The oldest bedrock visible in Jay Cooke State Park is the Thomson Formation, dating to the Paleoproterozoic era 1.9 billion years ago. The southernmost unit of the Animikie Group, the Thomson Formation is named for the nearby town of Thomson, as this is its 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.