2. ground stone assemblage variations and the role of grinding and pounding in plant food exploitation. In the Levant, true ground stone tools (in which abrasion plays a large role in lithic reduction) begin to appear in small numbers in sites of the Upper Paleolithic (45,000-22,000 b.p.), Kebaran (22,000- 14,500 b.p.) and Geometric Kebaran (14,500-12,500 b.p.) time ranges.