The Rocks That Rolled – Boulder Field, Hickory Run State Park

Written by Susan Gallagher,
Chief Naturalist at Carbon County Environmental Education Center and author of nature blog:

The Lonely Raven


Tucked away in the northeastern corner of Hickory Run State Park, Boulder Field, Carbon County’s hundreds of millions of years old geological oddity, is a bit off the beaten path. Though it can be reached on foot via Hickory Run State Park’s vast trail system, most people prefer to travel there by car. Signs beginning along State Route 534 and continuing through Hickory Run State Park direct visitors to a small parking lot and interpretive signs at Boulder Field’s edge. From the main highway, the drive to the entrance of Boulder Field is about four miles long, and this part of the trip is, in itself, worth the time. Though years ago Hickory Run’s trees were decimated by clear cutting and forest fires, many impressive specimens remain, and some can be seen along the first paved mile of the ride. After that, the forest opens up a bit and the road becomes a one-way dirt track. There is no winter maintenance, though visitors will have little trouble making the trek under normal weather conditions.


Wildlife sightings aren’t uncommon on the drive through Hickory Run to Boulder Field. Turkeys, grouse, warblers and woodpeckers of all sorts – it’s a birder’s delight! Drivers can also make a quick stop at Hickory Run Lake along the way. Though there is no swimming here, the place is a hotspot for local fishermen during spring stocking season. At other times of the year it’s usually a quiet place, and one of Hickory Run State Park’s better kept secrets. The best part of this ride comes at the end. After a few meandering turns, you swing around to the right and Boulder Field suddenly pops into view – a wide, flat expanse draped on all sides by lush evergreens. It seems starkly out of place, and immediately begs the question, How did these rocks get here?


How the rocks came to be, and how the field came to be are really two different stories. The rocks themselves are ancient, laid down as sediments which slowly turned to stone more than 300 million years ago. The rocks are older than the dinosaurs, a part of what geologists call the “Catskill Formation”. This is the type of bedrock underlying a large portion of northeastern Pennsylvania, including most of Monroe, Pike, Wayne and Susquehanna counties. Rub your fingers gently on one of the boulders, and you may be able to feel the fine grains of sediment which were cemented together eons ago. The red color and sandy texture of these grains give the rock its common name: red sandstone. Scattered about Boulder Field you’ll also find conglomerate sandstone – rock with chunks of white, milky quartz crystals embedded within.


Geologists don’t all agree on exactly how this sediment-turned-to-stone became Boulder Field in Hickory Run State Park. Signs at the edge of the field’s parking lot describe an ancient valley where the boulders now sit, a valley straddled on either side by cliffs made up of red and conglomerate sandstones. Others suggest the sandstone cliffs weren’t cliffs at all, but a single, gently sloping mass of bedrock situated to the east of the current field (to your left as you enter from the parking lot). Whichever form the sandstone took on, the consensus is this mass of rock was broken into boulders by the same kind of “freeze / thaw” process responsible for most of our potholes and cracked sidewalks; rain or melt water seeps into small crevices in the rock, then freezes solid. Because water expands as it freezes, it expands the crevice in turn. Repeated freezing and thawing continues to widen small spaces in the rock, eventually splitting cliffs or masses of bedrock into boulders, and boulders into smaller pieces over time. The freeze / thaw process was helped along by the area’s glacial climate. Scientists agree most of the rocks were broken apart about twenty thousand years ago, during North America’s last Ice Age. It was then that a massive glacier stopped just short of Boulder Field – only a quarter-mile to the northeast. Though the glacier didn’t deposit the rocks in their current position, its proximity was an important factor in shaping the weather, and therefore in shaping the Hickory Run State Park landscape.


At the time, the area that is now Hickory Run State Park looked much like present-day Greenland. This meant less plant life, which in turn meant more exposed rock. Temperatures fluctuated, things froze, things thawed, and the rocks rolled. Boulder Field was also shaped by “frost heave” – another process to wreak havoc on our present day roads and sidewalks. Water that has seeped into soil also expands when it freezes, sometimes uplifting that soil an inch or more. Some geologists think frost heave was responsible for the depressions found scattered throughout the field – ancient “potholes” that look like odd stone circles, with larger boulders on the outer rim, and smaller ones in the center. Though the glacier has long since receded, freezing and thawing still shape Boulder Field – though much more slowly today. Look closely and you’ll find evidence of recently fractured rocks, their sharp edges standing out in contrast to the more rounded forms worn smooth over time.



Other factors influence Carbon County’s Boulder Field as well. Some plants are able to make a living by growing directly on the rocks, without the benefit of soil. These plants secrete chemicals that allow them to stay fixed to the boulders. These chemicals are slowly wearing away fine bits of sediment from the stones. One of these plants is actually a type of algae called protococcos. It appears as patches of flat, circular green growths throughout the field. Plants called lichens (pronounced LIE-kens) also grow on the rocks, but take on a variety of forms. Some look like withered lettuce, others like simple black dots. One – known as reindeer lichen – resembles tiny little deer antlers, and can be found in abundance on the south side of the field. Your own footsteps, scraping away at the rocks as you teeter your way across the field, are slowly wearing away at the rocks, too, turning big boulders into smaller ones by the slightest of margins. That wear and tear is expected, and does little to take away from the natural beauty of the area. Vandalism, littering and the removal of rocks are more serious threats, and are strictly prohibited.


In addition to preserving the area as it is, visitors are asked to consider their own safety before hiking out on the rocks. Sturdy shoes are a must on this hiking trail. Sandals or flip-flops are an invitation to disaster. Even with the proper footwear, rocks are slippery when wet, making rainy day hiking a bad idea. Visitors are encouraged to take along extra water on warm, sunny days, since the temperature of the rocks may be higher than that of surrounding air. Plastic drink bottles are recommended, since glass should not be taken out onto the boulders.


To make the most of your visit to Boulder Field, consider attending one of the lectures offered by Hickory Run staff. The park’s Environmental Education Specialist may also be available to schedule hiking tours for school or scout groups. Visiting naturalists from Carbon County Environmental Education Center in Summit Hill can do the same.



Whether you make it a quick stop, or decide to spend the day hiking out on the rocks, be sure to include a hiking trip to Boulder Field in your plans when visiting Carbon County. It’s a truly unique way to connect with a geologic past full of sand and soil, ice and water – and rocks that rolled.

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