Located in Hickory Run State Park on the Pocono Plateau near the headwaters of Hickory Run, this landmark is one of the most striking geologic features in the state of Pennsylvania. A true relic of the past, it has remained relatively unchanged for more than twenty thousand years.
Measuring Approximately 400 feet by 1,800 feet and at least 12 feet deep, the boulder field is the largest of its kind in the Appalachian Mountains. Its large size and low gradient make it geologically significant. So much so that in November 1967 it was designated a National Natural Landmark by the National Park Service.
The Shrinking Boulder Field
An EarthCache about Weathering, Frost Action, and Ecological Succession
Introduction
At first glance, Boulder Field appears permanent. Thousands of large boulders cover the landscape, creating a seemingly timeless sea of rock. But despite its rugged appearance, this landscape is constantly changing.
The changes occur so slowly that they're difficult to observe during a human lifetime, yet two powerful natural processes continue to shape this site today:
- Physical weathering, which continues to break rocks apart.
- Ecological succession, which slowly transforms open rock into forest.
Together, these processes tell the story of Boulder Field's past, present, and future.
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How Boulder Field Formed
During the last Ice Age, approximately 17,000 to 22,000 years ago, this region experienced a much colder climate. Water repeatedly seeped into cracks within the bedrock.
When temperatures dropped below freezing, the water expanded as it turned to ice. This expansion increased pressure within the rock and gradually widened existing fractures. Over thousands of years, repeated freeze-thaw cycles split the bedrock into large angular blocks.
Gravity then moved the broken rock downslope. During seasonal freezing and thawing, the boulders slowly shifted within a mixture of ice, water, and sediment. The process was extremely slow, taking thousands of years to create the Boulder Field that exists today.
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Frost Weathering Continues Today
The same process that helped create Boulder Field continues to operate.
Many rocks contain small cracks, fractures, and pore spaces that collect water. During winter, water freezes and expands. Repeated cycles of freezing and thawing gradually enlarge these openings until pieces of rock separate from the parent boulder.
As you explore the field, look for rocks that appear to fit together like puzzle pieces. These rocks may once have been part of a single larger boulder. You may also observe fractures that suggest a rock is currently in the process of splitting apart.
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Succession: The Forest Moves In
Although the boulders dominate the landscape, the forest is slowly reclaiming the field.
Each autumn, leaves and other organic debris fall between the rocks. Over time, this organic material decomposes and forms soil.
The spaces between the boulders are a challenging environment for plant growth, but some seeds find suitable conditions for germination. Small plants become shrubs, shrubs become trees, and the forest gradually expands into the open rock field.
This process is known as ecological succession.
The change is slow, occurring over decades and centuries. However, if succession continues uninterrupted, future generations may see a much smaller Boulder Field than we see today.
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Logging Requirements
Email or message the cache owner with the answers to the following questions:
1. Evidence of Succession
Visit an area where vegetation meets the Boulder Field.
Describe two observations that indicate ecological succession is occurring at this location.
2. Soil Formation
Examine several spaces between the boulders.
What materials do you observe between the rocks, and how do these materials contribute to soil formation?
3. Evidence of Physical Weathering
Locate a fractured, cracked, or broken rock.
Describe the evidence you observe and explain how freeze-thaw weathering may have contributed to its appearance.
4. Connecting the Processes
How do physical weathering and ecological succession work together to gradually change the appearance of Boulder Field over time?
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5. Photo
Please post a photograph of yourself, your GPS, or a personal item at Boulder Field with your online log.
Do not include answers to the EarthCache questions in your public log.
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EarthCache Educational Summary
Boulder Field is more than a remnant of the Ice Age. It is an active outdoor laboratory where visitors can observe the effects of physical weathering and ecological succession. The same freeze-thaw processes that helped create the field continue to break down rock, while plants slowly establish themselves among the boulders. Together, these processes demonstrate that even landscapes that appear permanent are constantly changing.
