About 345 to 280 million years ago, the anthracite deposits beneath Scranton were formed. Slope 190, part of the Northern Coal Field, contains coal deposits with the highest carbon content and the greatest depth in the region. At one point, about half of Pennsylvania's total anthracite production came from this field. The Clark Vein in the Slope 190 mine is especially rich because an unusually heavy deposit of organic matter settled and decayed there. During its operation, the mine had two openings and was worked by both underground drilling and strip mining methods. It functioned for more than 100 years and was owned by the Continental Coal Company when operations ceased in 1966, making it the last deep mine in Lackawanna County to shut down. The Clark bed and the Dunmore #1 and Dunmore #2 beds extend for thousands of feet in all directions, and reaching these deep beds requires a descent of 300 feet down the mine shaft before following a circular route through mine tunnels beyond the main park road and toward the ventilation fan.