Geophysical studies in the southern Washington Cascades have outlined a possible, previously unrecognized sequence of sedimentary rocks. These postulated sedimentary units are interpreted to correspond to at least the upper section of a low-resistivity (high-conductivity) assemblage of rocks at depths of 1–10 km and with thicknesses up to 15 km that we call the southern Washington Cascades conductor (SWCC). Structure on the upper surface of this conductive assemblage correlates in some places with anticlines that bring Tertiary marine rocks near the surface. These anticlines have recently been imaged in detail with seismic reflection surveys. The conductive rocks have also been traced to the surface west of Morton, Washington, where they correlate with Eocene marine sandstones and shales. The geometry of the conductive rocks consists of a east-dipping, low-angle wedge that thickens to the north and with an undulating upper surface corresponding to the anticlines. We suspect that marine sedimentary rocks are the primary constituent of the SWCC, but recognize that parts of the conductive section may be composed of nonmarine and marginal marine sedimentary rocks like those in the upper part of the Puget Group. Geothermal fluids may be a contributing factor to low resistivities in the deeper parts of the conductive section (greater than 6–7 km). Other possible candidates for the conductive units are thick sections of highly altered volcanic flows and/or graphitic (conductive), pre-Tertiary, metamorphic rocks.