Road Cuts Provide Clues To Arkansas’s Deep Past
Article and Photos By Jim Taylor, travel writer
MARSHALL – By shearing edges off South Mountain and Devil’s Backbone in north central Arkansas, the builders of U.S. 65 just south of Marshall created a straighter, safer highway passing beside massive walls of stone. At Conway, some 70 miles south, a path for Hogan Lane was blasted through the rock of Cadron Ridge to provide northwest city traffic a direct route to U.S. 64.
At both locations, the road cuts – the removal of soil and stone – did more than hasten motorists’ travel. They also made it possible to journey deep into Arkansas’s past, through layers of time recorded more than 70 million years before the first dinosaurs walked the earth.
The rock walls near Marshall (see photo below) border U.S. 65 for almost a mile. Midway is a mountaintop pass with a highway turnout and scenic overlook of the town. Four picnic tables covered by small pavilions make the turnout a good vantage point for closer study of the cut into Devil’s Backbone.
Shale and limestone form the many, basically horizontal layers of stone that can be seen within the wall. For the most part, the strata of the two kinds of rock are what geologists call “interbedded.” That is, the layers alternate. Towards the bottom of the cut, shale predominates, while the top is almost exclusively limestone.
The 0.15-mile cut through Cadron Ridge at Conway reveals, by comparison, a geological jumble. Three major types of rock in strata of varied widths alternate throughout the cut. And, lying nowhere close to horizontal, these towering strata form severe diagonals between earth and sky.
A road cut and geologists’ subsequent study of the rocks it bares are, in effect, a biopsy of planetary tissue that enables a diagnosis of an area’s geological history. The stone chronicles at the Marshall and Conway cuts speak of very different circumstances. At Hogan Lane (see photo, below), the tale includes a particularly violent chapter in the making of Arkansas.
In very basic terms, the road cuts’ differences were products of two key factors: the conditions present when the sediments that formed their rocks were being deposited – “depositional environment” to geologists – and the forces to which those rocks were later subjected.
Though now rising to more than 1,300 feet above sea level, the shale and limestone at Marshall were formed on the floor of a prehistoric ocean. The clay minerals comprising the lower shale likely settled onto the outer edge of the submerged continental shelf between roughly 340 and 320 million years ago after being carried to sea by wind and distant streams, there being no nearby rivers emptying into the ocean.
The change to limestone in the upper level of the cut resulted from a change in the depositional environment. A decline in sea level left the area covered by shallower, clearer water inhabited by corals and other varieties of marine organisms that produce hard body parts such as shells. The limestone was formed from the remains of such creatures and contains many fossils.
The interbedded strata of shale and limestone in the Devil’s Backbone wall may have resulted from periodic storms that washed layers of limestone-forming materials into the deeper waters of the continental shelf where the clay minerals of the shale were being deposited. It has been hypothesized that the number and severity of the recurring storms may have been influenced by cyclical sunspot activity and/or cyclical changes in Earth’s orbit and axial tilt.
The Marshall cut and scenic overlook are located on the northern edge of the Boston Mountains, one of three eroding plateaus that make up the Ozark Mountains in northern Arkansas. The plateaus were, about 300 million years ago, part of a regional uplift of the Earth’s surface that created what geologists call the Ozark Dome. Because the area was lifted as a unit, its rocks – including those at Marshall – generally became only slightly tilted from the horizontal planes at which they were formed.
At roughly the same time the dome was raised, the prehistoric North American continent and a similar land mass began a violent, ultra slow-motion collision that would last perhaps more than 15 million years. As the two continents pressed against each other, the rock strata at their margins and lying on the ocean floor between, including those of the Hogan Lane road cut, were squeezed north and folded upwards. The collision also formed the Ouachita Mountains and may have been a factor in the Ozark Dome uplift.
Though folded severely upwards, the strata at Hogan Lane did not completely overturn as happened at some locations in the Ouachitas. Nonetheless, it is still difficult for the casual observer to discern that the younger rock is found at the cut’s southern end and becomes progressively older as one travels northward.
The other aspect of the geological jumble on Hogan Lane – the many and repeating strata – tells the story of a river emptying into an ocean some 10 to 20 million years after the rocks at Marshall were being deposited.
A river’s delta begins forming below sea level as the stream drops its loads of sediment onto the ocean floor. There the sediment settles in relatively homogenous groupings determined by the size and weight of its particles. The relatively heavy sand particles (which form sandstone) are the first to fall from the steam’s flow, followed by smaller silt particles (which form siltstone), and then by the extremely fine minerals of clay (which form shale).
As those materials build up, the fan-shaped delta rises above sea level and advances into the ocean, causing the river to dump its sediments farther offshore. Thus, the layers being deposited begin overlapping. Atop the original outermost array of clay, silt begins to deposit; atop the original silt, a layer of sand begins to accumulate. As the delta extends further into the sea, the pattern of overlapping repeats itself.
In very simplified terms, that was the process that formed the alternating strata of sandstone, siltstone and shale that span the length of the Hogan Lane road cut.
About midway through the west side of the Hogan Lane cut and on the east side at its south end (see photo, below), ripple marks are preserved on large, upturned slabs of shale, left there some 310 million years ago by currents on the ocean floor.
Throughout the Ozark and Ouachita Mountains and in the Arkansas River valley upstream from Little Rock, the story of Arkansas’s geological history is written in the stone revealed by man-made cuts for roads, dams, spillways and quarrying, as well as in such natural rock exposures as caves and stream-carved bluffs.
To reach Hogan Lane, take the U.S. 65 exit off Interstate 40 in northern Conway and head west about 0.4 mile to U.S. 64. Turn right on U.S. 64 and proceed for 3.5 miles. Hogan Lane intersects from the left and the cut begins immediately. The Marshall cut is on the town’s southern outskirts.
For those satisfied with drive-by geology, most of the features previously described can be observed from a moving vehicle. Parking at Marshall is provided in the highway turnout. At Hogan Lane, parking is limited to the roadsides and care should be exercised. At both locations, visitors should remain vigilant for falling rock.
For more information on Arkansas’s geology, visit the Arkansas Geological Commission’s web site at www.state.ar.us/agc/ or http://rockhoundingar.com, a site whose principal author is geologist J. Michael Howard of the commission’s staff.
(Author’s Note: This article could not have been written without the learned assistance of geologists John David McFarland and Charles G. Stone of the Arkansas Geological Commission staff.)
This release, along with others by the Department of Parks & Tourism, is available electronically through the Arkansas Press Association Bulletin Board: firstname.lastname@example.org (in-state only) and the Department’s Website: http://www.arkansas.com under ”media information.”
by the Arkansas Department of Parks & Tourism