The genitive expresses material or contents.
ἕρκος ὀδόντων the fence (consisting) of the teeth Δ 350, κρήνη ἡδέος ὕδατος
a spring of sweet water A. 6.4.4, σωροὶ σῑ́του, ξύλων, λίθων heaps of corn, wood, stones H. 4.4.12, ἑξακόσια τάλαντα φόρου
six hundred talents in taxes 2.13 (cp. 1296).