By Wendy Diaz EMGV
This article was first published in The Gardeners Buzz, a newsletter of the Extension Master Gardeners of Durham County in June, 2022.
On April 2, 2022 my husband and I joined the ‘Spring Botanizing at Penny’s Bend‘ tour organized by Duke Gardens Education Program for an easy hike at Penny’s Bend Nature Preserve[1]. Duke University, Department of Biology professors Paul Manos and Alec Motten guided us and answered our questions. The botany at Penny’s Bend is unique because of the geology of the site. The restoration of the rare Piedmont Savanna at Penny’s Bend is ongoing by the North Carolina Botanical Garden at the Preserve[2].
Water flows downhill, along a path of least resistance, and that is one reason rivers meander. Penny’s Bend is a local example of a pronounced sharp curve in the course of the Eno River. The reason for this abrupt change and subsequent re-correction of the river course is that it passes around the hard, erosion-resistant igneous rock called diabase. The easterly flowing Eno River encountered the diabase promontory exposed at the surface and abruptly changed course – flowing south and then for a short distance, north – along a diabase sill outcrop forming a pinched horseshoe shape.
From left to right: Photo of trail head map at the entrance Kiosk at Penny’s Bend Nature Preserve, Durham North Carolina. Looking north from the top of diabase sill outcrop along Eno River. All Photos by Wendy Diaz, April 2022
A sill is the term for a geologic structure forming a sheet-like igneous body that is parallel to the bedding planes and solidified between the sedimentary rock layers of the host rock; whereas, a dyke is a sheet of igneous rock that cuts across bedding planes of the host rock. There isn’t a lot of exposed bedrock in Durham County but there are a few outcrops of the diabase rock, such as along the Eno River at Penny’s Bend and the old rock quarry walls of the bear enclosure at the Museum of Life and Science. I also noticed there are some transported diabase boulders (not outcrop) along the side of Brigg’s Avenue Community Garden.
While the diabase rock may not be unusual, geologically speaking, the soil derived from it did stimulate the growth of a group of unique plants that flower in the spring and summer at Penny’s Bend. Dutchman’s Breeches (Dicentra cucullaria)[3], the endangered Smooth purple coneflower (Echniacea laevigata), Prairie blue wild indigo (Baptisia aberrans) and Hoary puccoon (Lithospermum canescens) are some of the unique plants that require neutral to alkaline soil conditions. While it was too early in the season to see the blooms of most of these unique wildflowers, we did see large patches of Dutchman’s Breeches along the north side of the rock face and the east bank of the Eno River.
Some spring ephemeral flowers that we saw on our walk were Spring Beauty (Claytonia virginica), which have pink stripes on the petals to guide pollinators to the nectar, Rue Anemone (Anemonella thalictroides) and Sessile Bellwort (Uvularia sessilifolia). A large colony of Painted Buckeye (Aesculus sylvatica) shrubs was also in bloom near the diabase cliff along the north end of the bend. There was also many Trout lily (Erythronium americanum ) seed pods and leaves along the hiking path.
A Brief Geologic History of North Carolina
In Durham County, we live among the rolling hills of one of three physiographic regions, known as the central Piedmont, which is located between the Blue Ridge Mountains to the west and the relatively flat Coastal Plain to the east. Each region can be divided into geologic belts or ‘terranes’ that have similar rock types and geologic histories[4]. Furthest west, the oldest Blue Ridge Belt is a mountainous region with rocks dating all the way back to 1.5 billion to 1 billion years. These rounded mountains were thought to have peaks as high as 20,000 feet before millions of years of erosion took their toll. The youngest belt is the Coastal Plain-a wedge of marine sedimentary rocks which thicken closer to the coast and formed from sediments eroded from the mountains to the west between 65 million to just a few million years ago[5]. The bedrock which underlies the Piedmont consists of several different rock types formed over several hundred million years of geological history. Piedmont bedrock age ranges from the metamorphic Inner Piedmont Belt of 500 to 750 million years old to the sedimentary rocks of the Late Triassic Basins formed between about 235 million and 200 million years old. Most of the southern half of Durham County is underlain by one of three Triassic Basins, named the Deep River Basin which extends from Granville to Union County. Younger still, are the diabase dykes and sills intruded into the Late Triassic sedimentary rocks during the Early Jurassic Age[6] around 200 million and 175 million years ago when the supercontinent Pangaea was breaking apart forming what would eventually become the Atlantic Ocean. (This is illustrated in the YouTube video by California Academy of the Sciences[7]). Yes, the Jurassic Period, known as the peak age of the dinosaurs. When the continental plates diverged, the earth’s crust thinned when it was stretched, then the crust over the mantle became less dense and the reduced weight promoted the uppermost mantle to partially melt producing basaltic magma which intruded into the Triassic sediments.
About 220 million years ago when the Deep River Basin (rift valley) began to form, the continental tectonic plates pulled apart, forming cracks in the earth’s crust, roughly perpendicular to the direction of the moving continents. This basin was bordered by faults. Over millions of years it gradually filled up with eroded sediments from the much higher Blue Ridge mountains and the Piedmont Plateau to the west of the basin. These sediments solidified and formed mudstones, siltstones and shale. Other cracks continued to widen until they filled with ocean water. The pulled apart edge of early North Carolina (minus the coastal plain which was deposited after the pulling apart of the continents that formed the supercontinent of Pangea) roughly fits with the coastline of Africa from Mauritania and Morocco. At this point in geologic history the North American continent was closer to the equator.
Rock Mineralogy
Diabase is a dark grey to black, medium-grained, mafic igneous rock composed of Iron and Magnesium-rich minerals such as pyroxene. The exposed rock of the diabase sill at Penny’s Bend weathered and formed a soil markedly different in chemistry and properties to the surrounding acidic soil which was derived from the Triassic sediments.
When the minerals of the diabase rock breakdown they form clay minerals that form ‘sweet’ or basic soil with high pH and richer in iron and magnesium that what many other native plants can tolerate2. The soils are also rich in calcium from the plagioclase feldspar mineral[8]. Medium-grain size in igneous rock indicates the basaltic magma intruded and cooled underground near the earth’s surface (microscopic grains would indicate the molten rock cooled quickly on the earth’s surface). The diabase rock is more resistant to erosion because it is igneous rock that solidified from molten rock and not from cemented together grains of the surrounding less-resistant sedimentary rock.
The old diabase bedrock beneath Durham formed millions of years ago, and is now the foundation of our current natural world and determines the native plants we enjoy today on our spring and summer walks through the Penny’s Bend Nature Preserve. If you have not yet visited this site, I recommend a short walk along the Eno River, especially in the spring time. You will be rewarded when you see the unusual flora and fauna – and don’t forget to take a closer look at the bedrock that is exposed along the river bank which makes this area so special.
DRIVING DIRECTIONS to Penny’s Bend from Durham (3710 Snow Hill Road, Durham, NC 27712)
Head north on Roxboro Street.
About 1.5 miles north of I-85, turn right onto Old Oxford Road.
Stay straight for 3.2 miles until you cross the Eno River.
Turn left onto Snow Hill Road and on the left is a gravel parking area on the left side of the road.
References:
[1] https://www.enoriver.org/what-we-protect/parks/pennys-bend/
[2] A River Runs Around It: Restoring the Rare Flora of Penny’s Bend by Emily Oglesby, Conservation Gardener Magazine, Spring/Summer 2022, North Carolina Botanical Garden, The University of North Carolina.
[3] https://www.wildflower.org/plants/result.php?id_plant=DICU
[4]Generalized Geologic Map of North Carolina, N.C. Department of Environmental Quality https://deq.nc.gov/about/divisions/energy-mineral-land-resources/north-carolina-geological-survey/ncgs-publications
[5] Crossroads of the Natural World. Exploring North Carolina with Tom Earnhardt. University of North Carolina Press: Chapel Hill, 2013, 314 pages.
[6] Geologic Map of the Southwest Durham 7.5 Minute Quadrangle, Durham and Orange Counties, North Carolina by Charles W. Hoffman and Patricia E. Gallagher, North Carolina Geological Survey Open File Report 2001-XX
[7] https://www.youtube.com/watch?v=ADsjdu27WaM California Academy of the Sciences
[8] Exploring the Geology of the Carolinas, Kevin G. Stewart and Mary-Russell Roberson. The University of North Carolina Press, Chapel Hill, 2007, 298 pages.