cl − in these deposits is influenced predominantly by molecular diffusion, apparently more so than by hydraulic flow.