Scanning tunneling microscopy has been used to study the reconstructions of Si-terminated SiC(0001) surfaces observed after heat treatment at increasing temperatures. For the root 3x root 3 reconstruction, obtained by heating at temperatures around 950 degrees C, the images can be explained by a model composed of 1/3 monolayer of Si or C adatoms in threefold symmetric sites on top of the outermost Si-C bilayer. For surfaces heated above 1050 degrees C the images show growing fractions of quasiperiodic 6 x 6 and 5 x 5 reconstructions. Heating above 1250 degrees C results in a partial graphitization of the surface which modifies the observed 6 x 6 structure.