  We present a structural analysis of the graphene-4HSiC(0001) interface using
surface x-ray reflectivity. We find that the interface is composed of an
extended reconstruction of two SiC bilayers. The interface directly below the
first graphene sheet is an extended layer that is more than twice the thickness
of a bulk SiC bilayer (~1.7A compared to 0.63A). The distance from this
interface layer to the first graphene sheet is much smaller than the graphite
interlayer spacing but larger than the same distance measured for graphene
grown on the (000-1) surface, as predicted previously by ab intio calculations.
