Pennsylvania isn't usually thought of as a climbing destination. It doesn't have cliffs like the Gunks or New River Gorge. What Pa lacks in rope climbing it more than makes up for in boudering. There are boulders all over the place and many are of extreme quality. PA may be the only state that has a good quantity of the STICKY 4. Sandstone, Gritstone, Gneiss, and Diabase. Because of the particular grain structure of these rocks, they offer amazing friction for hands and shoe rubber. Sticky is good for climbing. The better you stick, the more stuff you can get away with. The more "wow, I can't believe I just did that" moments you have. Sticky rock is addictive. Sticky rock is easy on the tendons.
Sandstone - For western climbers this is a word not associated with quality. Western sandstone is poorly cemented choss. Even the best Mill Creek or Joe's Valley sandstone can't compare to the sandstone in the East.

The Fontainbleau quality sandstone, found in central Pennsylvania is some of the best climbing rock in the world.

What makes this sandstone so good? Indirectly it's the purity. Take >99% pure quartz sand, add heat, pressure and H20 and the grains start growing together, solidifying into solid rock. If the process goes too far the rock turns into slick quartzite - essentially solid quartz. You want just enough quartz cement to stick the grains together but you don't want to fill in the spaces between the grains. With the proper amount of cementing you get grippy sand grains at the rock surface that won't break away.