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The steep hillside that gave way Saturday east of Arlington, killing 14 people, is a historic slide area that dates back to ancient times.

The most recent slide is a reactivation of previous slides, according to University of Washington geology professor David Montgomery.

“It’s part of an ancient landslide complex in glacial sediment,” he told .

Looking at high resolution data, Montgomery said, “There’s a number of fairly large slides like that all along the coastal bluffs and in some of the river valleys where rivers had cut into weak glacial sediments, like in this case.”

Montgomery said previous slides weakened the hillside near Oso, Washington, making it prone to slide again.

“Basically, the way you can think about it is that if a big part of a hill like that has slid, it has created a zone of weakness at the failure plane in the hill, so, yes a slide that has slid before can be prone to reactivation,” he explained.

Local leaders confirm the hillside collapsed Saturday in the exact spot of a huge landslide in 2006.

“Yeah, this is a big one,” declared Montgomery. Snohomish County Public Works Director Steve Thompson estimated the amount of material in the slide at 15 million cubic yards, the “biggest thing” he’s seen in 30 years.