The anomolously high surface temperature at the JET inner divertor target plates can be described by a layer of impurities having poor thermal contact with the substrate. During experiments with the direction of B¥B reversed, a similar thermally resistant layer was observed to grow on the outer target plates. Given that the outer divertor plasma density and temperature do not depend on the field direction, it is concluded that there was a net source of impurities flowing to the outer target. Gradual disappearance of the layer upon returning to forward field confirms that the outer target is a net erosion zone under these conditions. The implied impurity flow in the plasma boundary is consistent with the measured SOL flow, provided there is an impurity source at the low field side of the tokamak.