  A generic black hole merger occurs through a restructuring of creases (sharp
edges) on the event horizon. This process is studied for a black hole merger in
the limit of infinite mass ratio, for which constructing the event horizon
reduces to finding a null hypersurface that asymptotes to a Rindler horizon in
the Kerr spacetime. Geometrical properties of the creases on this horizon are
determined and the results are compared with the predictions of an exact local
description of the event horizon in a generic merger. The crease set is shown
to have finite area. A recently proposed expression for the gravitational
entropy of a crease is shown to diverge at the instant of merger. Caustics on
(and off) the event horizon are determined by exploiting the correspondence
with the problem of gravitational lensing by a Kerr black hole. Caustics form
an "astroid tube" in spacetime, two edges of which lie on the event horizon of
the merger. Perturbative expressions for the location of this tube are
presented, extending previous work to much higher perturbative order.
