type locality. It began as mud, silt, and clayey sand deposited by turbidity currents at the bottom of a deep sea. These sediments compacted into horizontal layers of shale and greywacke. Ripple marks are still visible in the resulting rock, but there are no signs of life as complex organisms had not yet appeared. 1.85 billion years ago a mountain-building event to the south called the Penokean orogeny subjected the sediments to intense pressure and heat. This metamorphosed the shale into dark, thin layers of slate. The slate also developed a pattern of vertical cleavage, but the more massive greywacke deposits were largely resistant to this foliation. Both types of rocks, though, were folded and fractured, giving them the wildly tilted appearance that distinguishes them today. 1.1 billion years ago the Midcontinent Rift System cracked the Thomson Formation, creating southwest-to-northeast trending fractures. Volcanic eruptions sent magma through these gaps, forming the distinctive flood basalts of Minnesota's North Shore. Magma remaining in the underground fractures cooled more slowly into gabbro. Several of these dikes are exposed around the swinging bridge and also just outside the park where Highway 210 crosses the river below the Thomson Dam. They are nearly the same color as the Thomson slate, but can be distinguished by their horizontal columnar joints and lack of slaty cleavage. 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. Glaciation Although the rocky riverbed around the swinging bridge is Jay Cooke's best-known feature, this Precambrian formation is only exposed in a portion of the park. Elsewhere the land