Excerpts from Books and Wikipedia
"In 1959 excavations at Erlitou (in the city of Yanshi not far from Luoyang and just south of the Yellow River) uncovered another site with large palaces that seems likely to have been a capital of the Xia dynasty. . . . Moreover, Xia, Shang, and Zhou centered in three different areas and seem to have co-existed. . . . Toward the end of the third millennium BC the making of bronze from copper and tin deposits widely mined in North China coincided with the rise during the Xia and Shang dynasties of the first central government over a broad area." [Fairbank: China, p. 33-7]