Excavation of a Chimpanzee Stone Tool Site in the African Rainforest Julio Mercader, Melissa Panger, and Christophe Boesch

I. Sediment influx through rainwash causes rapid deposition. Experimental sediment traps, active for 12 days through 9 episodes of rainfall, yielded a 7 mm build-up with 320 g of deposit on a 27 ( 20 cm trap surface. The starfish-like shape of the buttressed root system has created inter-buttress receptacles in which the behavioral remains were trapped and buried rapidly. On average, sediment within these receptacles was c. 12 cm thicker than outside.

II. Incipient color variations in the vertical dimension occur and are related to the migration of organic matter down the soil profile; from dark brown between 0-5 cm of depth to dark yellowish brown between 5-20 cm, and to light yellowish brown from 20 cm to the base. The imcomplete weathering of the soil and, also, the migration of the water table through the profile leads to patchy accumulations of oxides in the lower part of the profile. Color classification follows the Munsell system in which:

10YR 3/3: dark brown
10YR 4/4: dark yellowish brown
10YR 6/4: light yellowish brown
10YR 8/3; 10YR 8/4: very pale brown
10YR 7/3; 10YR 7/4: very pale gray
10YR 8/2: white
5YR 4/3: reddish brown
7.5YR 2/0: black

III. Excavation of P100 was performed in September 2001. Our decision to focus on P100 was determined by available behavioral data (2), the existence of nut shell remains around anvils, use-wear on four anvil complexes, and the stone pieces that visibly grouped around the anvils. Of the excavated surface, a total of 59 m² corresponded to the excavation of nut-cracking surfaces around anvils in arbitrary levels (i.e. spits) circa 3 cm thick each, with sediments wet-sieved through a 1.81 mm mesh. We also excavated a single 5 cm level across 119 m² outside the anvil zones, as off-site control units, in areas without any surface indication of chimpanzee nut-cracking. For these off-site control units we followed a random sieving strategy in which 10 kg of sediment were sieved out of every 10 m². An additional off-anvil trench of 2 m² was excavated to penetrate the site deposits to the water table.

IV. Rock colors (see section I above for Munsell classification): Granites are primarily very pale brown (41.54%) and very pale gray (32.15%), or, in the case of feldspar and quartz, white (4.59%). Various hues of reddish brown (e.g.: 10.43%) are common among the laterites, and the diorites are black (1.46%). The remaining percentage (c. 10%) include slight variations of the hues above.