Category: Stories in Stone Blog

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.

Good Old Cobblestones

President Obama has said that his stimulus plan will provide the “largest investment increase in our nation’s infrastructure since President Eisenhower created the national highway system half a century ago.” It will create hundreds of thousands of job for people rebuilding railways, bridges, and roads. While I applaud his concern about jobs and improved safety, I worry that in the mad rush to pave we may lose a bit of history—the handful of cobblestone roads still dotting Seattle.

A happy group of explorers trodding across one of Seattle’s last cobblestone streets

Between the 1890s and 1910s, sandstone cobbles were a a popular road-paving material in Seattle. The most commonly used varieties came from quarries in Wilkeson, a small town about 45 miles south of Seattle. Workers could easily cut the brick-sized blocks, which provided good traction for horses, although horse shoes did wear down the stone. And the stone cobbles lasted longer and created less of a mess than the mud or wood of the past. (Manhattan also has a few cobblestone streets made from the 450-million year old Quincy granite.)

Quincy Granite cobblestones in Greenwich Village

The Wilkeson sandstone lithified from thousands of feet of sand deposited 40 to 50 million years ago in the Eocene Period when western Washington was flat and subtropical. Palm trees, swamp cypresses, and tree-sized ferns grew in the moist (40-100 inches of rain), bayou-like environment. Waves from an ocean that spread to the west washed ashore on beachfront property, now covered by the urban metropolises along Puget Sound. The only mountains that existed rose far to the east along the Washington-Idaho border. Rivers and streams washed out of those mountains and dropped sand in a coastal lowland dotted with seasonal lakes, swamps, and lagoons.

In addition to sandstone cobbles, brick was another important paving material and can still be found on a few Seattle streets. Local manufacturers used a clay deposited in a proglacial lake, which formed in advance of the ice sheet that covered Seattle during the last Ice Age. A production capacity of 70,000,000 bricks a year made King County the largest producer of paving brick in the country. At one time, these bricks covered streets in Portland, San Francisco, Tokyo, and Los Angeles, as well as roads in Chile, Argentina, and India.

One other local building material, the Index granite, appears on local streets. It formed 34 million years ago during subduction of the Juan de Fuca plate under the North American continent. The Index was used primarily for curbs and can still be found in a few older neighborhoods of Seattle. (One way to get a feel for the age of a neighborhood is to look at the curbs. Old ones are granite; newer ones have concrete with a metal rim, particularly on corners; and modern ones are only concrete.)

According to the last official survey I could find, conducted in 1993, only 93 of these historic streets remain in Seattle; most have been paved over, their stories lost to drivers who don’t want the jarring ride. I do not mind them. I like the connection to the past and consider it a privilege to drive over the ancient cobbles and rustic bricks, although I try to take these roads when no else is in the car with me.