In general, as discussed elsewhere herein, a major limitation of remote mapping using VCGI as done for this report is that the method lacks the ability to discern vertical subsurface stratigraphic, structural, and textural differences. However, reports in the literature make reference to both oscillations and readvances. As discussed by Calkin, the distinction between a “readvance” and an “oscillation” is important but difficult to make. In general, the term “readvance” to me implies climatic significance, and should only be used when and where the evidence so indicates.
The T3/T4 and T8 ice margins are here regarded as indicating readvances. The evidence for a readvance of the T3/T4 margins comes from the correlation with the White Mountain Moraine System ice margins of Thompson et al. In the Missisquoi Basin, evidence of a readvance, which is here correlated with T8 ice margin and time was previously reported by Wagner(1972) and Cannon(1964). The scales of these readvances, meaning the distance that the ice margin readvanced is unknown. In the case of the T8 margin it appears that the recession following the T7 ice margin in Fort Ann time was sufficient to allow for the invasion of the Champlain Sea into the Missisquoi Basin prior to the readvance in T8 time, and that the readvance may have resulted in an elevated proglacial water body level. The elevation, nature, and extent of this raised water body are unknown.
The evidence reported by Wright et al in the Charlotte area and by Connally in the Bridport area, here correlated with T8 time, in my opinion, is consistent with either ice margin oscillations related to a calving ice margin, or a readvance, or both. In this report, the only indication that this evidence may represent a readvance comes from the correlation of the Wright and Connally features with the T8 margin in the Missisquoi Basin, and the suggestion that this was a readvance only comes from the evidence in the Missisquoi Basin. This correlation is quite significant because it implies a long convex shape to the Champlain lobe in T8 time, similar to the lobe convexity implied by the T7 margin evidence. As discussed elsewhere, the evidence supporting the correlations of the T7 and T8 margins, so as to indicate a long convex lobe in late deglacial time is substantial and is part of the recessional calving margin story in the re-entrant basins within the Middlebury Bench.
Apart from the issue of the readvance of the ice margin, the question of whether or not the interpretation given here of a long, convex lobe in the Champlain Basin at a late glacial time is correct is quite important. As discussed elsewhere herein, in contrast, Chapman in his classic Champlain Basin proglacial water body strandline report, and Parent and Ochietti in their reports on the deglacial history of southern Quebec, both suggest a flattened margin. Franzi fundamentally favors a flattened lobe. Franzi and I discussed this issue at length. In my case, whereas my earlier interpretations had flattened T7 and T8 margins, in the end I felt compelled to suggest a long convex lobe based on multiple lines of evidence, which fit together in a robust and compelling way. My sense is that Chapman and Parent and Ochietti both inferred the positions of flattened Champlain lobe as opposed to actual evidence of the ice margin itself, which is part of what is referred to here as a “Paradigm Trap.” However, this matter clearly is one of the major unresolved issues needing further study. For me personally, whereas resolving this matter is of interest related to our understanding of deglacial history is important, the implication for present day global warming concerns and the possibility that the convex lobe represents a “collapse” is more significant.