We present particle and magnetic field observations of the magnetic field line merging process in the earth's magnetotail. By analyzing the lunar shadow pattern of electron fluxes detected by the lunar-orbiting Apollo 16 subsatellite we are able both to identify the topology and to measure the velocity of the magnetotail field lines. The observations reported here were made as the moon crossed the separatrix between premerging and merged field lines. The field line velocities toward the merging region were measured to be 30--60 km/s, and the thickness of the separatrix was estimated to be ~2¿103 km. Most of the magnetic energy released in the merging process appears to go into the energization of particles. The length and the thickness of the merging region are inferred to be of the order of ~10RE and ~4¿103 km, respectively. The energized particles travel away from the merging region along the separatrix. Those headed earthward may form the plasma sheet by being trapped on closed field lines. The rate of energization and the energy spectrum of those particles are consistent with those required for formation of the plasma sheet.