Oldowan stone tools were made by striking flakes by hard-hammer percussion, mostly from water-rolled pebbles of volcanic stones.  Stone-flaking was well-controlled and many of the flakes were expertly struck, showing that these earlier hominin stoneworkers knew to strike near the edge of the stone behind zones of high mass on the core, to choose a platform angle of less than 90 degrees (the ‘acute angle rule’), and to strike the core with a glancing blow. 

Many of the cobbles were reduced bifacially—a scar produced previously was used as the platform to strike a flake off of the opposite face.  Some cobbles were rotated frequently during reduction and many platforms struck, resulting in multiplatform cores.  Some archaeologists argue that the hominins removed flakes to deliberately produce designed (if rather crude-looking) tools, but a more parsimonious explanation is that flaking was algorithmic, driven by the mechanical restrictions of stone-working combined with the configuration of the particular stone. 

Cobble cores may have been used as heavy-duty tools, but the hominin stoneworkers were most likely after the sharp-edged flakes.  Current thinking is that these sharp-edged flakes allowed access to meat for the first time, which in turn gave our ancestors and adaptive edge, and allowed for brain growth during subsequent evolution.  Oldowan knappers trimmed the margins of some of these flakes, a process called retouching, perhaps to resharpen them.  They also smashed pebbles and flakes on anvils, creating more flakes—a process called bipolar flaking.  Cobble hammerstones are also common at Oldowan sites, probably to break open bones for marrow, as well as to use as hammers in stone-flaking.  Heavily worn hammerstones in Oldowan assemblages are called ‘spheroids’, and some archaeologists have suggested that these were deliberately-shaped tools.