The Skagit Valley Herald reported that no homes were threatened and cool, damp weather was 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 said the two-acre fire was 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 dropped water on the hillside.
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