  The nonlinear stage of development of the spatiotemporal instability of the
monochromatic Townes beam in a medium with self-focusing nonlinearity and
normal dispersion is studied by analytical and numerical means. Small
perturbations to the self-guided light beam are found to grow into two giant,
splitting Y pulses featuring shock fronts on opposite sides. Each shocking
pulse amplifies a co-propagating X wave, or dispersion- and diffraction-free
linear wave mode of the medium, with super-broad spectrum.
