A few examples of natural fracture are shown here, all from San Diego river beds and terraces. It is a section that will hopefully grow. Currently a lot of decisions regarding natural versus intentional fracture are carried out by consensus, a show of hands. Until recently it kind of seemed that, depending what side of the "Pre-Clovis Early Man" issue you were associated with, an internal bias would follow you wherever you went. This problem of recognition and discernment, which is world-wide, was one of the initial reasons bipolar experimentation was investigated in the first place. The question, "How do I know it is an artifact or naturally fragmented?" is better answered with a familiarity with the full scope of the lithic variability associated with the intentional side of the issue. Overlaps do occur, as some have already realized while looking at the experimental specimens. We may never get to the point where we can always discern human fracture versus nature, but we can probably get closer than we are now.
A common confusion resides around the occurrences of spalls on pebbles and cobbles. Three of the pieces shown here all have overt evidence if chemical weathering and attrition. The fourth (below right) shows the after-effects of the spalling phenomena, along with an undisclosed period of weathering producing rounded features. Thermal alteration such as brushfires and campfires can produce similar kinds of spalling.
In the future, this section will be appended by other specimens.