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Pictured above is a weathering style called honeycomb weathering. Honeycomb is a type of differential weathering that produces a Swiss cheese appearance on the rock’s surface that can be quite striking. This example is from a sandstone outcrop (Prairie Grove member) at Devil’s Den State Park in northwest Arkansas.
Honeycomb weathering has been observed in many rock types, from igneous to sedimentary, forming in wet and dry environments. It has even been noted in man-made structures. Despite being common, the causes of honeycomb weathering are poorly understood. Some studies have linked its formation to exposure to salt in coastal regions, but that doesn’t explain its occurrence in north Arkansas.
In this instance, some clever bats have taken advantage of one of the pits in the rock and are using it as a sleeping shelter.
For more views of honeycomb weathering click here.
The Arkansas Geological Survey hosted a field trip to Petit Jean State Park for 26 Fountain Lake High School seniors and science club students. The high school seniors are currently in a college geology course taught by Mrs. Jennifer Cox, a former geologist with the AGS. As far as we could tell, these seniors were ready to show off their geologic knowledge. Two students, whom I understand are brothers, were excited enough to buy Muscadine Grape Juice from the Visitor’s Center prior to the start of our trip. Nothing says geology like a good swig to start your day.
Our first stop was to Seven Hollows Trail. Along the trail, we first looked at liesegang banding and a natural shelter within the Hartshorne Sandstone. Liesegang banding (aka box-work) is created when water percolates through the sandstone and comes in contact with the iron minerals present causing the iron to go into solution. As the rock is exposed to air, oxygen is added to the solution, oxidizing the iron and causing it to precipitate out of solution along exposed joints and/or bedding planes in the rock formation. The iron sometimes precipitates out as box-shaped and triangular patterns. The natural shelter within the sandstone was created as a result of weathering. Again, water percolates through the sandstone and between individual sand grains, causing the grains to loosen and separate from the rock. After millions of years of weathering, large voids are created within the rock. This large void appears to be a prime location for the first of many class photos.
Senior class photo in a natural shelter within the Hartshorne Sandstone. The natural archway is lined with great liesegang banding features.
Liesegang bands, or carpet rocks based on their square pattern, adjacent to natural shelter.
Our last stop was to Natural Bridge and the turtle rocks above natural bridge. We had some excited young geologists who immediately began to climb on top of the natural bridge.
Natural Bridge (left). Again notice the great liesegang bands in the archway. Young adventurous, soon-to-be geologists climbing above natural bridge to the turtle rocks above (right).
Another senior photo op at natural bridge.
Turtle rocks above natural bridge are some of the best features in Petit Jean State Park. “Turtle Rocks” are unique, mounded polygonal structures that resemble turtle shells. These features are found along the Arkansas River Valley in the Hartshorne Sandstone deposited during the Pennsylvanian Period by ancient river systems. The processes that generate “turtle rocks” are not clearly understood. One explanation suggests that these features were created by a process known as spheroidal weathering, a form of chemical weathering that occurs when water percolates through the rock and between individual sand grains. These grains loosen and separate from the rock, especially along corners and edges where the most surface area is exposed, which widens the rock’s natural fractures and creates a rounded, turtle-like shape. Additionally, iron is leached from the rock and precipitated at the surface creating a weathering rind known as case hardening. These two processes along with the polygonal joint pattern contribute to this weathering phenomenon.
Exploring these great turtle rocks. Everyone was thinking that these features were definitely worth the hike.
After exploring these sedimentary features, we headed back up the trail toward the bus, ready for lunch. I’m not sure how anyone had any energy left after the hike, but it seems most of the students finished their lunch pretty quickly so they could play around on the playground.
After lunch we headed to Rock House Cave, a large rock shelter within the Hartshorne Sandstone. Honeycomb weathering and cross bedding features are easily visible around Rock House Cave. Honeycomb weathering is created very similarly to how the natural shelters are formed (e.g. Rock House Cave, natural shelter along Seven Hollows Trail), in that water percolates through the sandstone, loosening and separating the sand grains from the rock creating a void. Cross beds are diagonal lines that represent movement of large ripples within the sandstone deposited by an ancient river system that existed here 300 million years ago. These cross beds indicate the direction the river once flowed.
Notice the nice cross beds in the middle section of the large boulder above Ms. East’s head.
We ended the day with a final photo session in both Rock House Cave and on the turtle rocks located on the trail.
There are those Sig Figs (FLHS Science Club). They are reminiscing about the day’s awesome geology field trip.
Another great week in the field! Temperatures are coming down some which makes things much more bearable. Monday we came down from Fox and crossed the Middle Fork at Lydalisk. Went as far south on the old railroad grade as we could then worked our way back, getting points along the way. Still finding the Imo near the bottom of the valley. The river was so low that we could actually cross it on foot to get a drainage on the east side. The first good outcrops we found there were the lower Cane Hill. There were good examples of taphoni in the rocks there. Commonly known as “honeycomb weathering”, taphoni is caused by weathering processes in many rock types and under many conditions. In the Cane Hill it typically affects the thin-bedded sandstones in dry areas beneath overhangs and is probably due to differential dissolutioning of the cement by groundwater. This process also causes the concave structures as noted in previous blogs.
The next two days we started at the upper end of Indian Creek and worked our way down. Lots of Fayetteville shale production going on in this area, which means good roads for us! Seemed to be in Witts Springs sandstone the entire stretch, so not a lot of contacts. Definitely had a lot of dipping rock and signs of a large fault further south including deformation bands and non-vertical joints. Located a couple small faults with what we think is minimal offset along the creek and adjacent hollows.
Did have a few encounters with our reptile friends. We always leave them be so maybe word will spread among their kind that we mean no harm. It’s worked so far!
See you on the outcrop!
Snake count: 2
Tick attacks: severe