14-3 Cracks and Tools: The Influence of Natural Structures on Stone Tool Source Rocks in Metavolcanic Rocks of the Carolina Terrane, North Carolina

Session: New Research in the Appalachian-Ouachita Orogen: Integrated studies from the Foreland to the Hinterland

Presenting Author:

Authors:

Abstract:

Quarrying of metamorphosed felsic volcanic rocks (rhyolite to dacite) for stone tools in the Uwharrie Mountains of the central Piedmont in North Carolina extends from Paleoindian through at least Late Woodland, with especially strong activity during the Archaic (~8,000-1,000 BCE). Quarry and reduction sites in the North Carolina Piedmont commonly target greenschist-facies felsic metavolcanic rocks of the Carolina Terrane (Albemarle Group), a peri-Gondwanan arc, in the Tillery and Cid formations of the Uwharrie National Forest.

In ridge-capping metarhyodacite–metarhyolite, toolstone availability is based on bedrock exposure, phenocryst content, and knapping quality, yet the near-surface fracture architecture produced by mechanical weathering superimposed on anisotropic fabrics may have been equally important in shaping how cores were extracted and reduced.

These rhyodacites contain an axial-planar cleavage overprinted by joint sets and abundant curviplanar discontinuities (cracks and unloading fractures). Mode I thermal and unloading cracks exploit the rock’s pre-existing anisotropy but are not confined to cleavage planes; instead, they propagate normal to evolving exposure faces and coalesce into scalloped, concave-outward detachment surfaces that shed sharp-edged fragments.

Fresh fracture surfaces commonly display semi-conchoidal to conchoidal curvature. Where cleavage intersects these curved breaks, it produces step-like margins and thin, planar “facets” that mimic intentional flake scars. In effect, the tectonic cleavage acts as a stress guide and edge template: it localizes microcracking, straightens portions of otherwise curved fractures, and promotes acute dihedral angles along fragment margins – exactly the kind of sharp, durable edges useful for initiating reduction.

Some natural joint and detachment surfaces and worked boulder surfaces are ornamented by plumose structures, hackles, rib marks, and arrest lines that radiate from crack initiation points. Debris fields have been identified, but natural processes also reduced outcrops, boulders, and cobbles to “flakes”.

Toolmakers may have recognized and leveraged naturally “pre-flaked” fracture systems when extracting cores. Rather than beginning reduction on massive, unfractured rock, knappers could detach cores along predictable discontinuities to obtain steep, sharp-edged platforms. Reading fracture spacing, continuity, and edge quality would reduce effort and could help explain artifact-rich colluvial aprons even where bedrock exposure is limited. Mechanical cracking continues to operate on cobbles and boulders embedded in colluvial blankets, renewing a supply of core-sized fragments whose fracture geometries blur the boundary between natural disintegration and early lithic reduction.

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