Street Appeal

Down and dirty and rockin' the streets

First curbs, now streets, and a second discussion of urban geology. I will return to a non-geological theme next week.

One of the world’s great bike races started last Saturday. For three weeks riders course through Italy in the Giro d’Italia. My favorite Giro spectacle came on the final day in 2009 during a dramatic, geology-influenced finish. Race leader Denis Menchov was a 1,000 meters from the end of a time trial and looked ready to hold onto and earn the final maglia rosa, the fabled pink jersey of the race leader. Traveling about 50 km/hour, Menchov hit a long stretch of black cobbles and went tumbling, as his front tire slid out from under him. In the ensuing “absolute pandemonium,” his mechanic in the car behind quickly got him a new bike, pushed Menchov along, and he won the Giro d’Italia.

What made Menchov’s ride geological were the cobbles, or what locals call “San Pietrinis,” or little Saint Peters, in reference to his role as the rock of Christianity. The cobbles (technically called setts) come from the Capo di Bove lava flow, the product of a 277,000 year old eruption from the Alban Hills volcano, southeast of Rome. Famous since Roman times, building stone from the basalt flows was described by the great architectural writer Vitruvius in The Ten Books of Architecture, written between 31 and 27 BCE.

“I thought that I should expand on their [Roman stones] varieties and the criteria for their use, as well as what qualities they have in building, so that when this information is known, those who are planning to build will avoid mistakes and assemble supplies suitable for buildings,” he wrote. Vitruvius discussed three types of rocks. The volcanic tuff that comprises Rome’s seven hills was soft and yielding but fire resistant. Basalt was hard and enduring, which contributes to their slipperiness, particularly on rainy days in the Giro. Between these end points was travertine, a type of limestone, which withstood compression, making it suitable for the keystones of arches. Travertine is the stone of the Colosseum, and several Seattle buildings.

Seattle also has sett-covered streets. Between the 1890s and 1910s, sandstone was a popular road-paving material. Setts often came from 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 horseshoes did wear down the stone. In case you were interested, and I know you are, 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. For some unexplained reason, many drivers don’t like these streets; of course I seek out setts!

Brick can also be found on a few Seattle streets. Local manufacturers quarried two clay layers: one deposited about 10,000,000 years ago (quarries near Auburn and Taylor) and one deposited during the last Ice Age (quarries along Duwamish River and near Renton). Production capacity of 70,000,000 bricks a year made King County the largest producer of paving brick in the country, some of which paved streets in Portland, San Francisco, Tokyo, Los Angeles, Chile, Argentina, and India.

And, a follow up to last week about curbs, sent in by a reader. Down in California, there was a curb that illustrated the offset produced by movement of the Hayward Fault. Sadly, but understandably, it was altered to make way for an ADA curb cut.


In case you missed it, you can catch my book launch for Homewaters with Mary Ann Gwinn, via the Seattle Channel. Or if you still want to see me at a live virtual talk, here’s a link to my upcoming talks.

Plus, I still have copies of Homewaters for sale. This is the best way to get a signed or inscribed copy.