  We report measurements of the interaction-induced quantum Hall effect in a
spin-polarized AlAs two-dimensional electron system where the electrons occupy
two in-plane conduction band valleys. Via the application of in-plane strain,
we tune the energies of these valleys and measure the energy gap of the quantum
Hall state at filling factor $\nu$ = 1. The gap has a finite value even at zero
strain and, with strain, rises much faster than expected from a single-particle
picture, suggesting that the lowest energy charged excitations at $\nu=1$ are
"valley Skyrmions".
