Footprints in the Ash

Over the past couple of years, a controversy has arisen over possible human footprints in central Mexico. The disagreement centers on age and maker. Is the ash where the prints are preserved 40,000 or 1.3 million years old and did people out for walk produce the foot-shaped impressions or did more recent people inadvertently alter the rock and subsequent weathering and erosion form the marks? The simmering debate raises many questions but to me the focal point is the fact that the tracks occur in an abandoned building stone quarry.

Footprints in ash layers, from from web site of Sarah Metcalfe, University of Nottingham

The debate began in July 2005 when Dr. Silvia Gonzalez and her colleagues at Liverpool John Moores University reported that they had found 40,000-year-old human and animal footprints preserved in volcanic ash. The ash had fallen along a shoreline of a shallow lake in the Valsequillo Basin, about 80 miles southeast of Mexico City. If the dates are correct, it would make them the earliest evidence for people in the Americas. Gonzalez’s team based their analysis on accelerator mass spectrometry dating, electron spin resonance, Argon-Argon dating, and optically simulated luminescence dating. Knowing that the dates would be controversial, Gonzalez said at the time “It’s going to be an archaeological bomb and we’re up for a fight.”

By the end of 2005, a team at Berkeley Geochronology Center at the University of California Berkeley had responded. They reported that the ash dated to 1.3 million years old. Either the tracks belonged to our ancestor Homo erectus or, more likely, the impressions were not footprints but marks that resulted from quarrying. The California team also relied on Argon-Argon dating, as well as a paleomagnetic data. Their most recent and detailed analysis of the quarry site ash appeared in the March 2009 issue of Geology.

Numerous small quarries dot the Valsequillo Basin. The ashy layers split into thin sheets that can be used in roads and walkways. The track-rich quarry was abandoned relatively recently, according to Joshua Feinberg, formerly a graduate student at UC-Berkeley and now a professor at the University of Minnesota, but not before workers had removed several feet of lake sediments to get to the underlying hard beds of ash. As happened with the quarries examined by Gerta Keller for evidence that gases released by voluminous basalt flows led to the extinction of the dinosaurs, the Mexican quarry workers made the story possible by exposing the ash.

Quarried stone from web site of Sarah Metcalfe, University of Nottingham

“[T]he past quarrying activity really is an essential part of this story,” wrote Feinberg in an email to me. Not only did it reveal the tracks but he suggests that quarry workers left the marks in the ashy layer and that over time they eroded into shapes that Gonzalez’s team misinterpreted as human footprints. I will let the scientists duke out the when and who of the story but I do take pleasure in knowing that yet another building stone quarry makes the world a bit more interesting.

Building Stone Census of 1880

Every ten years the federal government conducts a census of the people. Census takers spread across the country ferreting out the vital statistics of America. For the 1880 census, the government also took stock of building stones and the quarry industry. I was recently lucky enough to acquire a copy of the 410-page report. (It came to me because my brother-in-law bought a bar in Arizona, which had the book in it, and his seven-year-old son was kind enough, with a bit of coercing, to give me the tome.)

As one might expect of a census, the book provides a minutia of facts. For example, 1,525 quarries operated and generated 115,380,133 cubic feet of stone, roughly enough to build one and a half Great Pyramids. Or you can learn that a David Williams owned a slate quarry in Slatington, Pennsylvania. The quarry was in Lower Silurian beds, produced black slate for roofing and school slates, and opened in 1864. Sounds like an industrious fellow.

Of more interest is the section that describes the building stones used in 137 cities, ranging from Akron, Ohio to Zanesville, Ohio. The listings reveal the classic story of most cities and towns, which generally use local stone, if available. Some people also import outside rock, most often for monumental buildings such as banks or churches. The story changes slightly in the bigger cities of Boston and New York, which still use local rock but also pull in stone from around the world. For example, exotic marbles came from Italy, France, Portugal, Switzerland, Spain, and Germany. That being said, 80 percent of the stone fronted houses in New York City used brownstone, most likely quarried in either Connecticut or New Jersey.

A hornblende granite from Grindstone Island in New York, some of which was used as paving material in Chicago

Reading through the census book, I am reminded of other books like this. These are the older scientific books that seemingly attempt to put in everything known about a subject. They are clearly labors of love of the writer and detail not just the science but also the history of a subject, such as the origin of a name of a species or who first used a particular building stone. In delving so deeply into the finer points, the authors make their subjects so interesting that the reader cannot help but take a deeper interest.

A channeler, used to cut stone into blocks in the quarry

Perhaps that is why my new book from 1880 survived its long life at a small bar in the middle of nowhere Arizona. I like to imagine that the drinkers of yore gathered around the book late into the evening and regaled each other with the many fine facts. I am sure they were better people because of what they learned.