McConnell's Mill Historic Site, Gristmill, and Covered Bridge

Slippery Rock Creek and the university in neighboring Butler County ( BU15) are named for an exceptionally slick rock six miles south of the mill near a Native American fording site that was naturally covered by an oil seep. Although the region's oil has been depleted, the scenery of the forty-nine-mile-long rocky gorge remains an attraction. The gristmill was constructed by Daniel Kennedy after fire destroyed the original of 1852. Standing three-and-one-half-stories tall, the lower one-and-one-half stories are built of local sandstone salvaged from the original structure, and the remainder is of wooden board-and-batten construction. The upper stories store grain to keep it dry for grinding. The mill was purchased by McConnell, Wilson and Company in 1875, and updated by the replacement of the old waterwheel with water turbines, and the grinding stones with rolling mills. A dam in Slippery Rock Creek pools the water then releases it through a raceway to turn the mill's turbines. The mill operated profitably until 1928, when it closed due to the retirement of James McConnell, decreased profits, and the advancement of milling technology. Over 2,500 acres of the rocky gorge and its surrounding park were sold to the Western Pennsylvania Conservancy in 1946, and ultimately deeded to the Commonwealth of Pennsylvania. In 1957, it opened as McConnell's Mill State Park.

On the west side of the mill site is a covered bridge of 1874 crossing Slippery Rock Creek. It is one of only three Howe truss bridges remaining in western Pennsylvania. Howe trusses were the first to use wood connected by iron rods, and were the forerunner of all iron truss bridges. Lawrence County's other covered bridge, a 121-foot Burr truss built in 1889, crosses the Neshannock Creek in Wilmington Township. Bridges are often associated with gristmills, as they provide critical access to the mill from both sides of the waterway.