Before and After: Satellite Photos of Vast Big Sur Landslide

L

ast Saturday night, a mountainside gave way along Highway 1, a mile or so down the road from the hamlet of Gorda on the southern end of the Big Sur coast.

You can say lots of things about the landslide (depicted above in before-and-after satellite images from San Francisco's Planet Labs): That it was vast, sending millions of tons of rock, soil and mud down the mountain, across the precariously situated roadway and into the Pacific Ocean. That it's historic -- almost certainly the largest since a giant slide near the entrance to Julia Pfeiffer Burns State Park in March 1983 closed the coast highway for 13 months. That it's going to take a long, long time -- Caltrans says at least a year -- before this stretch of Highway 1 is clear again.

One thing you can't say, though, is that the monster slide was a complete surprise. Weather and history both foretold something big happening along Highway 1 -- perhaps close to Gorda.

Rain fell long and hard on the slide area at Mud Creek and the mountains above it, with some highland locations getting as much as 117 inches from last October through early April. Caltrans engineers say that springs and seeps that came to life near Mud Creek saturated the mountainside and started several separate slides, occasionally closing the highway, before the entire slope collapsed last weekend.

Mud Creek is also in the middle of a zone that one might call particularly slide-prone. A 2003 Caltrans report described the terrain where the latest slide happened as "very active" with "high landslide potential."

History bears out the truth of that statement. The latest slide occurred on a 5-mile stretch of Highway 1 that landslides have closed for a month or more on at least half a dozen occasions since 1982 -- typically during or after unusually wet winters.

Caltrans spokeswoman Susana Cruz said Friday afternoon that engineers are continuing to assess the extent, composition and behavior of the slide -- investigating the area with drones and helicopters and drilling through the mass of rock and muck to see how deep it actually is.

Cruz said the agency will use ground-penetrating radar to assess how much the slide is still moving. The technology has been used to monitor a slope blocking Highway 1 at a location called Paul's Slide, near Limekiln State Park, 13 miles northwest of the Mud Creek slide.

Caltrans managed to build a temporary lane around Paul's Slide, only to discover the earth is continuing to move there. The agency closed the location again earlier this week.

One immediate impact of the Paul's Slide closure is a potential delay in work to replace the Pfeiffer Canyon Bridge -- severely damaged by a slide, then demolished -- further north in Big Sur.

Caltrans contractors have been trucking construction supplies from U.S. 101 in the Salinas Valley over steep, narrow, twisting Nacimiento-Fergusson Road to Highway 1, then hauling them north to Pfeiffer Canyon. Paul's Slide is now blocking that route indefinitely, and Cruz said contractors could be forced to fly supplies to the bridge site by helicopter.