Around Jay Cooke State Park, the basalt from the Midcontinent Rift eruptions eroded away long ago. In fact the rock layer immediately above the Thomson Formation is the Fond du Lac Formation, which dates from the late Mesoproterozoic era, an unconformity of about 800 million years. This red and brown sedimentary layer is composed of sandstone, siltstone, and shale. The bottommost layer is a conglomerate of quartz pebbles originating from quartz veins in the Thomson Formation. Above, cross-bedding of the sandstone, several layers of mud-chip conglomerate, and mudcracks indicate deposition in a broad river plain. Only small portions of the Fond du Lac Formation are visible within Jay Cooke, primarily along the Little River in the northeast part of the park. Better exposures are found in Fond du Lac, a neighborhood on the outskirts of Duluth just outside the park's eastern border. Many of Duluth's brownstone buildings were constructed from quarries there.