  We calculate the evolution of the geometric entanglement entropy following a
local quench in the D1D5 conformal field theory, a two-dimensional theory that
describes a particular bound state of D1 and D5 branes. The quench corresponds
to a localized insertion of the exactly marginal operator that deforms the
field theory off of the orbifold (free) point in its moduli space. This
deformation ultimately leads to thermalization of the system. We find an exact
analytic expression for the entanglement entropy of any spatial interval as a
function of time after the quench and analyze its properties. This process is
holographically dual to one stage in the formation of a stringy black hole.
