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What caused the Lilydale landslide?

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Light rain falls on emergency crews
As light rain falls, emergency crews work in shifts to secure the area around a deadly rockslide in Lilydale Regional Park in St. Paul, Minn., on May 22, 2013.
MPR Photo/Nikki Tundel

Recovery efforts have resumed at the site of a fatal landslide in a St. Paul park along the Mississippi River.    A fourth-grade field trip to a Lilydale Regional Park turned deadly Wednesday when a hillside, saturated by persistent rain gave way, killing one child and injuring two others. One child is still missing.

Satish Gupta, a professor in the University of Minnesota's Department of Soil, Water and Climate, described how mudslides happen on MPR's Morning Edition. 

Gupta said the rainy weather likely played a role.

"The rain was trickling and not coming at a high intensity, so it was soaking in," he said.

The park's landscape includes buried layers of rock left by glaciers, so rainwater can't go directly to the groundwater and instead travels toward the hill slopes, he said. 

If the hill slopes get too saturated, they can fail. Tree roots can help hold the slope, but clay in the soil acts like a lubricating agent, he said. 

"Roots can only hold so much," Gupta said.

Gupta said mudslides are common in the Minnesota River Valley along hillsides and bluffs.