A combination of electrical sounding methods has been used to study the vertical resistivity structure of the crust on the southern extension of the Canadian Shield in northern Wisconsin. Direct current dipole-dipole resistivity soundings were made at transmitter-receiver separations of 1 m to 40 km. Electromagnetic transient soundings were made at ranges of 5 to 40 km and with frequency bandwidths of about 0.5 to 10 Hz. The soundings were made in a region where the gross subsurface structure is laterally uniform, so horizontal, plane-layered models were used to interpret the data. Layered earth models were randomly generated and tested against the observed data to determine the range of models that fit the data. A four-layer model with the following range for the resistivities and thicknesses fits the data: (1) A surface layer, comprised mainly of glacial till, has a few hundred ohm meters resistivity down to depths of a few tens of meters. (2) A bedrock layer has a resistivity in the range of 3000 to 7000 ohm m down to depths of 4.5 to 11 km. (3) A deeper, high-resistivity layer has resistivities of greater than 100,000 ohm m down to depths of 14 to 22 km. (4) A lower layer has resistivities of from 50 to 1500 ohm m. The interpreted resistivities down to depths of about 5 to 10 km are consistent with resistivities of wet rocks containing microfractures. Between 5 to 10 km and 15 to 20 km, we interpret much higher resistivities, which may indicate fewer original microfractures or healed microfractures at these depths. The low resistivities interpreted at depths below 15 to 20 km cannot be explained by usual models for the temperature and composition of the lower crust. The low resistivities can be explained by a higher temperature than is usually postulated for the lower crust, or by water of hydration in the rocks.