A half-million years ago, during one of several ice ages in North America, ice accumulated across Canada and formed glaciers that slowly advanced southward into Pennsylvania's valleys. One glacier crept into what would become Tioga County, Pennsylvania, blocked the original easterly flow of Pine Creek, dammed it, and forced it to spill over into the mountain gap near Ansonia. Around 15,000 years ago, as the climate warmed and the glacier melted, Pine Creek, fed by melting ice, carved a deep new channel, or gorge, now known as the Pennsylvania Grand Canyon, and it never again flowed east to the Tioga River.