Ripples over Bedrock № 1

ripples over bedrock 21
Shallow water ripples over jointed bedrock with algae.

Water ripples are forming in very shallow water of a Piedmont stream as it flows over orange bedrock. The rock is a metamorphosed argillite (fine-grained sedimentary rock) whose present color is the result of weathering in an oxygen-rich environment. Algae has grown preferentially in planar, nearly vertical fractures in the rock called joints.

The stream is in Uwharrie National Forest, North Carolina.

Image © Andy R. Bobyarchick.

Meander and Point Bar in Flat Creek, SC

Flat Creek southeasterly through Lancaster County, South Carolina. The stream feeds into Lynches River, that farther southeast joins the Great Pee Dee River. The Pee Dee joins the Waccama River at Georgetown on the SC Atlantic Coast.

Flat Creek carries a heavy load of sediment. Where this image was made, the stream crosses the Pageland granite, a pluton that is about 300 million years old. The granite is coarse-grained and sheds considerable quartz and feldspar grains as it weathers and erodes.

This image is a 180° panorama around a meander bend in Flat Creek. Lower water velocity on the inside of the meander causes the coarser sediment load to drop out to form a sandy point bar. The near bank of the river is being undercut by erosion on the outside of the meander where water velocity is greater. In time, the stream channel will migrate toward this cut bank.

It’s difficult to see in the shadows, but in the lower left corner of the photograph a tributary trickles into the main channel, and there are abundant beaver tracks there. Beavers use the tributary mouth as a slide.

In the lower right corner of the photograph you can see a boulder of Pageland granite. It’s rounded shape is characteristic of spheroidal weathering of a larger mass of granite bedrock.

Not far to the southeast, Flat Creek crosses from Piedmont igneous and metamorphic rocks into sedimentary rocks of the Sandhills.

This image was made late on a Spring afternoon under a brilliant blue sky reflecting off the water’s surface and under the long shadows of hardwood trees yet to put out theire leaves.

This image is ©Andy R. Bobyarchick

meander and point bar

A meander bend and point bar in Flat Creek near Taxahaw, SC.

Invasive Gastropods in High Rock Lake, NC

High Rock Lake is one of several impoundments on the Yadkin-Pee Dee river system in the North Carolina Piedmont. Flat Swamp Creek is, or was, a tributary to the Pee Dee River. After the High Rock reservoir filled Flat Swamp Creek became a narrow, northeast-trending arm to the larger lake. This arm is so linear because it follows a geologic contact between metasedimentary rocks to the east and metavolcanic rocks to the west. The stratification in these rocks is tilted by folding, and differential erosion created narrow ridges and valleys. Flat Swamp Creek occupies one of those valleys.

At the Flat Swamp access area off Highway 8 the shoreline is littered by abundant gastropod shells. These are the remains of an invasive snail Bellamya japonica. It’s reported that their presence is the consequence of snail farming (they are pretty large). The day I was there in February, 2018, the waterway was hosting hundreds of water birds.

During low water, there are great exposures of stratification and cleavage in tuffaceous siltstone and sandstone of the Cid Formation. Nearby, to the southwest and below the High Rock dam, there are volcaniclastic rocks in the Flat Swamp member of the Cid Formation. This formation and the adjacent older Tillery Formation are part of the Ediacaran to Cambrian Albemarle Group in the Carolina Terrane. These are peri-Gondwanan sequences, attached to Laurentia (ancient North America) only since the Ordovician period.

All photographs ©Andy R. Bobyarchick.

flat swamp access

Flat Swamp access area at sunset. This is a popular swimming and boating area in summer.

gulls

Gulls huddling on perimeter floats around the swimming area.

Sedimentary bedding under shallow water at the Flat Swamp access area.

bedding

A closer view of bedding.

three shells

Evacuated and beached shells of B. japonica.

B. japonica.

many shells

A wave-washed collection of B. japonica shells.

tree roots

Life is hard for a sweetgum tree when the water level fluctuates.

sunset

Sunset across Tuckertown Reservoir below High Rock dam at the Bringle Ferry Road access area.