Two unusually clear mesospheric gravity wave events were observed by the Boston University all-sky imager and the Argentine airglow spectrometer on two consecutive nights at the El Leoncito Observatory, Argentina (31.8¿S, 69.3¿W), during August 2001. Both events exhibited brightness amplitudes an order of magnitude above typical values. The first event had the appearance of a large-amplitude (>60% in OH(6-2), 22% in O2, and 37% in O(1S) emissions (peak to peak), compared to 1--5% typically), upward propagating gravity wave with pronounced nonlinear behavior. The waves also showed noticeably curved wavefronts, indicating that they had originated from a relatively small source region within ~180 km of El Leoncito. Estimates of the vertical flux of horizontal energy and momentum for the wave event in the OH and O2 emissions were very large compared to typical values, and they also indicated a high degree of flux divergence over the nominal 8 km altitude between the two layers. The second event occurred on the following night and exhibited a frontal morphology and behavior consistent with an internal ducted gravity wave showing nonlinear behavior akin to a mesospheric bore. The disturbance also had several interesting characteristics; in particular, the propagation speed decreased during its passage across the sky. In addition, a strong vertical temperature gradient due to the semidiurnal tide appeared to influence the occurrence of the bore event.