Ancient Mesopotamians made rock from silt.

Inhabitants of ancient Mesopotamia cultivated crops in the rich soil of the Fertile Crescent. They also took that soil and, thanks to a surprising technological innovation, transformed it into the slabs of rock that they desperately needed for grinding grain and constructing buildings, according to a new study.

Pieces of gray-black rock previously excavated at the second-millennium--B.C. Mesopotamian city of Mashkan-shapir, located in Iraq, look like natural basalt but were actually manufactured by melting and slowly cooling silt, reports a team led by anthropologist Elizabeth C. Stone of the State University of New York at Stony Brook.

"We were surprised to find this level of technological achievement in ancient Mesopotamia," Stone says. "Potters and metalsmiths probably pooled their knowledge to develop an experimental process for making synthetic basalt that eventually yielded a consistent product."

She and her coworkers first noticed signs of intentional manufacture on several large, basalt blocks that had been found near temple remains at Mashkan-shapir. The slabs were all flat on the bottom and bumpy on top.

Several hundred rock fragments from around the site displayed the same smooth bottom and uneven top surfaces. The chemical composition of this material is unlike known basalts but resembles that of riverborne silt in the region, the scientists report in the June 26 Science.

Both synthetic basalt and silt samples from the Mashkan-shapir area melt at temperatures close to 1,200 [degrees] C, according to experiments conducted by Stone's group using a covered furnace. When the silt was melted, then slowly cooled at a rate of 1 [degrees] C per minute, the process yielded a hard material like that in the ancient slabs.

Only further excavation at the Iranian site, which has been off-limits to archaeologists since the Gulf War in 1990, will determine how ancient Mesopotamians made synthetic basalt, Stone says. For now, she suspects that charcoal and lumps of silt were melted in large furnaces and left to cool for 20 to 40 hours, the time needed for the crystal growth observed in the blocks.

Many researchers hold that Mesopotamian metalworkers did no more than refine and cast imported pieces of copper and bronze. But evidence of at least smallscale copper production exists at Mashkan-shapir and a nearby site from the same time period, Stone maintains. Ceramic and metallurgical workers may have provided innovations, such as large, high-temperature furnaces, for synthetic basalt production, she theorizes.

It remains unclear why synthetic basalt production emerged in the second millennium B.C., comments anthropologist Guillermo Algaze of the University of California, San Diego. The mixing of ceramic and metallurgical techniques suggests "a measure of desperation" in the need for basalt, he says.

Rock has long been scarce in southern Mesopotamia, says David J. Killick, a historian of technology at the University of Arizona in Tucson. It's not surprising that inhabitants of Southern Mesopotamia modified different lines of technology to cook up a basalt substitute, an advance that proved "elegant and exciting," Killick remarks.