This one is another leftover from the glacial period up in Butler County. Back in the day, glaciers dumped and packed down a bunch of sentiment, and as they retreated and the country warmed and dried a massive prairie emerged. Much of it still exists out in the Midwest, but from Ohio eastward wetter weather allowed forests to dominate except in a few isolated pockets, many with an underlying layer of clay that kept trees from successfully taking root.
Jennings Environmental Education Center, a few miles north of Moraine State Park along Rt. 528 (and just south of Route 8), is home to one such "Relict" Prairie. It contains hundreds of plant species, many of which - including the pictured blazing star - don't exist in many (if any) other spots in Pennsylvania. Peak bloom ranges from late July through August. Butterflies and Goldfinches (especially when the thistles are out) also abound.
There are easy hiking trails mowed through the prairie, which make for nice, leisurely strolls that are accessible to anyone that can move under his or her own power. Interpretive signs and scheduled guided tours are also available.
