BIPOLAR UNIFACES derive from an indirect fracture technique that logically follows from split pebbles or cobbles. When a pebble is split, the fragments can be hemispheric, or plano-convex. Stabilize the flat surface on the anvil, and strike it above with the maul. A smaller maul is used so that the entire core does not fragment.
Using this technique, chances are that most flakes are initiated along the distal platform. The size of the maul should be smaller or else the force will tend to split the plano-convex core in half, or worse. Note that the proximal surface that receives the maul can undergo damage, most often in the form of the battered texture you have come to expect from a hammerstone.
Is this a familiar combination to you?
SPLIT PEBBLES and cobbles generate a wide array of general forms that archaeologists refer to as plano-convex. Many types of scrapers fall under this general term. Given a sphere, half of it would be a hemisphere, the classic plano-convex shape. (In contrast, bifaces are most often associated with the term bi-convex.)
During the course of experimenting, and with the pile of plano-convex pieces lying about, the idea arose to place their relatively flat surfaces on an anvil and strike the convex (dome) surface with the maul. In science we call this kind of decision: hit it and see what happens.
The forms and phenomena associated with this reduction technique are showcased in the pages ahead. Materials vary, ranging from industrial glass to cryptocrystallines to quartzites and igneous rocks. Each specimen is positioned in the way it was struck: the top of the core is the proximal region, the bottom of the core is the distal. None of the pieces were ever used. The edge patterns were all generated by production vectors.
Fracture patterns generated by the distal rebound force will be chronicled first, with an emphasis on the edges in contact with the anvil. A range of flake scars along the periphery of the distal edge could be mistaken for evidence of function, e.g. use-wear. In the cases of serrated, denticulated or notched edges -- all of which occur naturally and occasionally in multiple flake releases -- these forms could be mistaken for styles. The traditional way of looking at denticulated edge types is that each flake was removed intentionally, one at a time; hence subject to style. The results of bipolar experiments on plano-convex cores shows that edges can mimic signatures of use and style.
There were occasions where fracture was instigated from the proximal region, often leaving behind bulb remnants and features extending toward the core's distal region. These specimens will be compared to several possible artifacts picked up along the terraces of San Diego's river beds.
The first example was made from industrial glass (1, 2), or slag. It approximates obsidian, with analogous fractures and force features.
Another instructional piece is called the breadloaf with all sections generated from the proximal region as shown by the series of "sliced bulbs." The material is basalt.
Georgetown flint from Texas was some of the sweetest material around. It did not have to be heat-treated. The quarry was blown up by miners during the late 70s. Chunks and pieces were picked up by the local knappers at the time who generously gave me a few pieces. Like most cryptocrystalline materials subjected to this process, a denticulated or notched appearance commonly developed along the distal periphery, occasionally generating spurs.
A complete listing of the photos of these materials begins on the next page.
A series of reductions of quartzite pebbles and cobbles provide a more complete context of the reduction process.