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C. W. M. Swithinbank: We have interpreted similar steps in radio-echo bottom profiles of ice shelves as representing the confluence of streams of ice from discrete sources. Are your steps oriented along a flow line and would you interpret them in the same way ?
C. R. Bentley: I find that interpretation difficult in this case. The steps appear to lie within the margins of the outflow from ice stream B, although they might possibly occur at the boundary between that ice and the “turbulent” zone down-stream from Crary Ice Rise. Flow lines are not yet well defined in this region; the two steps do lie on a flow line in my interpretation, but in that case they represent opposite displacements: east side up in one case, and west side up in the other. In Neal’s flow-line interpretation (Neal, 1979) they do not lie on the same flow line, in which case they represent a double step toward thinner ice from grid west to grid east.
J. G. Paren: Your radio-echo depths have been calculated using a constant deep-ice velocity. You have suggested earlier that there may be a progressive change of velocity with position on the Ross Ice Shelf. Is there any correlation of velocity with the slight bumps you have found in an otherwise flat ice shelf? Is the velocity variation large enough to affect the map ?
Bentley: The maximum deviation of our measured velocities from that used in the mapping was about 3%, corresponding to a thickness variation of 10 to 20 m over the ice shelf. That implies that some of the shallowest features could marginally be explained that way, but none defined by more than one contour line.