In talking about “end moraines” as a map unit, one needs to be careful about nomenclature, because end moraines which are generally recognized come in various shapes, materials, and origins, with built-in historical meanings and implications. Whereas discrete, narrow, ridge-shaped end moraines composed of till or gravel that formed at the frontal or lateral ice margins of the ice sheet have been reported in Quebec, New York, and New Hampshire, such ridge-like moraines in Vermont generally are absent.
The State surficial geology map and accompanying report by Stewart and MacClintock (“S & M”) identify “moraines” which they regard as particularly important. In their report (p. 81), they state: “There are only three regions in Vermont in which the moraines are of enough significance or size to warrant discussion. These are the St. Johnsbury area in northeastern Vermont, the Rutland area in the west central region, and the Manchester-Bennington area of the southwestern corner of the State.” Whereas VCGI mapping here interprets the Rutland and Manchester-Bennington deposits as examples of massive stagnant ice deposits, and the St Johnsbury features as “Scabby Terrain” having a very different, but quite significant origin related to “Disconnections,” as discussed below, these three locations have merit as major ice margin markers. The Rutland and Bennington features in my opinion are substantial sprawling stagnant ice deposits which formed at the ice margin, and in a sense are thickened welts on the terrain, but they are not narrow ridge-like features as end moraines are commonly and classically (though not always) regarded. Regardless of the name, these mark the positions of significant ice margins.
Stagnant ice features, including kames, kame fields, kettles, kame terraces, eskers, and kame deltas, are numerous and widespread, and commonly occur together in various combinations and ways. In general, in mapping such features on VCGI, this interpretation was made primarily using LiDAR, based on characteristic hummocky topographic expression. 1It is interesting to note that in some places field examination of suspected stagnant ice deposits shows strong hummocky topography and yet LiDAR imagery, despites its wondrous ability to see through vegetation, fails to detect and display such features. This speaks to both the strength and limitations of LiDAR imagery, and underscores a complexity of LiDAR signal detection. In addition to LiDAR, VCGI’s tabs for surficial geology, aggregate, and on- site sewage disposal were also helpful for determining the physical soil type make up of these features.
As stated above, Stewart and MacClintock, 1969, pp 44-45, 2Stewart, D.P. and MacClintock, P., 1969, The Pleistocene Geology of Vermont, Vermont Geological Survey, Bulletin No, 31, 239 pages. suggest that the mountainous and hilly terrain of Vermont tended to favor ice stagnation as the ice sheet thinned rather than classic end moraine ridge formation composed of till, such as Stewart experienced in the midwestern States. My mapping in the 1960s and 1970s, and as well VCGI mapping here, supports this view. Such deposits tend to be sprawling masses, not narrow ridges(although collectively, as just stated, they commonly represent topographic welts of thicker surficial material). And they range in scale from scattered small and local deposits to much more substantial deposits, in places representing massive valley fills. Stagnant ice deposits are marked and identified on VCGI maps by gray shading, with the term “stagnant ice,” or as related specific deposits such as kames, eskers, kame deltas, kettles, etc. 3The term “kettle” or “kettle holes” is used very broadly. Some depressions marked as “kettle holes” on VCGI maps are strictly speaking not kettle holes, and instead are ponds in depressions in till ground moraine, as for example in the Memphremagog Basin These are not true kettle holes but nevertheless were occupied by ice blocks during deglaciation in areas along or near stagnant ice margins. In a further examination of this issue below it is suggested that the absence of end moraines may have to do with the reverse gradient environment and the direction of ice flow relative to the grain of the topography.
In general, the ice sheet was adventitious. Stagnant ice deposits formed wherever and whenever the terrain resulted in ice thicknesses which locally were insufficient to sustain flow, resulting in localized stagnation, and where a necessary sediment supply was available. This observation helps to explain why ice margin features are found in some places but are curiously absent in places nearby where such features would be expected.
In the interpretation of such deposits using the Bath Tub Model a distinction was made between ordinary, scattered, local stagnant ice deposits along hillsides versus much more substantial and massive valley fills, recognizing the latter as more significant ice margin markers which likely formed perpendicular to flow lines at lobe frontal margins or tips. Obviously, this distinction is a highly subjective judgment matter, but the more substantial valley fills suggest more significant ice margin positions, which in fact is corroborated by the correlation of diverse types of ice margin features in different locations in multiple diverse basins with different physiographic settings, all at similar isostatically adjusted elevation levels.