“Drainage Lines” are important “connective tissue” along, within, and between active, stagnant, destabilized, and calving ice margins marking the path of sub-aerial, free-flowing, meltwaters. Drainage Lines generally are marked by red colored arrows on VCGI maps based on evidence of surface water drainage along the sides of ice margins, with or without elevation information which, where and when used, tends to be consistent with expectable directions and gradients. Drainage Lines often but not always are associated with more granular soil as evident on SCS (sewage soil), Highway Department aggregate, or surficial geology tabs on VCGI. They in some places are associated with Bedrock Grooves, and Ice Tongue Grooves, and often lead to or are associated with kame terraces and kame deltas. Many Drainage Lines are local features, but in some cases extend for many miles, extending across drainage divides in some but not all cases marked by proglacial lake spillways. Drainage Lines are exceedingly important to the deglacial history as links between drainage basins, which greatly strengthens the case for the validity of the “Bath Tub Model.”
In general, “Drainage Lines” are numerous features mapped on the VCGI project sheet, as linear, curvilinear, or irregular but nevertheless narrow and elongate ice marginal features identified and mapped on VCGI topographic maps and on LiDAR imagery. They are difficult to characterize as their expression on VCGI maps is very variable and, in some cases, quite subtle, but in all cases their linear and fluvial-like character is suggestive of and consistent with surface water drainage along ice margins or across divides beteen drainsge basins They commonly are identifiable by long, linear tonal differences, patterns, or streaks on LiDAR imagery. In places they form in and are associated with distinct topographic channels, as on the floors of Bedrock Grooves and Ice Tongue Grooves, or simply as topographic channels in stagnant ice deposits, or in multiple, semi-parallel clusters. In places, they are associated with tracts of sand and gravel deposits, commonly in conjunction with kame terraces and leading into kame deltas, but in many places are in areas designated as till ground moraine on the State surficial geologic map. Drainage Lines in some cases are documented by patterns on the Aggregate or Soil Sewage favorability tabs on VCGI, even where and when not documented on the State surficial geology map tab.
Drainage Lines are identified in different locales and topographic/geologic settings, as for example:
- At or near the base of sloping hillsides, commonly but not always in areas of till, along identified or suspected ice margins, and close to but above stagnant ice deposits, commonly helping to identify discrete lobes.
- In stagnant ice deposits and kame deltas, in some cases suggestive of recessional lobate ice margin patterns.
- On interfluves at upland fronts between neighboring basins, where the ice margin served to block drainage, diverting flow laterally from one drainage basin to its neighbor.
- Across cols at opposing drainage basin divides related to drainage between these drainage basins, in some cases as distinctive channels in stagnant ice deposits in such divides, indicative of flow across present day divides in directions consistent with but in some cases in opposition to present day drainage.
As noted, some Drainage Lines are major features, providing important links between ice margins in adjoining basins, helping to establish ice margin correlations directly by themselves or in conjunction with associated standing water body features such as proglacial lake deposits, deltas, or other features. For example, the Drainage Line described above near East Hardwick is about 60 miles long, linking the Memphremagog Basin with Lake Winooski in the Lamoille and Winooski Basins, which in turn extends a link southward about 40 miles. Again as noted above, in this way, Drainage Lines aid correlation and greatly strengthen the case for a “Bath Tub Model.”
Because Drainage Lines are so numerous but diverse in detais, no examples are given here. However, these can readily be seen as marked on the VCGI Project Sheet in many places as shown in illustrations provided in this report.