When road builders blasted through the rock of Cadron Ridge in Conway, AR, to extend Hogan Lane and provide northwest city traffic a direct route to U.S. 64, they did more than hasten motorists’ travel. They also made it possible to view Arkansas’s geological history as written in the stone of that highway cut.
REQUIRED INFORMATION
In order to gain credit for completing this EarthCache, you should read the information provided below and then go to the posted coordinates and make some observations of the rock sides of the highway cut. You will need to carry with you a simple protractor and a compass.
Parking is not allowed at the posted coordinates, so please park at either end of the highway cut (see waypoints) and walk on the shoulder about 500 feet to the location. Care should be exercised due to passing traffic. Also, remain vigilant for falling rock.
Please complete the following and provide me with the required answers within 7 days of your find. DO NOT post your answers in your log. Instead, send them to me by email through my profile page OR through the Message Center using the link found near the top of this cache page, under the cache name. Found It logs that are not supported by the required information will be deleted, as I must have proof that you successfully completed the requirements.
1) In the highway cut, you will see several layers of rock tilting upward, evidence of some past extreme geological action. Using your protractor, measure the angle of this tilt from the horizontal. What is this angle?
2) Using your compass, in what direction is the rock tilting upward?
3) Provide a simple explanation for the cause of the tilt in the rock you see.
4) How many major types of rock in strata are visible?
5) Name the kind of rock that makes up each of these layers.
INTRODUCTION
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 Hogan Lane cut tell a tale of a particularly violent chapter in the making of Arkansas.
The 0.15-mile cut through Cadron Ridge reveals 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.
GEOLOGIC HISTORY
Imagine yourself in this area, hundreds of millions of years ago, when a prehistoric river flowed into an ocean that once covered this area. A delta at the river’s mouth begins to form 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 the three distinct layers 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, ripple marks are preserved on large, upturned slabs of shale, left there hundreds of million years ago by currents on the ocean floor.
But how did the layers of rock become upturned, so that they are no longer horizontal as they were after their original formation? At the time of the prehistoric river and ocean, 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 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.
ACKNOWLEDGEMENTS: The geological information and some of the text contained in this EarthCache description were obtained from the online article “Road Cuts Provide Clues to Arkansas’s Deep Past” by Jim Taylor, Arkansas Department of Parks and Tourism. Other supporting geological information 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.