Analysis of the free air gravity field between Quebec City and Lake St. John indicates that there is an uncompensated load, apparently resulting from dense topography, directly over the region of observed vertical movement of the crust. A finite element calculation for a viscoelastic model, in which Logan's Line is included as a zone of weakness, yields a profile of surface velocities which approximates those obtained by geodetic relevelling. It is proposed that the vertical movement represents the decay of an enhanced proglacial forebulge which was formed in a region partially decoupled from its surroundings by major faults.

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