A wild fire burning east of Mount Vernon has closed part of Highway 9.

The Washington State Department of Transportation and a witness said the Big Rock Fire was burning along State Route 538 near State Route 9, about four miles east of Interstate 5.

The Skagit Valley Herald reports no homes have been threatened and cool, damp weather is assisting firefighters.

The Big Rock fire is named for a 500-foot-high local landmark, which is popular spot for hiking and mountain bikers, although it is not technically open to the public.

The Washington Department of Natural Resources reports the fire is burning in trees on rocky, steep terrain.

Fire officials have not determined the cause but suspect it might have started with a campfire.

To facilitate firefighting, Highway 9 was closed at times on Thursday. A helicopter is dropping water on the hillside.

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