A torrent of water from an overflowing lake sliced open the earth in 2002, exposing rock formations, fossils and even dinosaur footprints in just three days. Since then, the canyon has been accessible only to researchers to protect it from vandals, but on Saturday it opens to its first public tour.

"It exposed these rocks so quickly and it dug so deeply, there wasn't a blade of grass or a layer of algae," said Bill Ward, a retired geology professor from the University of New Orleans who started cataloging the gorge almost immediately after the flood.

The mile-and-a-half-long gorge, up to 80 feet deep, was dug out from what had been a nondescript valley covered in mesquite and oak trees. It sits behind a spillway built as a safety valve for Canyon Lake, a popular recreation spot in the Texas Hill Country between San Antonio and Austin.

The reservoir was built in the 1960s to prevent flash flooding along the Guadalupe River and to assure the water supply for central Texas. The spillway had never been overrun until July 4, 2002, when 70,000 cubic feet of water gushed downhill toward the Guadalupe River for three days, scraping off vegetation and topsoil and leaving only limestone walls.

"Underneath us, it looks solid, but obviously it's not," said Tommie Streeter Rhoad of the Guadalupe Blanco River Authority, as she looked out over a cream-colored limestone crevasse.

The sudden exposure of such canyons is rare but not unprecedented. Flooding in Iowa in 1993 opened a limestone gorge behind a spillway at Corvalville Lake north of Iowa City, but that chasm, Devonian Fossil Gorge, is narrower and shallower than Canyon Lake Gorge.

Neither compares to the world's most famous canyon. It took water around 5 million to 6 million years to carve the Grand Canyon, which plunges 6,000 feet at its deepest point and stretches 15 miles at its widest.

The more modest Canyon Lake Gorge still displays a fault line and rock formations carved by water that seeped down and bubbled up for millions of years before the flooding.