The Slippery Rock Gorge Trail at McConnell's Mill State Park in Ellwood City, Pennsylvania is a difficult 14.3 mile moderately-trafficked out-and-back trail. It's the longest trail at McConnell's Mill State Park and is one of the more popular day-hike trails; however, due to its length you often don't pass other hikers.
You can start the trail at two different places - near the Hell's Hollow Trail parking lot along Shaffer Rd in Ellwood City or the covered bridge along McConnell's Mill Rd. The GPS data here starts from Shaffer Rd and runs along Hell Run for about 2.2 miles before turning left and heading northeast along Slippery Rock Creek.
Once the trail makes it to Slippery Rock Creek, it will start to ascend hillsides and eventually has a couple of switchbacks before reaching Eckert Bridge. The trail crosses the road at Eckert Bridge and continues on for another 1.1 miles before reaching the covered bridge, mill, and waterfall.
This trail is great for fishing and dogs are also allowed here.