And if you listen closely, you might sometimes hear water flowing beneath the rocks.
The outline of the field is irregular, but the surface is surprisingly flat.
The rocks were broken down from the surrounding ridges by the continual freeze-thaw cycle.
During summer, water seeped into the porous rocks and froze in winter.
Freezing water expands generating a force powerful enough to break rock into pieces.
When the glaciers melted, the melt water carried the boulders and deposited them on the valley.
When the glacier vanished from Pennsylvania and the climate warmed, all movement ceased.
The boulder field has been lying stationary for tens of thousands of years.
The Hickory Run Boulder Field was designated a natural national landmark in 1967.
