A rock approximately the size of a Volkswagen Beetle came down from a low place in Glenwood Canyon, about two miles west of Hanging Lakes Tunnel at mile marker 122.9.
Crews are breaking up the rock and hauling it away. The rock caused only minor roadway damage (1-foot wide pothole) and did not impact vehicles.
The outside right westbound lane - closest to hillside - is currently closed and both westbound lanes will be closed only intermittently later as crews haul the rock pieces away and patch the hole.
Colorado Department of Transportation spokesman Mike Murray says no one was injured when a rock fell on the highway on Monday.
Murray says the westbound lanes may be closed temporarily for rock removal.