  The initial data of gravity for a cylindrical matter distribution confined to
a brane are studied in the framework of the single-brane Randall-Sundrum
scenario. In this scenario, the 5-dimensional nature of gravity appears in the
short-range gravitational interaction. We find that a sufficiently thin
configuration of matter leads to the formation of a marginal surface, even if
the configuration is infinitely long. This implies that the hoop conjecture
proposed by Thorne does not hold on the brane: Even if a mass $M$ does not
become compacted into a region whose circumference ${\cal C}$ in every
direction satisfies ${\cal C}> 4\pi GM$, black holes with horizons can form in
the Randall-Sundrum scenario.
