but only four have been big enough to block the highway in the past half-century, Prof. Clague said. In the 1960s, a falling rock killed a motorist within a kilometre of this week's slide.

For 130 years, the flow of Big Canoe Creek was obstructed by a man-made hunk of rock and mortar called Goodwin's Mill Dam in Springville, wreaking havoc on the fish, wildlife and land surrounding it.