Examples of grinding tools in the San Diego. County. archaeological record, 7000 years ago to present

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1 Examples grinding tools in the San Diego County archaeological record, 7000 years ago to present Over 17,000 sites in San Diego County rinding acorns, Santa Ysabel, From Hedges and Beresford Flat/concave metate Mano Basin metate Pestles Mortar Bedrock mortars, basins, and slicks 1

2 Processing clay on a flat metate, Santa Catarina Pai Pai (Campbell 1999) Acorns Quercus kelloggi Laguna Mountain, CA Chia seeds Salvia columbariae Purchased Experimental tool surfaces were described and photographed prior to use. Clay Residual granitic clay Cuyamaca Rancho State Park, CA Experiments were observed and monitored by project consultants. Clint Linton, Santa Ysabel Band Kumeyaay Indians Jenny Adams, Desert Archaeology, Inc. Experimental grinding session October 2008 San Diego Archaeological Center 2

3 Each experimental tool set was used by multiple users for a cumulative period 6 hours. Product Yield Rate=Weight finished product/processing time Product Loss= Difference between weights intermediate substance consumed and finished product expressed as a percent Average Processing Efficiency (over 6 hours) for Tool Shapes and Intermediate Substances (n=3 for each cell) Acorns Average Yield (g)/hour Product Loss (%) Mortars Basin Metates Flat/Concave Metates Chia Seeds Average Yield (g)/hour Product Loss (%) Clay Average Yield (g)/hour Product Loss (%) All tool shapes were capable processing all intermediate substances (Form Function). Manufactured concavities improved yield rates by up to 50% for acorns and small seeds. Working surface area = entire area modified by use. For basins and mortars, this is estimated using the formula for surface area a cone. S B B S B S 1. Experimental yield rates are tightly clustered for mortars and basin metates with manufactured concavities. Therefore, the size and shape manufactured concavities in netherstones appears to play an important role in determining tool efficiency. B S S B B S 2. Increasing working surface area may have the potential to improve processing efficiency. 3. Tool sets made from aphanitic basalt rocks are frequently less efficient than granitic and sandstone tool sets, but not always. (B=Basalt, =ranitic, S=Sandstone) 3

4 Number Leveled processing relief wear zones Scratches Fractures/ Cracks/ Chipping Pits Task 10 Flat/concave Acorns Acorn processing 10x Mortars 2 X X X Basin metates 1 X X Before Flat/concave metates 1 X X Chia seed processing Mortars 2 X X Basin metates 1 X X Flat/concave metates 1 X X Clay processing Mortars 2 X X Basin metates 1 X X Flat/concave metates 1 X X All showed leveled relief characteristic stone-on-stone processing. Acorn and seed processing tools showed sheen formation on high areas. Clay processing tools had a frosted appearance resulting from fractures/chipping. After Fractures Low areas Flat/Concave Metates Microscopic Observations 10-90x Use surface (20X): Intermediate Substance Number processing wear zones Leveled rock grains/ crystals Scratches/ striations Fractures /cracks/ chipping Pits/rock grain removal Acorns (shelled, skinned) 1 X X X X X Chia seeds (dried) 1 X X X X Clay (residual granitic, dry clods) 1 X X X X Crystal edge rounding was observed on tools used to process acorns but not chia seeds. Acorn flour and small seeds and seed coats are pushed into low areas and protect these areas from wear. Rock grain/ crystal edge rounding Alteration interstitial lows Surface Task 11 flat mano used to process chia seeds (10x) Use surface (90X): Rounded edges 4

5 Use surface (20X): Basin Metates Microscopic Observations 10-90x Use surface (90X): Intermediate Substance Number Leveled Scratches/ Fractures Pits/rock rock striations /cracks/ grain processing grains/ chipping removal wear crystals zones Acorns (shelled, skinned) 1 X X X X Chia seeds (dried) 1 X X X X Clay (residual granitic, dry 1 X X X X clods) Rock grain/ crystal edge rounding Alteration interstitial lows Basin tools used to process acorns and chia seeds showed the same kinds wear traces. Acorn processing Mortars Microscopic Observations 10-90x Intermediate Substance Number processing wear zones Leveled rock grains/ crystals Scratches/ striations Fractures /cracks/ chipping Pits/rock grain removal Rock grain/ crystal edge rounding Alteration interstitial lows Acorns (shelled, skinned) 2 X X X X Chia seeds (dried) 2 X X X X X X X Task 13 Flat Metate Task 4 Basin Metate Clay (residual granitic, dry clods) 2 X X X Mortars have two distinct wear zones that exhibit different use-wear traces for the same task, striations, and sheen develop first on the concavity walls Fractures, cracks, and chipping are observed in the bottom the concavity Intermediate substances tend to protect the bottom the concavity from use-related alterations Small seeds and seed coats crushed and abraded against the concavity walls are forced into low areas, resulting in rounding rock grain edges and wear within interstices 5

6 Task 23 Sandstone mortar used to process chia seeds Processing acorns with mortar and pestle Processing chia seed with mortar and pestle Task 23 Sandstone pestle used to process chia seeds Wall (20x): Wear in low areas Bottom (20x): Crushing Upper distal end (20x): Rounding within interstices Rim (20x): uneven topography Comparison Experimental Hide-Working Wear and Hand Wear Edges (20x): Lower distal end (20x): Crushing Hide-working experiment by J. Adams 40x (photo by T. Buonasera) Hand wear on Task 23 sandstone pestle 40x Both hides and hands are st and oily and produce similar wear traces. 6

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