U.S. 40 is one of two main detour routes around a large rockslide onto Interstate 70 in Glenwood Canyon.

The Colorado State Patrol said the woman was the only person injured in the incident that happened just after 7 a.m. on the highway between Steamboat Springs and Hayden in northwest Colorado.

The coroner identified her as Karen Lynn Evanoff, 55, of Craig, a passenger in the 2004 Buick. The unidentified driver was unhurt.

The vehicle was traveling on U.S. 40 near Milner, a CDOT spokeswoman said.

"This is a very, very rare location for a rockslide," CDOT spokeswoman Mindy Crane said.

Crane said she hesitated to even call the incident a rockslide.

"It was a rock. One rock," Crane said.

Crane said a check of CDOT records back to 1998 did not indicate any other rock incidents in that area.

A CDOT maintenance crew was on scene, Crane said, and a geologist was headed to the area from Denver to survey the situation.

U.S. 40 remained open following the incident.

(Copyright 2010 by The Associated Press. All Rights Reserved.)