It broke out just before 5 p.m. at 10 acres and had grown to 20 to 30 acres by 6 p.m., according to Contra Costa Fire.
No injuries or threatened structures have been reported but crews say the fire is difficult to access because of its location.
The cause of the fire is still under investigation.
Stay with ABC7 Eyewitness News for the latest details on this developing story.
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