Two doors from the Sun Crest Lounge, man and machine have excavated a gaping canyon, nearly a square city block filled with backhoes, dump trucks, power shovels and cement mixers.

Attempting to lay the foundation for a 22-story office tower, work crews have unearthed many 19th century artifacts, including a bottle dated 1869, spurs, horseshoes, even a pistol clogged with dirt.

"Everything you'd expect to find in a 19th century dump," said Charles Ball, the project director for Trammell Crow Co.

But on a recent day, they found prehistoric history: a brittle 3-foot-long ivory mastodon tusk, leading on-site archeologists to two more tusks, some teeth, a hip bone and other fragile fragments of the extinct elephant-like animal.

"It's a once-in-a-10,000-year deal," said security guard Cedrick Burdett, who found himself warding off curious citizens eager to see the bones 18 feet below street level.

Alton Briggs, chief archeologist on the site, said the bones may be up to 100,000 years old. They will be analyzed at the University of Texas and returned to Trammell Crow.

The backhoe operator who unearthed the first 3-foot length Dec. 30 might have caused its subsequent splintering, Briggs said, but few such ancient finds are intact in the earth.

Ball said Trammell Crow makes a point of hiring an on-site archeologist when potentially historic excavation takes place. Thousands of artifacts, ranging from tools to a Civil War cabin, have been uncovered at five construction sites around Austin.