Felsenmeer - a "sea of rock"

Felsenmeer is a German term meaning “sea of rock” and is used to describe blockfields that formed during glacial epochs when temperatures just south of the glaciers fluctuated frequently. Repeated freezing and thawing of water trapped in small cracks in the bedrock broke the rock into angular boulder-sized rocks. 

At that time there was little vegetation on the ground and what there was resembled the vegetation now growing in the high arctic.

Two of Pennsylvania's best known felsenmeers are the Hickory Run Boulder Field in Hickory Run State Park and the River of Rocks in Hawk Mountain Sanctuary. Felsenmeers occur throughout much of Pennsylvania’s Ridge and Valley Region, some fairly small and some quite large. Little of the Ridge and Valley Region in central Pennsylvania was glaciated in either the most recent glacial era or the preceding one. But most or all of the region had the tundra-like conditions in which these block fields were created.

Now, thousands of years later, these areas of broken rock remain, some on steep hillsides and some on fairly flat areas.

The blockfields, as barren as they appear, do provide habitat for a few species of wildlife. Turkey vultures raise their young in voids between the rocks and Allegheny wood rats build their bulky nests in narrow spaces between the boulders. Unfortunately, Allegheny wood rats are considered threatened in Pennsylvania as their populations have fallen drastically due to the raccoon roundworm, an internal parasite. Here's a photo of the only one I’ve ever seen – in a felsenmeer. 

A number of years ago members of our hiking group explored a rather large felsenmeer on a steep hillside –

Over time vegetation is gradually creeping into the felsenmeers as the spaces between the boulders accumulate decayed vegetation –

But the felsenmeers will still be gracing the Ridge and Valley Region for thousands of years.