Petit Jean State Park, established in 1923, is Arkansas’ first State Park and contains some of the most scenic geologic features in the state. This EarthCache is established to reveal some of these geologic features and encourage EarthCachers to explore the area on their own and view other features not covered here. The posted coordinates will take you to the Visitors Center, which is a good place to start your exploration of the geological wonders of this park. Inside, you will find information that may help you successfully complete this EarthCache. However, you will also need to visit a few points within the park to gather other information. Enjoy your visit!
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It is highly recommended that you read the description following this section, as it contains some of the answers to the required information and will also provide you with an explanation and better understanding of what you will see at the waypoints.
1) Visit Waypoint A (N 35 07.211 W 092 56.044): What is the geological structure just to the south of the boardwalk? Name the material that caused the veins in the rock and describe the process that caused these formations.
2) Visit Waypoint B (N 35 07.229 W 092 56.071): From this point, you will have a good view of Cedar Falls and Cedar Creek Canyon. Below the falls, you will see the dividing point between two different geologic layers. What are the two layers (or formations) called, and which is the lower layer? (NOTE: The correct answer is not the rock types you see, like shale or sandstone, but the names of the geologic formations.)
3) Still at Wapoint B: If you look at the top of the falls (the point where the water begins its free fall) and follow the edge of the canyon up and to the left of the falls, you will see a small overlook platform across the canyon from your location. If the height of the waterfall between the top of the falls and the dividing point between the two geologic layers is 50 feet, provide an estimate (in feet) of the thickness of the sandstone formation between the top of the falls and the overlook platform across the canyon. What is this layer called?
4) Do ONE of the following:
(a)(Wheelchair Accessible) Visit Waypoint C (N 35 07.040 W 092 56.175): What is the geological formation is located just north of the roadway? Name and describe one process that may have created this formation.
(b)(Non-Wheelchair Accessible) Visit Waypoint D (N 35 07.317 W 092 56.151): What is the geological formation that is all around you? Name and describe one process that may have created this formation.
Petit Jean State Park is located on Petit Jean Mountain, the highpoint of Conway County, and contains many exceptional geological landscapes as a result of weathering and erosion that has taken place during the last 250 million years. Drainage from this bowl-shaped mountain formed scenic Cedar Falls, which carved a steep canyon that is considered the centerpiece of the park. Surrounding the canyon are numerous overlooks and trails containing rock shelters and passageways cut by Cedar Creek long ago. Weathering also produced curiosities called “turtle rocks” and “carpet rocks. These are just a few of the features preserved as illustrations of the geologic story seen in the rocks at Petit Jean State Park.
The rocks exposed in Petit Jean State Park formed during the Pennsylvanian Period of geologic time around 315 million years ago. Underneath these sandstones and shales are older rocks, predominantly limestones and dolostones that formed during the Cambrian through Mississippian Periods. The area known as the Arkansas River Valley has been above sea level and eroding since the beginning of the Permian Period. Therefore, no rocks were preserved until the Quaternary Period when the Arkansas River deposited sediment in the form of terraces close to the mountain.
During the Pennsylvanian Period, the area now known as the Arkansas River Valley was a basin sitting between the slightly uplifted Ozarks to the north and the slowly rising Ouachita Mountains to the south. These higher areas from the surrounding regions supplied sand, silt and clay that was carried into the basin by ancient river systems. Periodically, much of the basin was covered by a deepening ocean. Swamplands populated by ferns and trees unlike any today developed around bays and inlets. Sea level fluctuated during this time and as sediment subsided in the basin, sea water spread into the area covering the bays and inlets burying the swamplands. Several cycles of deposition created layers of clay and sand which contained thin layers of plant material. These layers became buried and compressed by overlying deposits to form shales and sandstones with an occasional thin coal bed. These layers of rock are known as the Atoka Formation.
Eventually the ocean retreated west and the in-filled basin became exposed. A large river system flowing east to west developed in the low-lying area. Swamplands developed on adjacent flood plain deposits and later became buried when the river meandered and changed its course. This river deposited sandy sediment that became buried and compressed to form sandstone sequences that in some areas contain plant fragments. This is the Hartshorne Sandstone seen today on Petit Jean Mountain.
Around 365 million years ago, North America was attached to the continents now known as Greenland and Europe to form one large landmass called Laurasia situated along the equator. South of the equator a larger continent called Gondwana, made up of the continents now known as South America, Africa, Antarctica and Australia had formed. These two landmasses moved toward each other and finally collided around 340 million years ago. During this collision some of the continental rocks of Gondwana became attached to the southeastern portion of Laurasia in what is now Arkansas. This collision first affected the sequence of rocks in what is now known as the Ouachita Mountains. These once flat-lying rocks were compressed into tight folds, downwarps and upwarps, called synclines and anticlines. Eventually deformation spread to the flat-lying shales and sandstones of what is now called the Arkansas River Valley. These rocks were compressed into more gentle folds that extended far above the current land surface. During the last 250 million years these rocks have eroded to form the present landscapes within the Arkansas River Valley and Petit Jean State Park.
The area now known as Petit Jean Mountain was compressed into a downwarp or syncline. The limbs and younger rocks that would have existed on top of the mountain have been eroded. The lowest portion of the syncline has been preserved as a mountain today.
GEOLOGIC FORMATIONS PRESENT WITHIN THE PARK
Shales and sandstones in the Atoka Formation and the Hartshorne Sandstone underlie Petit Jean Mountain. The Atoka Formation is divided into upper, middle and lower members based on regionally mappable shale or sandstone intervals. Only the upper member is exposed in the park. In some areas fossiliferous beds containing mostly gastropods and bivalves are present. The upper member of the Atoka Formation can be seen when beginning the climb to the top of the mountain from either direction on Highway 154. The best exposures are found along the highway on the east side of the mountain beneath Stouts Point. At this location dark gray to black shales are interbedded with thin layers of tan to gray sandstone. Thicker sandstones appear along the road at the top of the mountain. These sandstones are part of a different rock formation called the Hartshorne Sandstone.
The Hartshorne Sandstone caps Petit Jean Mountain and forms the near vertical ledges and bluffs around the top of the mountain. This rock formation also contains most of the geologic curiosities including Rock House Cave and Bear Cave. The sandstone is brown to light gray, forms thick beds, and is usually cross-bedded. Scenic exposures of this sandstone are found behind Mather Lodge and many of the cabins. This formation is easily viewed at any of the overlooks for Cedar Falls and the canyon formed by Cedar Creek.
Stouts Point is located on the eastern end of Petit Jean Mountain. Here, the bluff forms an enclosure resembling a nose and extends westward creating an elongate mountain. The structure creating this “nose” is called the Pontoon Syncline.
One interesting feature that formed from weathering of the Hartshorne Sandstone is abundant at this overlook. Some of the best examples of color banding, also referred to as liesegang banding, occur in the rocks used to build the abandoned CCC structure at this overlook. Liesegang banding is a result of rhythmic precipitation of iron hydroxide within a fluid-saturated rock.
Cedar Creek developed due to the geologic structure of Petit Jean Mountain. Petit Jean Mountain is a syncline or downwarp of the rock formations creating the mountain. The top of the mountain forms an elongate bowl and the rock formations are slightly tilted toward the center from all sides. As a result, water collects in Lake Bailey then drains from the mountain by Cedar Creek. In the past, Cedar Creek flowed in basically the same channel it occupies today but was 50 to 400 feet higher than at present. During that time, the rock layers forming the ledges and cliffs were continuous across the canyon. The down cutting action of Cedar Creek dissolved portions of the cement holding the sand grains together and eroded passageways through the sandstone in the Bear Cave area and left rock shelters like Rock House Cave, and even a natural bridge on the north side of the canyon.
Cedar Falls formed downstream on the west side of the mountain where Cedar Creek now widens into the Petit Jean River bottom. Here, the falls would have flowed over the thicker and more resistant sandstone layers that formed the caprock of the mountain onto the thinner sands and shales that formed the slope and valley. The shale layers were more easily excavated by the scouring of sand and gravel in the falls. As shale eroded from the base of the falls, overlying sandstone was left unsupported. The sandstone then became more susceptible to erosion from the pounding action of water from the falls and eventually fell to the stream bottom below. Continuing migration of the falls upstream to its current position and dissolution of the sandstone caprock eroded the canyon that now forms the centerpiece of the park.
Today, Cedar Falls cascades over a thick sequence of resistant sandstone of the Hartshorne Sandstone onto less resistant shales of the Atoka Formation. The contact between the two formations is approximately 20 feet above the base of the waterfall. Steep walls dominate the canyon just downstream from the falls. Farther west, Cedar Creek has eroded a wider canyon with more gently sloping sides. Finally, Cedar Creek reaches the lower elevations of the mountain and flows into the Petit Jean River.
Box-shaped and triangular patterns are abundant in the sandstones on top of Petit Jean Mountain. These patterns form when iron present in the rock is oxidized. Iron exists as the minerals siderite, magnetite, hematite and some clay minerals that are present in the Hartshorne Sandstone. At some point in geologic history water fills the pore spaces of the rock formation and comes into contact with minerals made up of iron. This causes the iron to go into solution. If the rock becomes exposed to air then oxygen is added to the solution and will cause the iron to oxidize and precipitate out along exposed joints in the rock formation. Sometimes color bands result from the different oxidation states of iron. These bands are also referred to as Liesegang banding or box-work by the scientific community. Often the joints form interesting triangles referred to as “carpet rock” by the park community.
Some of the most unique features seen around the park are those called “turtle rocks”. Mounded polygonal structures resembling a turtle shell were carved in thick sandstones of the Hartshorne Sandstone. These features are unique to this sandstone but not to the mountain itself. More turtle rocks can be found throughout the Arkansas River Valley where the Hartshorne Sandstone is exposed. The exact processes that create “turtle rocks” are poorly understood. One explanation involves spheroidal weathering. This process occurs when water percolating through cracks and between individual grains in the rock loosens and separates layers of the rock. The weathering acts more rapidly on the corners and edges of the rock producing a rounded shape. Another theory concerns the amount of calcite present in the matrix of the rock holding the grains together along with the size of the grains that allow for this type of weathering. Either way, the weathering of the rocks is strongly influenced by the polygonal joint pattern seen in all “turtle rocks”.
ACKNOWLEDGEMENT: The information on the geology of Petit Jean State Park provided in the description above was obtained from “The Geologic Story of Petit Jean State Park”, State Park Series 02 (SPS-02), by Angela Chandler, Arkansas Geological Survey, State of Arkansas, and is reproduced here with the permission of the author.
This EarthCache is registered with Petit Jean State Park, Permit # PT 41-51.
Congratulations to idratherbehiking for being the first to complete this EarthCache!