Observations by GEOS 1, supported by data collected by IMP 7, IMP 8, and ATS 6 during the July 29, 1977, sudden storm commencement (SSC) have been used to derive the magnetopause's position (7.8RE before and 6.1 RE after the SSC), velocity (inward speed of about 95 km/s by two independent methods) and thickness (~500 km measured by the change in drift pattern). Particle, field and wave data from GEOS 1 have been used to establish the satellite's position with respect to the magnetopause. After the initial crossing of the magnetopause, the satellite remained in the magnetosheath for most of the interval considered, occasionally encountering what are interpreted as 'open magnetic field lines.' When the spacecraft was in the magnetosheath, duskward plasma flows tangential to the magnetopause are derived from our electric field measurements. The flow velocity was higher during periods when the interplanetary magnetic field assumed a southward component as compared to periods when a northward component was present. On three occasions, an environment was encountered which was neither typical magnetospheric or magnetosheath-like. It is argued that in the instances GEOS 1 either reentered the magnetosphere or encountered a 'magnetic island,' caused by a tearing mode instability at the magnetopause. The electric field dropped to zero or very small values in these instances.