A 320-acre fire crackling through brush and trees has forced authorities to shut down a stretch of the main route linking Northern California with South Lake Tahoe. U-S 50 is closed in both directions between Pollock Pines and Meyers. That same area was hit by the 25,000-acre Cleveland fire in October of 1992. The CHP responded to the blaze in the El Dorado National Forest around four yesterday afternoon. The California forestry department says more than 600 firefighters now are on the scene and the fire is 70% contained. The fire has caused trees and rocks to crash down hills and into the highway. No structures are threatened. The cause of the blaze is unknown.