During excavations at Ryckholt-St. Geertruid (1964-1972) we examined more than 60 flint mines (shafts and their galleries). Radio-carbon dates indicate these mines were in use between 3150 and 3050 B.C. At Grimes Graves we examined (1973-1976) about 20 flint mines. Flere, Radio-carbon dates gave only a general indication of a date between 2200 and 1800 B.C.