DryStoneGarden

Devil's Postpile National Monument

Probably the most striking geological feature on the eastside, an area full of geological features, is the Devil’s Postpile, one of the world’s best examples of columnar basalt. Columnar basalt is one of those natural elements that looks unnaturally geometric. It forms when it lava cools very, very slowly and evenly. The lava starts to contract and then crack, and because it cools so evenly the cracks form into hexagons, the most stable and efficient shape. As the Park Service page about the formation of the Postpile explains, “a hexagonal system provides the greatest relief with the fewest cracks.” (Bees use the same hexagonal system in their honeycombs because it forms a matrix with the largest amount of storage space for the least amount of wall.) Devil’s Postpile isn’t the only place where this has happened — the Wikipedia entry has links to many other columnar basalt cliffs, and pieces of columnar basalt regularly show up at stoneyards in the Bay Area –but the columns at the Devil’s Postpile are especially long and regular.

The Devil's Talus

The base of the cliff has the world’s coolest talus in my opinion. My crew was so well trained/indoctrinated that their first comment was how great these pieces would be for building steps. The park service doesn’t even let you climb on the talus, though, let alone build with it.

The Devil's Patio

Possibly the best part is that a glaciar carved off the top and made a natural patio.

The Devil's Paver Pattern

It’s uncanny how much they look like rough-cut natural stone pavers. Hexagons are stable because three joints come together at every vertex, making for a nice oblique 120 degree angle. The park service says that at Devil’s Postpile 55% of the pieces have 6 sides, 37% have 5, 5% have 7, and the remaining 3% have 4 sides or fewer. That’s a high percentage of hexagons compared to other sites in the world.

The Devil's Grading

The grading is a bit extreme on parts of the patio.

Surprisingly Comfortable

Ahhh, nice, soft basalt. My crew never saw a flat surface that couldn’t be slept on.

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Three Rivers Flagstone

One of our clients calls Three Rivers flagstone “purple zebra,” which is a pretty accurate description. It always catches our clients’ eyes when they visit the stoneyard, though they often balk at the price, as it became really expensive a few years ago and is probably the most expensive flagstone commonly available in the Bay Area. It’s from one of the largest flagstone quarries in the United States, located up in the Sawtooth Mountains in Idaho. The quarry wanted to expand and the Bush administration, in their inimitable way, told them to go right ahead and not worry about doing an environmental impact study. An environmental group, the Western Watersheds Project, which owned a wildlife preserve adjacent to the quarry, then sued, and a judge agreed with the environmentalists that yes, U.S. law does require environmental impact studies, and temporarily shut down work at the quarry. Everyone settled out of court and the price then went up a couple of hundred dollars to around $750/ton.

I’ve seen the stone described as a type of quartz-sandstone and as argillite. Answers.com defines argillite as “an intermediate between shale and slate, that does not possess true slaty cleavage,” which sounds about right, except that I would add that Three Rivers is really hard and heavy. (Can I say that I prefer my stone with a bit more cleavage? It’s true. Cleavage is the tendency of stone to break cleanly.) The swirls of color in Three Rivers come from irregular mineral layers which look cool but make the stone inclined to break irregularly. The patio in the photo, for instance, has rougher, wider joints than I would do with a cleaner-breaking stone like a sandstone. To get tight joints with Three Rivers, you pretty much have to cut everything with a saw. We usually only use Three Rivers for stepping stones, paths, and small patios; it can look too busy when used for larger areas.

Quarriesandbeyond.org has links to info about the Three Rivers quarry on their list of quarries in Idaho.

Three Rivers Flagstone Patio