St. Louis ? Inspectors were shocked to discover that the collapsed portion of a mountaintop reservoir was made of rocky “fill” instead of the granite that was assumed for decades to be the main material, the state’s chief reservoir inspector said Thursday.

James Alexander, director of the Missouri Department of Natural Resources’ Dam and Reservoir Safety Program, said the broken portion of retaining wall – 70 to 80 feet high and about two football fields wide – appeared to consist entirely of soil and smaller rock.

“We were shocked,” he said, to see the “rubble material.”

Alexander inspected the dam after the accident Wednesday that released a billion gallons of water down the side of Proffit Mountain.

The 50-acre upper reservoir of the Taum Sauk Lake hydroelectric plant in southeast Missouri breached shortly after 5 a.m. The torrent of water ripped through a state park, then down along the Black River, knocking cars and trucks off a rural highway.

The water tore from its foundation the home of park superintendent Jerry Toops, his wife, Lisa, and their three young children. All five survived, but the children were being treated at a St. Louis hospital for hypothermia.

A hospital spokesman said 5-year-old Tanner was in critical condition. His 3-year-old sister Tara and 7-month-old brother Tucker were upgraded to fair condition.

The breach apparently occurred after an automated system malfunctioned and pumped too much water into the reservoir. A backup system that should have caught the problem also apparently failed, said Gary Rainwater, chairman and chief executive of St. Louis-based utility AmerenUE.

Inspectors from AmerenUE and the state were assisting the Federal Energy Regulatory Commission, which oversees the plant, with the investigation.

If a large part of the retaining wall was mostly soil and smaller rock, it likely was doomed once too much water was pumped into the reservoir, said Charles Morris, an environmental engineering professor at the University of Missouri-Rolla.