  Ordering of the geometrically frustrated two-dimensional Heisenberg
antiferromagnet on a pyrochlore slab is studied by Monte Carlo simulations. In
contrast to the kagom\'e Heisenberg antiferromagnet, the model exhibits locally
non-coplanar spin structures at low temperatures, bearing nontrivial chiral
degrees of freedom. Under certain conditions, the model exhibits a novel
Kosterlitz-Thouless-type transition at a finite temperature associated with
these chiral degrees of freedom.
