There isn't much evidence of last Friday's rush-hour rockslide anymore. The three tons of boulder and mud have been pushed to the side of the George Washington Memorial Parkway, and all lanes are expected to be open tomorrow just north of the Key Bridge in Arlington.

But even as remnants of the huge hunk of rock that fell onto the scenic highway are broken into smaller pieces and dumped into a shallow gully there, federal authorities are investigating the cause of the incident and trying to make sure that it doesn't happen again.

"There's no formula for what causes a mudslide," said Harold Rohde, an engineer for the Federal Highway Administration. "But it's pretty incredible that in the middle of the time when people are going home, on a Friday, there was no major damage or injuries to anyone."

Experts said that weather and age played roles in the slide. The long process of rock and dirt repeatedly freezing and melting over time caused much of the rock formation to contract and expand. Then, officials said, "gravity took over" as the layers of rock loosened and plunged to the highway. While they were unsure exactly why the slide happened last week, officials all but ruled out the recent damp weather.

"This kind of thing really develops over a number of years," said Dottie Marshall, deputy superintendent for the parkway.

"It's almost like trying to stack wooden blocks on top of one another when one is slanted," said Rohde, who has been at the site all week. "Eventually, it's going to collapse" under its own weight, he added.

Parkway officials said they do not see the potential for similar slides elsewhere along the 38-mile roadway. They said they couldn't remember a rockslide of such magnitude on the parkway, although there have been similar incidents along Skyline Drive in the Shenandoah National Park and in North Carolina. No one was hurt in those slides, officials said.

Work crews and geologists from the National Park Service, which manages the road, and the Federal Highway Administration continued to shore up the site on the northbound side of the parkway yesterday. On Jan. 8, federal authorities plan to bring in an outside geologist to examine the site and ensure that pieces of red clay and gray rock on the 40-foot cliff cannot dislodge.

The consultant will also look at several other spots on the cliff face to make sure that there aren't any other weak spots.

Tomorrow, authorities plan to open the far left lane, closest to the cliff's edge. It has been closed since last Friday as work crews repaired a gutter along the parkway. Jersey barriers were erected to prevent sediment from running onto the highway.

The slide left about three tons of debris -- from fist-size fragments to a large boulder close to the size of a small pickup truck -- on the left and middle lanes of the parkway south of the Spout Run Parkway exit just after 6 p.m. Friday.

The parkway usually carries tens of thousands of cars each day, officials said. Emergency crews cleared one lane of debris immediately, but for a week, the far left lane has been blocked as crews removed loose rock from the formation and tested the remaining rock for sturdiness.