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 huddling on perimeter floats around the swimming area.
Sedimentary bedding under shallow water at the Flat Swamp access area.
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 across Tuckertown Reservoir below High Rock dam at the Bringle Ferry Road access area.

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