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The evolution of Si(100) surfaces has been studied during oblique high-fluence ion sputtering by means of atomic force microscopy. The observed surface morphology is dominated by nanoscale ripples and kinetic roughening at small and large lateral scales, respectively. The large-scale morphology exhibits anisotropic scaling at high fluences with different roughness exponents alfan = 0.76 ± 0.04 and alfap = 0.41 ± 0.04 in the directions normal and parallel to the incident ion beam, respectively. Comparison to the predictions of single field and two-field ("hydrodynamic") models of ion erosion suggests the relevance of nonlinearities that are not considered in the simpler anisotropic Kuramoto-Sivashinsky equation.