  Giant planets embedded in circumstellar discs are expected to open gaps in
these discs. We examine the vertical structure of the gap edges. We find that
the planet excites spiral arms with significant (Mach number of a half)
vertical motion of the gas, and discuss the implications of these motions. In
particular, the spiral arms will induce strong vertical stirring of the dust,
making the edge appeared `puffed up' relative to the bulk of the disc.
Infra-red observations (sensitive to dust) would be dominated by the light from
the thick inner edge of the disc. Sub-millimetre observations (sensitive to gas
velocities) would appear to be hot in `turbulent' motions (actually the ordered
motion caused by the passage of the spiral arms), but cold in chemistry.
Resolved sub-millimetre maps of circumstellar discs might even be able to
detect the spiral arms directly.
