The geological process that produced the region's coal seams also created a layer of sandstone ideal for rock climbing. Called Nuttall Sandstone, this layer originated in the ancient formation of the Appalachian Mountains. As those mountains eroded, a high percentage of quartz sandstone remained within the Nuttall layer. These hard and, in climber's terms, reliable sandstone cliffs rise 80 to 150 feet high and provide some of the finest rock climbing in the United States. More than 2,000 named climbing routes exist in the park, with even more in neighboring areas outside it. In the gorge, tourism and outdoor recreation have replaced mining and logging as major industries. Although outdoor recreation can be sustainable and environmentally friendly, it also has impacts, and proper management and Leave No Trace ethics help protect the land from overuse and reduce those effects.