  We investigate classical formation of a trap surface in $D$-dimensional
Einstein gravity in the process of a head-on collision of two high-energy
particles, which are treated as Aichelburg-Sexl shock waves. From the condition
of the trap surface volume local maximality we deduce an explicit form of the
inner trap surface. Imposing the continuity condition on the fronts we obtain a
time-dependent solution for the trap surface. We discuss trap surface
appearance and evolution.
