  Ordering of the geometrically frustrated two-dimensional Heisenberg
antiferromagnet on a pyrochlore slab is studied by Monte Carlo simulations. The
model is expected to serve as a reference system of SrCrGaO compound studied
extensively. In sharp contrast to the kagom\'e Heisenberg antiferromagnet, the
model exhibits locally non-coplanar spin structures at low temperatures,
bearing nontrivial chiral degrees of freedom. We find that under certain
conditions the model exhibits a novel Kosterlitz-Thouless-type transition at a
finite temperature associated with these chiral degrees of freedom.
Implications to experiments are discussed.
