B.  Tabular Summary of Ice Margin T times and Levels

The following table summarizes the elevation levels and times used for mapping of ice margins:

These T times and levels were determined by application of the “Bath Tub Model.” The mapping of ice margin features on VCGI has repeatedly shown in many Locales that in general the lowering and recession of the ice margin was progressive and incremental, as repeatedly stated for emphasis in this report, “like multiple rings on a slowly draining “Bath Tub”.” As noted above, this simple statement has profound significance. Recession was in fact progressive and incremental, but whereas the deglaciation, as just stated may be likened to a slowly draining “Bath Tub”, certain “rings” are more prominent and substantial, suggesting perhaps pauses or stillstands in the lowering of the ice sheet. These are at the above T levels and times. However, it is likely that the ice margin recession was mostly steady, progressive and continuous, such that T times, levels, and margins are merely time markers, having  no particular significance otherwise.

With regard to the concern that the application of the Bath Tub Model might not be appropriate or accurate for the early times of deglacial history when the ice sheet was relatively thick, it needs to be said that the Scabby Terrain tracts associated with T1 and T2  times represent the closest approximation to moraines found in this VCGI study. Whereas Scabby Terrain is mapped in some places as isolated patches, in southern Vermont, Scabby Terrain can be traced as tracts which are extensive for many miles across the state. These tracts show a relationship with stagnant ice features at slightly lower elevations, making it clear that they are ice margin features for the en masse stagnation in the Connecticut Basin. Scabby Terrain tracts show a relationship to regional physiography, less so for local topographic variations. However, Scabby Terrain tracts, when examined closely in detail on VCGI imagery show a remarkable relationship with cols on the physiographic divides indicative of ice flow from parent ice in the Champlain Basin and Memphremagog Basin for the ice masses in the Connecticut Basin (less so for the divides between the Winooski Basin and the Middle Connecticut Basin which requires further study). Also, stagnant ice deposits extend across divides between the Memphremagog and Upper Connecticut Basin which clearly establish ice supply pathways.

These observations indicate that by the time of the deglacial recession for T1 time the ice thicknesses were not so great, relative to the physiographic terrain differences, to invalidate the applicability of the “Bath Tub Model.” This observation applies to early T1 and T2 times which both entailed Disconnections leading to en mass stagnation of ice masses in the Connecticut Basin. The T3 ice margin in southern Vermont is marked in the Vermont Valley where the physiographic relief is very substantial, again conforming to the Model for southern Vermont. The T3 margin is traced northward using the Bath Tub Model along correlative  elevations in the Lobate phase (the highest and oldest features in this phase) to central Vermont, north of which the T3 margin is in the Nunatak Phase.

As noted previously, ice margins in the Nunatak Phase were mapped but not correlated or traced. However, Ice Marginal Channels which occur in both the Nunatak and Lobate Phase might be amenable to further, future study to delineate times, margins, and levels in the Nunatak Phase. The adherence of ice margins to physiography becomes increasingly apparent through time, with the T8 ice margin showing remarkable conformance to even minute physiographic differences..

The above chronology does not reflect stratigraphic information, for example specifically related to readvances or oscillations of the ice margin, in as much as this type of information is not provided by VCGI mapping. However, evidence from other sources is used to include reference to oscillations, as identified by others. And, some limited field reconnaissance was done for specific features, as identified in the text.

T levels, times, and margins are given as elevation ranges, as shown above, reflecting fundamentally the fact that ice margin features by nature are formed over a range of elevations, plus the uncertainty and imprecision of the elevations assigned to ice margin features as discussed above. The development of T levels, as shown on the tabulation above, was done as the mapping progressed, with the recognition that ice margin features tend to be grouped at certain elevation ranges, reflecting vertical scatter and ice margin range, again as discussed above.

T levels, times, and margins were identified progressively. As more was learned about the ice margin features, their correlation, and relationships, these elevations and the associated ice margins were modified. Suffice it to say, this was an evolving mapping-, learning-, and history-making process, resulting in repeated modification of ice margin levels and times, the delineation of ice margins, and the interpretation of their meaning in terms of deglacial history, Styles, and Glacial Dynamics. It is entirely possible, and indeed seems likely that further study will result in modifications of this chronologic framework. As stated above, deglacial ice margin history was spatially and temporally complex such that simple two dimensional graphics can never be perfect. As noted previously, the usage of T levels and times was followed more as a general guide.

In the final analysis, the establishment of T levels and the assignment of ice margin features to specific levels is as much art as science, having to do with what makes geological sense. In fact, this is the joyous discovery part of the mapping and learning process. In the course of mapping, certain ice margin features were identified which initially were puzzling. The nature, origin, and historical meaning of many features, such as Scabby Terrain, Ice Tongue Grooves, and Ribbed Lacustrine deposits, was unfamiliar territory, leading to multiple working hypotheses, again often in the context of trying to understand what makes sense. The identification and mapping of T levels and times was not just a process of identifying ice margin features, determining their elevations, and then correlating these across the State. Rather it was, as just stated, a thought process as to what makes geological sense. In the end, the coming together of what at first appeared as many disparate pieces of information into a story in which all the pieces fit together and make good geological sense is the litmus test for the validity of the deglacial history story coming out of this process.

Footnotes:

• 1
  Isostatically adjusted ice margin elevations using the Canadian border as the reference, with local elevations of features to the south of the border decreased by an amount determined by an isostatic slope of 5 feet per mile for the distance measured along a north-south transect to the Canadian border.