This is an EarthCache, not a traditional geocache. There is no physical container to find here. Instead, you’ll visit this beautiful location, make some observations, and answer a few questions to learn about the geology that created this natural feature.
Please only visit during daylight hours, and enjoy your time at this scenic spot! In warmer months, you’ll often see people swimming and relaxing in the cool water.
Earth Science Focus
Discover how erosion and rock resistance shaped this striking natural waterfall in the Atoka Sandstone Formation, revealing how layers of ancient seabed have endured—and continue to evolve—over millions of years.
The Geologic Story
The geological formation known as Natural Dam is a rock wall of Atoka sandstone about 200 feet wide that spans Mountain Fork Creek, creating a natural waterfall and pool within the Boston Mountains of northwest Arkansas.
This easily accessible feature is composed of Carboniferous-era sandstone, part of the Atoka Formation, which dates to the Pennsylvanian sub-period of the Carboniferous Period (approximately 320–300 million years ago).
During that time, this region was a shallow marine and swampy environment. Over millions of years, sediments accumulated, were buried, and were compacted into layers of sandstone and shale. The harder Atoka sandstone now forms this impressive rock shelf.
Photo credit: Mike Newman/Arklahoma Hiker
The rock barrier known as Natural Dam is entirely natural—not man-made as some might assume. Its appearance results from differential erosion: the Atoka sandstone is more resistant to weathering than the softer rock that once surrounded it. Over time, water eroded the weaker layers away, leaving the sandstone ledge that acts as a natural waterfall and low dam today.
Because erosion is an ongoing process, the shape of Natural Dam continues to change slowly as water and sediment move across it.
📋 Logging Tasks
To claim credit for this EarthCache, please visit the coordinates and send your answers to the questions below.
If you’re with a group, one person may send the answers for everyone—just include all names in your message.
Also include a photo of yourself or an object at the site. If you prefer not to post it publicly, you may send it to me privately. Logs without a photo (posted or sent) will be deleted.
1. Look closely at the rock wall that forms the dam. Describe what you see in the rock layers (for example: cracks, layering, color differences, smooth vs. jagged areas).
B. Based on your observations and the description, what do these features tell you about how erosion has shaped this formation?
2. Observe the water as it flows over the rock ledge. How does the water’s movement appear to affect the rock below the falls compared to the rock above it?
B. What does this tell you about how erosion continues to change Natural Dam over time?
3. The Natural Dam exists because Atoka sandstone is more resistant to erosion than the softer rock that once surrounded it. Based on what you see at the site, where do you think erosion is happening most quickly today, and why?
B. What might eventually happen to this feature in the distant future if erosion continues at this rate?
4. Remember to post or send a photo of yourself or an object at the site. Logs without a photo either sent or posted will be deleted.