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.