  An effective field theory is derived that describes the low-frequency spin
dynamics in the low-temperature orthorombic phase of La_2CuO_4. Restricted to a
single CuO_2 layer the effective theory is a simple generalization of the
relativistic nonlinear sigma model to include all spin interactions allowed by
symmetry. Incorporating a weak interlayer interaction leads to two coupled
nonlinear sigma models which provide an efficient description of the complete
bilayer dynamics. Particular attention is paid to the weak-ferromagnetic and
spin-flop transitions induced by external magnetic fields. The main features of
the observed (covert) weak ferromagnetism are thus accounted for in a
straightforward manner but some of the finer theoretical predictions would
require further experimental investigation. The derived framework is also
suitable for the study of the structure and dynamics of magnetic domains in
undoped La_2CuO_4.
