Stanley Boulevard, a major east-west thoroughfare between Pleasanton and Livermore, had one eastbound lane reopened to vehicular traffic early Monday evening after both lanes were closed for more than a day as a result of serious trail erosion adjacent to that side of the roadway.

The eastbound direction was first shut down on Sunday near Shadow Cliffs Regional Recreation Area, and those lanes remained closed through the morning and afternoon commutes on Monday until one lane reopened that evening. The westbound lanes were not affected.

That portion of Stanley Boulevard is in Alameda County's jurisdiction. The Alameda County Public Works Agency reported its crews were reopening one lane between Valley Avenue and Lilienthal Road, while the city of Pleasanton would be concurrently closing 300 feet of roadway, making two turn lanes unusable.

A county engineering team was out at the scene assessing the situation, but the cause of the erosion had not yet been determined, county officials said late Monday afternoon.

According to photographs from the scene provided by city and county officials, several square yards of paved trail and adjacent natural landscape between Shadow Cliffs Lake and Stanley Boulevard buckled and fell down the embankment, leaving a large sinkhole. A significant crack also resulted across eastbound Stanley Boulevard.