Beyond the issue of the validity of the Bath Tub Model as a general proposition is a complicating aspect, which pertains to ice margin feature elevation data uncertainty, variability, and imprecision. If we were to make a graphical frequency plot of ice margin feature elevations the result would show a considerable variation or scatter owing to uncertainty and complications associated with isostatic rebound and imprecision related to the identification and mapping of ice margin feature remnants as markers of T level and time elevations.
Statistical scatter reflects the imprecision and uncertainty about the elevations represented by ice margin features, much like water body strandline features. Imprecision and uncertainty stems from different factors. A major factor is the fact that all ice margin features, by their nature, individually span a range of elevations. For example, stagnant ice deposits, which in Vermont are by far the most common type of ice margin feature, commonly range over substantial elevations as much as one hundred vertical feet or more. In the selection of elevations for stagnant ice deposits in the VCGI mapping reported here, peak elevations of individual deposits, or in some cases predominant elevations, were utilized to establish their representative ice margin elevations. However, this entails subjective judgement, and adds imprecision and therefore variability in the data for deposits formed at multiple locations at the same levels or times. This is further complicated by the fact that post glacial erosion has removed large portions of stagnant ice deposits, which adds scatter to the elevation data base.
Another example of the same type of problem is provided by kame deltas. Such deposits are very important ice margin features, but these generally are sprawling type deltas(as opposed to Gilbert type deltas), with broad, extensive sloping planar surfaces, commonly spanning fairly substantial elevation ranges. Whereas it is preferable to utilize the elevation of topset/foreset strata contacts in deltas, such information is not available in remote mapping by VCGI. Moreover, deltaic deposits commonly were subsequently eroded with only remnants remaining, commonly across a relatively substantial range of elevations. Picking the elevation of individual kame delta deposits was again a matter of subjective judgement, which unavoidability adds to scatter of elevations for individual levels.
Adding to the imprecision is the usage of contours for elevation information, which is provided from USGS data, generally at 10 or 20 foot contour intervals. Whereas this is a relatively small factor as compared to the intrinsic variability in the elevations of ice margin features, this adds to the imprecision.
As a consequence of the scatter resulting from imprecision and uncertainty, the ice margin levels identified in this report are necessarily and intentionally established as elevation ranges. Thus, we have, for example, the T4 ice margin at the elevation range of 1200 to 1400 feet(366- 427 m). The same applies to all eight ice margin levels and times identified in the deglacial history section below.
Added to this, beyond the issue of imprecision and uncertainty as just discussed, an additional point needs to be made, which is that ice margins themselves were not simple knife edges but involved both active and stagnant ice margins, the latter as broad zones, which developed progressively over more prolonged times while the active ice margin was receding. VCGI mapping suggests deglacial ice margins were complex, broad zones marked by both active and stagnant ice margins, with overlapping spatial and temporal relationships, the nature of which varied in different drainage basins in accordance with different Ice Margin Styles in response to Glacial Dynamics, all as dictated by different physiographic settings. This observation is important because we all of us in the deglacial history analysis “business” tend to think of ice margins as sharp lines on a map. The evidence here suggests otherwise.
Glacial Dynamics and Styles add to the complexity and therefore uncertainty and imprecision of the historical analysis given here. This underscores a major point relative to the usage and validity of the Bath Tub Model, which is that the major “rings” on the Bath Tub Model were not narrow or thin marks but rather more like wide or thick blurry blotches extending around a remarkably irregular “Bath Tub” in which the “suds” formed in different ways according to local Style conditions and according to complex Glacial Dynamic. This was a very complex “Bath Tub” and historical process. However, interestingly and importantly, in a sense this complexity ultimately adds strength to the validity of the usage of the “Bath Tub” analogy as a model for deciphering deglacial history.