The purpose of this paper is to analyze and interpret the onset of some well-characterized transient inertio-gravity waves, observed during the PYREX (Pyr¿n¿es Experiment) campaign. The data used are the high-resolution soundings, launched every 6 hours on both sides of the ridge. In agreement with other observations made during the field campaign, certain among those soundings often show steady mountain waves. It appears that those waves are often transients and are followed by well-characterized inertio-gravity waves, propagating upward and downward from the level where the initial mountain waves are large. To interpret those observations, it is suggested that the observed inertio gravity waves result from the geostrophic adjustment of the large-scale flow that follows the transit and eventually the breaking of the main mountain wave. This hypothesis is supported by an analytical calculation of inertio-gravity waves emission from a momentum deposit that lasts a finite time. The spatial extent of this forcing are also finite, and its maximum amplitude is a fraction of the measured mountain drag. ¿ 1998 American Geophysical Union