Tag Archives: Hartshorne Sandstone

New Publication: Geologic Road Guide to Highway 10

 

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The Geologic Road Guide to Arkansas State Highway 10, a Geotour of the Southern Arkoma Basin Fold Belt and Related Ouachita Mountain Tectonic Zones by Drs. Richard Cohoon (Emeritus), Jason Patton (Associate), and Victor Vere (Emeritus), Professors of Geology at Arkansas Tech University, is now available for download on the Arkansas Geological Survey’s website.  Here’s the link:

http://www.geology.ar.gov/roadside_geology_series/rgs02.htm

The route begins at Petit Roche Plaza in the River Market District of downtown Little Rock. “Petit Roche” was the name given to the first rock outcrop early explorers encountered on their way up the Arkansas River.  It is near this outcrop that the eastern end of Arkansas State Highway 10 (AR-10) is now located.  From here, you will tour the 139-mile length of AR-10 to its western terminus at the Oklahoma state line, just past Hackett.  This route traverses a beautiful and geologically diverse cross section through the mountains of western Arkansas.  The stretch from Ola to Hackett is designated as an Arkansas Scenic Byway.

An overview of the physiography of Arkansas, the concept of geologic time, and the rock formations and structural regions encountered along AR-10 introduce the reader to the detailed Road Guides that follow.  The Road Guides describe the rock outcrops and geologic features along particular sections of the route.  They contain many wonderful color photographs and color-coded geologic maps to help travelers understand the landscape passing outside their windows.  Travelers are encouraged to get out of their vehicle at several places to have a look at the rocks, perhaps gaining a new appreciation of their significance.  An illustrated glossary defines words and concepts that may be unfamiliar to those without an earth science background.  Appendices direct the traveler to several interesting side trips just off the main route and detail the characteristics of the gas and coal resources in the Arkoma Basin.

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This Geotour is written to be of interest to the general public, to students of geology, and to professional geologists who want to gain a more in-depth understanding of this beautiful and geologically complex region.  So the next time you’re thinking of taking a scenic drive through the mountains of western Arkansas, consider traveling AR-10.  And don’t forget to take along the Geologic Road Guide to make your drive more enjoyable and informative.

Richard Hutto

GeoPic of the Week: Turtle Rocks at Petit Jean State Park

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“Turtle Rocks” are unique, mounded polygonal structures that resemble turtle shells. These features are found along the Arkansas River Valley in the Hartshorne Sandstone, a brown to light gray, massive, medium-grained 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 creating 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.

Fountain Lake High School Petit Jean Field Trip, May 2, 2014

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.HartshorneShelter_ClassPhoto

Senior class photo in a natural shelter within the Hartshorne Sandstone. The natural archway is lined with great liesegang banding features.

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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.

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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).

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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.

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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.

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Notice the nice cross beds in the middle section of the large boulder above Ms. East’s head.

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We ended the day with a final photo session in both Rock House Cave and on the turtle rocks located on the trail.

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There are those Sig Figs (FLHS Science Club). They are reminiscing about the day’s awesome geology field trip.