On Tuesday of this past week, a boulder, roughly 25 million years old, give or take, began to show some signs of instability. At some point in the past—we’re not sure whether it was decades, centuries, or millennia—it broke off the parent rock that it was once a part of, rolled once or twice, and to the best of anyone’s knowledge, never moved again.
For all of us, this king size piece of rhyolite has been a fixture of the local scenery at the top of the switchbacks on the Main Trail, only a stone’s throw from Picket Post House. It is big, probably 12 feet in its longest dimension and weighing countless tons, all of it bearing down on a slope that angles towards Queen Creek below. The Main Trail zig zags its way down the slope, and makes its first turn a few dozen feet below, passing directly in front of the boulder.
Growing on top of the rock in pockets and cracks is a horticultural diorama of tortured native plants, including a gnarly dwarfed palo verde, a rotting Ferocactus, a small, two-armed hedgehog—and, appropriately, a small plant with the common name of rhyolite bush, Crossosoma bigelovii, which considers this kind of rock its favorite substrate. Why any plant would bother taking root on such an inhospitable hunk of rock is a valid question, but I suspect that birds and their penchant for defecation (and a liking for the fruits of the aforementioned species) had something to do with getting it all going.
A familiar face
Everyone who walks the entire Main Trail passes right by this boulder, including the recent attendees of our once-a-month geology tours. It is huge but nondescript—not invisible (of course}, but unassuming, casual, not appreciably different in appearance than the massive rock cliffs and outcrops that mingle within the 360-degree scene on this section of the trail.
But a week or two ago, this volcanic castoff caught the eye of the leader of our geology tour.
With all of the rains that we had in the last week of June, it looked like the forces that have kept it immovable might be losing to the will of gravity that wants to send it rolling down the hill. Some erosion under the boulder had taken place, making it less stable, so the Arboretum decided to take action.
The contracting firm that successfully helped us remove some unstable rock along the High Trail in 2015, received another call from us last week. On Tuesday, July 12, at 6 a.m. a four-man crew was ready for some demolition work.
They came armed with heavy bars to chip away at the rock, a compressor to operate a jackhammer, and a hydraulic jack to encourage the rock to roll when they were ready. Plenty of our staff were on hand to watch the spectacle. A new history was in the making, and for several hours, we watched four men do their level best to reduce this boulder down to a manageable size. However, it resisted.
Pièce de résistance
Chunks broke off and small chips flew. Pea-sized gravel broke off and rolled down the hill, occasionally followed by amorphous chunks the size of a bag of ice. But if the goal was to have this monster break up like a watermelon dropped on the sidewalk, well, it was not to be. With all of their prying and hammering, it was probably internally weakened, but from the outside, it looked much the same as when they started.
Dynamite was ruled out. And getting heavy machinery into this location was impossible. Cursing usually helps me solve these sorts of problems, but these guys were professionals, so finally, they decided to roll it. It was more square than round, so there was a good chance that it would simply flop over once or twice and settle itself into a nice, safe, stable position.
They dug around the base of the boulder and used a heavy duty hydraulic jack to give the boulder some added incentive. They repositioned the jack several times for better leverage, and I found myself whispering to the few bystanders next to me: “You know, if that boulder jackknifes and falls back on them, there is nothing we’ll be able to do.” When the jack didn’t work, they put a rock in its place as a fulcrum and one of the guys inserted a long bar between it and the rock while the other three pushed.
Get out of the way
Then someone yelled, “It’s moving!” It started to slowly slide forward then flopped on its side and rolled, gradually picking up momentum. It did two full rotations and traveled about 150 feet before it broke into three Smart Car size pieces at the end of its final roll. One of the three chunks kept moving and stopped just 6 feet from a 15- foot saguaro, and if you listen closely to the video, you’ll hear me pleading for it to be missed.
The only major casualty was damage to the trail as the boulder used its heft and might to crush parts of it as it rolled. The four plants that grew on top of the boulder were wedged in too tightly to be saved, so they rode the boulder down to the bottom.
By the next day, the trail had been completely rebuilt and rerouted to go around the fractured remains of the original boulder. The three constituent parts that remain are now permanent parts of the landscape. It was nothing that earthly forces wouldn’t have accomplished, eventually. We just accelerated the process and gained two more boulders in the proc