Projectile points made from Ob2 obsidian

Eleven obsidian projectile points (4%) were made from Ob2 obsidian; a surprisingly high number under the operating assumption that fracture and visual quality of the material were important characteristics in bifacial tool production. Briefly exploring these eleven Ob2 projectile points may shed light on the characteristics that guided material selection in prehistory.

Ob2 materials form a much higher percentage (15%) of the obsidian flake surface collection than do Ob2 bifacial tools (4%) of the obsidian tool collection, which suggests that Ob2 material was being knapped but apparently not bifacially retouched.

The projectile points made from Ob2 material tend to have small or low-density heterogeneities that do not appear to greatly affect knapping quality, although visually the pieces appear mottled. These points were found in the south-eastern part of the study area in the San Bartolomé area (including one from a Late Formative excavated context), and in the reconnaissance blocks 4 and 5.

ArchID	Block	Period	PPt Type	Weight (g)	Length (mm)	Retouch Index
953.1	2	M. Archaic	2c	4.1	38.28	0.9375
820.1	2		3b	4.1	45.82	1
918.1	5		2c	6.9	48.73	0.96875
818.1	2	Late(2) - T. Archaic	4f	2.9	31.13	1
231.10	4	T. Archaic - Late Horizon	5	5.3	37.62	0.84375
994.1	2		5d	0.5	21	1
1014.3	2		5	2.5	25.52	0.9375
1026.9	2		5	1.9	Broken	1
1038.3	2		5	11.3	19.9
2061.3	2		5d	1.2	Broken

Table 6-9. Projectile Points made from obsidian containing heterogeneities (Ob2).

Period	Ob1	Ob2	Percent with Heterogeneities	Total
Middle Archaic	18	3	14.3	21
Late Archaic	4	1	20	5
T. Archaic - Late Horizon	221	7	2	227
Total	243	11	3.9	253

Table 6-10. Ratio of Obsidian Projectile Points with heterogeneities.

Due to low cell counts, conducting a chi-squared test required aggregating the counts from the Middle and Late Archaic Periods. A chi-squared test on the aggregated table (Table 6-10) showed that the difference between projectile points from Group 1: Middle and Late Archaicand Group 2: the Terminal Archaic through the Late Horizonwith respect to the use of obsidian with heterogeneities is very significant (c²= 9.976, .005 > p> .001). It appears that Ob2 was very significantly less used for point production in the later time period.

Note that the material used for projectile point production in Block 2 was at times the cloudy Ob2, and it is likely that this reflects, in part, the availability of this material on the southern and eastern flanks of Cerro Hornillo. However, the vast majority of the Ob2 obsidian flakes are actually found in Block 3, at the site of Taukamayo.

Obsidian source material with and without heterogeneities

A number of the lag gravel deposits encountered in Blocks 4 and 5 of the survey are Ob2 material. Accordingly, obsidian artifacts from these blocks are higher in heterogeneities, indicating that there was a utility for this type of obsidian despite the imperfect matrix of the material. Investigating the distribution of Ob1 and Ob2 material across all obsidian artifacts (primarily flakes) shows that the Ob2 make up approximately one half of the obsidian artifacts even in Block 3 some distance from the Maymeja zone where Ob1 was observed in situ.

The mean size of Ob1 flakes is notably smaller, which suggests that more advanced reduction was occurring on the Ob1 material. There may be some size bias occurring with observations of heterogeneities because small flakes struck from Ob2 nodule will often appear relatively homogenous and clear if few bubbles or particles are included in the glass in that portion of the flake.

	Homogeneous (Ob1)			Heterogeneous (Ob2)			Total count
Block	No.	m Length (mm)	m Weight (g)	No.	m Length (mm)	m Weight (g)
1	315	40.6	18.8	24	40.5	25.4	339
2	240	25.7	3.6	21	33.8	10.5	261
3	62	30.1	12.4	32	30.1	6.2	94
4	104	35.1	18.3	38	36.8	20.7	142
5	134	23.2	6.2	43	25.5	6.5	177
6	12	25.2	3.1	3	31.7	6.0	15
Total	867	30.0	10.4	161	33.1	12.6	1028

Table 6-11. Obsidian: mean sizes of complete Ob1 and Ob2 artifacts, by Survey Block.

These data, show patterns in terms of the mean length and weight differences between Ob1 and Ob2 artifacts. In all blocks, Ob1 artifacts are on average lighter than their Ob2 counterparts except for in Block 3. Furthermore, in most blocks the mean lengths of Ob1 and Ob2 material are very close but as the weights are different and therefore width or thickness must vary between Ob1 and Ob2 material. Further investigation of the metric data shows that, indeed, Ob1 artifacts have narrower and thinner medial measures, on average, than do Ob2 artifacts except for in Block 3 where Ob2 materials are thinner.

It appears that throughout the study region, Ob1 materials were preferentially knapped into artifacts that were narrower and thinner, but not necessarily shorter, than the Ob2 materials except for in Block 3. Ob2 material was much more common in Block 3, as will be discussed below, and it appears to have been used for more immediate butchering needs rather than for production of bifacial tools, a pattern that is consistent with the later date of the Callalli occupation. There is also a possibility of size bias where smaller Ob2 flakes are classified as Ob1 because no heterogeneities were evident in that particular small flake.