Proton emission from solid foil targets irradiated by relativistically intense femtosecond laser pulses is studied experimentally. Broad plateaus in energy spectra are measured from micron-thick targets when the incident laser pulses have relatively low intensity contrasts. It is proposed that such proton spectra can be attributed to the combined processes of laser-driven collisionless shock acceleration and target normal sheath acceleration. Simple analytic estimation and two-dimensional particle-in-cell simulations are performed, which support our interpretation. The obtained plateau-shape spectrum may also serve as an effective tool to diagnose the plasma state and verify the ion acceleration mechanisms in laser-solid interactions.