The photo above shows trace fossils that record the travels of two trilobites. Trilobites are an extinct group of marine invertebrate animals, resembling horse-shoe crabs, that flourished for 100s of millions of years in the Paleozoic Era (540-250 mya). The tracks the animal left are known as the trace fossil, Cruziana. It appears that one traveled from the right side of the photo, the other from the left, until they met in the middle where they rested for a while. At the center of the photo are resting traces known as Rusophycus. Perhaps they became friends or maybe they were even more than friends? It is Valentine’s Day. Their traces are preserved in the Atoka Formation of west-central Arkansas.
This rather handsome outcrop of the Wilcox group consists of alternating layers of sand and clay of the Eocene Epoch which lasted from about 56-34 million years ago. The Wilcox Group is a non-marine unit mostly composed of sand with lesser clay, silt, gravel, and lignite (low-grade coal).
This geologic unit is part of a larger sequence of loosely-consolidated sedimentary rocks exposed in south central Arkansas, south of Pulaski county. These rocks are the northern extent of the West Gulf Coastal Plain, a physiographic province that stretches from central Arkansas, south, to the Gulf of Mexico.
In the picture above, large black rectangular aegerine crystals are prominent in a rock type known as a pegmatite. Pegmatites are igneous rocks characterized by extremely large crystals. Sometimes they also contain unusual mineral species. This sample was collected from Magnet Cove, Arkansas. Magnet Cove, which is approximately 10 miles east of Hot Springs, is one of the few places in Arkansas where igneous rock is exposed at the surface.
Between 84 and 100 million years ago, magma was injected into the earth’s crust under central Arkansas where it slowly cooled and crystallized into igneous rock. Millions of years of erosion eventually unearthed that rock. Despite only being exposed over approximately 5 square miles, the rocks of Magnet Cove have yielded more than 100 different minerals. Rare minerals have been discovered there including a new variety of zirconium-rich garnet called Kimzeyite.
Why do rocks have beds? Are rock beds where geologists sleep? Sometimes, but that’s not the point of this article. The picture above, taken on the Goat Trail at Big Bluff, overlooking the Buffalo National River, is a great example of a sedimentary rock composed of many individual beds (layers). The reason that rocks are bedded is due to either gaps in deposition or abrupt changes in the grain size of sediment being deposited in an environment.
Here’s an example; when a storm causes a river to flood its valley, the water deposits sediment as the flood recedes. Typically, there’s a period of non-deposition before the next flood event deposits a new layer of sediment over that one. This time between floods allows weathering to alter the character of the first flood deposit. That weathered surface will eventually differentiate the flood deposits into distinct beds of rock.
Bedding can also form as a result of flowing water gaining or losing velocity. The size of sediment that water carries (and eventually deposits) is directly related to flow rate. A sudden change in flow rate creates bedding distinguished by differences in grain size.
A sinkhole is an area of ground that has no external surface drainage. Water that enters a sinkhole exits by draining into the subsurface. Many people are leery of sinkholes because of the damage they sometimes cause. Every now and then, a catastrophic sinkhole-collapse makes headlines, typically by swallowing someone’s house, or even draining an entire lake.
Not every kind of sinkhole is the dangerous kind though. The picture above shows a solution sinkhole. Unlike the feared collapse sinkhole, the solution sinkhole forms by chemical weathering of rock at the ground surface resulting in gradual lowering of the surface to form a depression. Solution sinkholes form in areas where fractures and joints in the bedrock create pathways through which rainwater can infiltrate the ground.
In, Arkansas, sinkholes are common in the northern part of the Ozark Plateaus where much of the bedrock is limestone or dolostone. These types of rocks are notorious for sinkhole development because they are soluble in weakly acidic rain water.
Geologic mapping of the Japton and Witter 7.5-minute quadrangles was recently completed by the Arkansas Geological Survey’s STATEMAP field team. In Arkansas, the STATEMAP Program is currently focused on detailed 1:24,000-scale mapping in the Ozark Plateaus Region, located in the northern part of the state.
Figure 1. Japton and Witter Quadrangles on the 1:500,000-scale Geologic Map of Arkansas (Haley et al., 1993)
Geologic Map of the Japton Quadrangle, Madison County, Arkansas. Download a digital copy at:
Geologic Map of the Witter Quadrangle, Madison County, Arkansas. Download a digital copy at:
STATEMAP is a cooperative, matching-funds grant program administered by the U. S. Geological Survey. The goal of the program is to classify surface rocks into recognizable units based on a common lithology–basically, an inventory of surface materials. Also, we strive to locate and depict any structural elements that may have deformed the rocks. The rock units are classified into formations and members, and structures are described as synclines, anticlines, monoclines, and faults. During the project, a rich dataset was recorded in the field using a portable data collector/global positioning satellite receiver as well as by traditional methods. This made possible a more detailed depiction of geological and structural features and a more comprehensive description of lithology than previous studies had done. Data collection included:
629 field locations recorded and described in detail
3,385 photographs taken at recorded field locations
72 strike and dip measurements, most depicted on the maps
950 joint orientations, depicted in a rose diagram of strike frequency
1 shale pit
8 springs, previously undocumented
108 rock samples collected and described
The new map is useful to landowners interested in developing their land for personal or commercial purposes, to scientists seeking a better understanding of landscape evolution and geologic history, and to planners responsible for resource development and mitigating environmental impacts.
Angela Chandler, Principal Investigator for the project, wrote the grant for fiscal year 2018 and we received funding adequate to produce two maps. Two geologists, Richard Hutto and Garrett Hatzell, began their field season last July and after putting in 76 days in the field, concluded that portion of their work in February of this year. The area of investigation lies within the Interior Highlands Physiographic Region in north Arkansas, specifically the Boston Mountains Plateau portion of the Ozark Plateaus Province. Previous work by the AGS in this area had been done in preparation for the 1:500,000-scale Geologic Map of Arkansas by Haley et al. circa 1976 (see Fig. 1). That mapping project delineated five stratigraphic units in this area, but through extensive field reconnaissance, we were able to define ten units on these maps at the 1:24,000 scale. Further division is possible, but several units were considered too thin to depict on the 40-foot contours of the topographic map currently available, or too difficult to delineate by lithology alone.
Several tributaries of the White River are located on these quadrangles including Lollars Creek, Drakes Creek, and War Eagle Creek. The White River is a major water resource in Arkansas and southern Missouri, and as such we need to learn as much as we can about this important watershed. Included in the field work was hiking, wading, or swimming the entire 13-mile stretch of War Eagle Creek located within the Witter quadrangle, the 10 miles of Lollars Creek within the Japton, and many smaller drainages. The reason we concentrate our efforts on stream beds is that there, erosion has typically removed soil and loose rock leaving well-exposed outcrops of bedrock for us to study. Also, being able to see the entire stack of the rock sequence as we move up or downstream helps put each formation in context with the others. Discovering where one formation contacts another is one of the most important things we do while mapping. Because formations are laterally extensive, similar contacts can be connected into a contact line separating one formation from another. Figuring out where to draw these lines on the map is a major goal of the project.
From mid-February through the end of June, we analyzed field data, classified rock specimens, drew formation contacts and structures on the map, then handed it off to our cartography staff to digitize. Final layout and production of the maps was accomplished by the geologists, after which they were subjected to an extensive review and editing process by fellow staff.
The following images were taken during this year’s field season. Hopefully, they will provide a small glimpse into some of what we were privileged to experience in the field this year. They are arranged in stratigraphic order from youngest to oldest:
Alluvium in War Eagle Creek (left). Landslide on Highway 23 above Dry Fork (right).
Ball and pillow structures in the Atoka Formation in Drakes Creek.
Sequence of photos zooming into herringbone cross-beds in the Greenland Member of the Atoka Formation.
Large blocks of Kessler Limestone sliding into Lollar’s Creek.
Sequence of photos zooming into oncolitic limestone of the Kessler Member of the Bloyd Formation. The oncolite pictured far right is nucleated on a tabulate coral.
Lycopod (tree-like plant) fossil weathering out of the Dye Shale.
Top of the Parthenon sandstone (Bloyd Formation) in Lollar’s Creek (left). Parthenon resting on the Brentwood Limestone (Bloyd Formation) with travertine precipitating at the drip line (right).
Siltstone unit in the upper Brentwood Limestone. Cross-bedded (left) and bioturbated (right).
Biohermal mounds in the Brentwood Limestone in Jackson Creek.
Massive bluff of limey sandstone in the Prairie Grove Member of the Hale Formation.
Sandy limestone in the Prairie Grove. Stream abrasion has revealed cross-bedding (left) and an ammonoid (right).
Typical thin-, ripple-bedded sandstone of the Cane Hill Member of the Hale Formation (left). A basal conglomerate in the Cane Hill contains fossiliferous and oolitic limestone pebbles and fossil fragments (right). This unit probably rests on the Mississippian-Pennsylvanian unconformity.
The Pitkin Limestone in War Eagle Creek.
This year we will be mapping the Weathers quadrangle which is just east of the Witter, and the Delaney quadrangle which is just south of the Durham (which we mapped two years ago). The Kings River flows through Weathers, so this should be a good place to start while river levels are low (and it’s so hot!). I will update you as I can, but until then, I’ll see you in the field!
Recently, we posted a blog explaining that the Ozark Mountains are actually incised plateaus and that the hills are remnants standing between the incised river valleys. If you missed that one you can see it here. Now, we will talk about how a river is able to erode solid rock.
The picture above is of the Buffalo National River in its valley. As you can see, an impressive volume of rock has been excavated by this little river. A common misconception is that the water is carving the rock. Water is soft and softer things generally do not abrade harder things. Slightly acidic water can dissolve rock very slowly, particularly carbonate rock like limestone, However, the majority of the erosion in a river is due to the sediment suspended in the flowing water. As the sediment – which can range from tiny grains of silt to boulders– is carried downstream by the current, it skips along the channel, colliding with the bedrock. The repeated collisions break down the sediment, chipping off edges and rounding it. By the same process, new sediment is ground away from the bedrock and the valley is slowly enlarged.
The same thing is true of wind erosion such as in a desert setting. The wind itself really can’t erode the rock. The erosion is due to strong winds lifting loose sand and blasting it against the solid rock, slowly wearing it away.