Flooding caused by Taum Sauk reservoir breach reveals area's history to geologists

LESTERVILLE, Mo. -- The raging flood from a December 2005 breach in the Taum Sauk reservoir not only ripped trees and soil from the side of Proffit Mountain, it sliced through centuries of time.

The 1.3 billion gallons of water stripped all the dirt in its path, revealing Taum Sauk rhyolite rock formed 1.4 billion years ago when volcanoes created the St. Francois Mountains.

The uncovering of the large bed of rhyolite is a silver lining in the flood that injured a ranger's family, destroyed their home and a state campground and permanently changed Johnson's Shut-Ins State Park.

The scour channel also holds rocks from at least three other geological eras, as well as a 530-million-year-old beach near the top of the mountain.

"We have 900 million years of the earth's history right here," said Cheryl Seeger, geologist for the Missouri Department of Natural Resources. "We get to see what has been going on underground at different developments."

Scientists use radiometric dating to determine the age of rocks, said Joe Gillman, DNR division director. "That is a technique that is based on naturally occurring radioactive isotopes in the rock," he said. "We know what the half-life of radioactive isotopes is. We can age-date the rock based on how much of the radioactive isotope has decayed over time.

"By the product of decay and amount of isotopes still there, we can determine how old the rock is," he said.

The discovery has drawn attention from geologists who are hoping to get a close look at the scour channel. The upper half is owned by Ameren. The DNR hopes to make its half of the channel available to the public next year.

"We plan to have a trail from Route N to an overlook about midway up the channel," said Sue Holst, spokeswoman for the DNR's Parks Department. "Plans call for it to be accessible to the public when we open the rest of the park in 2009."

Safety concerns are one reason the scour channel is off limits. Parts of the rock-strewn channel are treacherous to walk on. Some areas of rock are fragile and break even under small amounts of weight. In some places, the rock cover feels like walking on a pile of marbles.

Another reason is the channel's scientific value, especially to geologists.

"It's a major area of interest," Seeger said. "We're uncovering an ancient mountain range."

The uncovering of the scour channel rhyolite gives geologists a first-hand look at part of the geological map of the St. Francois Mountains. It lets them see what they have only read about, and gives them a chance to see if the physical evidence substantiates the theories they have read about.

Now that the rocks are exposed to the environment, weathering has caused small pieces of the more fragile types of rock to crumble. The weather will effect the rhyolite as well, but Seeger said will not interfere with the study of the rocks.

"In terms of geologic time, it will speed up the erosion," Seeger said. "But we're never going to see a difference in our lifetimes."

It is a cloudy, rainy summer morning when Seeger arrives to escort a reporter and Parks Department employees Rose Pollard, Pamela Kugel-Rolls and Hannah Memhardt on a tour of the scour channel.

The bottom of the channel slopes almost imperceptibly at first, and then rises steadily until mid channel, where the slope steepens sharply. Small rocks lay across the lower stretch, but the rocks grow larger and cover the channel as the trail begins to climb.

"The water came down the steeper slope very fast and when it hit the flatter valley floor, it immediately began digging out and scouring a big hole," Seeger said. "Then as the water moved past that point, it started slowing just enough that it started dropping a lot of the material that it had been carrying down the hillside."

Wildflowers and other plants now grow throughout the lower third of the channel. Most of the irregularly shaped rocks are baseball size or smaller and walking is easy.

Around the bend, the channel slopes upward. Here, walking is more difficult among the angular, basketball-sized rocks. Seeger stopped to pick up samples of rock, including rhyolite, dolomite, granite, sandstone and chert. Many of the rocks came from inside the reservoir walls, carried down by the water after it knocked down a 656-foot section of the dam.

Pollard talked about the way the park looked right after the flood.

"Trees were mangled and left in piles up to 15 feet high," she recalled. "Rebar, rocks and concrete from the broken dam were mixed in with the trees."

Ameren, which owns the reservoir and a utility plant at the top of Proffit Mountain, has been working with the DNR to repair the harm done to the park. Not much has been done in the scour channel.

However, the channel is healing itself. Wildflowers and tall grasses thrive in pockets throughout the boulder strewn slope. A tiny creek, some of which runs under the bedrock, flows down the mountainside, feeding tiny pools along the way. Near a cluster of cattails, a snake warms itself on the rocks then slithers off as the group hikes nearer.

"It's amazing to watch this landscape recover already," Seeger noted. "We have not planted any of these trees or wildflowers. This is all Mother Nature."

An orange plastic fence separates Ameren property from DNR's section of the scour below a deep gouge where the slope gets less steep. The landscape here is far different from the rest of the treed wilderness area. Bedrock in colors ranging from orange to gray stretches across the width of the channel and up to the top, where Ameren crews are building a new reservoir. There are hills and valleys among the rocks, which change in texture and color depending on minerals and the type of rock.

Seeger stops where a tiny stream bursts out from under a shelf of fragile layers of shale. She points to a series of tiny ridges on a section of harder rock.

"These were created by waves from a huge saltwater sea that lapped the shore," she explained. "This was a beach about 530 million years ago."

The entire St. Francois Mountain range, once higher than the Rocky Mountains, eventually was covered by sea. That was long before the glaciers melted, fish swam in the sea or dinosaurs roamed the earth, she explained.

"When the sea was here, there were mostly algae-like creatures," Seeger added. "At that time, a lot of the earth was sea and the supercontinents were pulling apart. Most of mid-continent Midwest was underwater."

Above the pink granite and dolomite lies the rhyolite outcropping. It has different shades of purple and is the largest exposed outcropping of Taum Sauk rhyolite in the area, Seeger said.

Some of the rhyolite is solid in color; some is speckled with flecks of pink. The coloration is affected by how quickly the rock cooled and what minerals are includes, Seeger explained.

Almost 1.5 billion years ago, "caldera" volcanoes spewed forth hot gases and materials from under the earth, creating mountains. Eruptions left holes underground, which eventually caused the mountains to collapse. This pattern continued for centuries.

"The best known caldera is at Yellowstone. It is 35 miles by 25 miles," Seeger said. "Right here, we're on the edge of the Taum Sauk caldera, which is about 12 miles in diameter."

Unlike Hawaiian volcanoes that spew liquid lava, calderas erupt explosively.

"Think Mt. St. Helens, only huge," Seeger related. "If you think of St. Helen's (eruption) as the size of an espresso cup, Yellowstone was a 50-gallon bag and ours was a 30-gallon bag."

As the seas climbed the mountains, they deposited sedimentary rocks. When they receded, the rhyolite was covered. The rocks lay hidden from human eyes as dinosaurs were born and died out, the continents slid around the globe and people began to populate the "New World."

Later, erosion revealed small patches of rhyolite on nearby Taum Sauk and Church mountains.