Trade and communications, which had expanded through the millennia, must have provided a tremendous cross-fertilizing impetus to cultural growth. The archaeologist can infer the existence of far-ranging trade from the wide dispersion of obsidian, alabaster, marble and Spondylus shell. The seas and inland waterways, doubtless served as primary routes of communication, and obsidian was being transported by sea as early as the seventh millennium BC. The use of sailing-boats is attested from the sixth millenia onwards by their incised depictions on ceramics.