“Streaks” are LiDAR curvilinears associated with the late T6 and early T7 time interpreted as representing destabilization of the Late T6 and T7 ice margin triggered by a) the lowering of Lake Vermont from Coveville to Fort Ann levels, associated with b) the progressive transformation of the Champlain Lobe from a Cold-based to fully Warm thermal condition, and c) the increasing presence of meltwater both along ice margins and beneath the ice. This destabilization triggered calving in the re-entrant basins of the Middlebury Bench, as marked by Headless Deltas, Ribbed Lacustrine Deposits, and Thickened Bouldery Lacustrine deposits in Middlebury Bench re-entrant basins. This calving then continued in T7 time with the ice margin receding in a downgradient direction in each of these re-entrant basins. Destabilization was also associated with the transformation of the eastern lobe margin from a lateral toward a frontal margin.
The finding of Streaks and their association with late T6 and T7 ice margins and elevations is both surprising and remarkable, providing an important part of the story of the recession and demise of the Champlain lobe. Many Streaks have been identified in the Champlain Basin. Whereas these features are quite subtle and this study has only scratched the surface (pun intended), with many more such features likely yet to be identified, nevertheless the recurrent pattern of their association with the late T6 and T7 elevation (adjusted for rebound) is remarkable. Streaks mark the eastern lobe margin and as well the margins of nunataks within the lobe, with the Champlain lobe at these times standing as a long convex ice mass. Streaks occur in clusters across a range of elevations at late T6 and T7 times, levels, and margins, in close association with the Coveville and Fort Ann strandlines, but are more pronounced at the latter. Streaks are interpreted as representing ice margin destabilization associated with the sudden and substantial change in water levels as a Glacial Dynamic.
Again, Streaks are associated with late T6 and early T7 time when Lake Vermont lowered from the Coveville to the Fort Ann level. The Fort Ann level is marked by the blue line on the above map and is taken from Chapman’s depiction and my own previous findings (Wagner, 1972). Whereas Chapman believed that the Fort Ann strandline was limited and excluded from the northern Champlain Basin by the ice sheet prior to the opening of the basin for the Champlain Sea, my mapping identifies Fort Ann features further northward, extending to the Quebec border. And as is well known, research in Quebec has likewise identified the Fort Ann strandline, referred to as Lake Candona. The Fort Ann strandline in New York is taken from Franzi et al, 1 In a presentation entitled “Bridging the gap: glacial geomorphic mapping in the Adirondack Upland, New York.” referred to as Lake Akawasasne. As discussed below, Franzi’s et al likewise present information suggesting a close relationship between the Akawasasne strandline and the ice margin. Again, it is not known whether or not Streaks likewise occur in New York and Quebec at this level and time.
It is emphasized that just as Franzi has recognized that the Fort Ann strandline was diachronic, so too was the corresponding ice margin associated with this strandline. Thus, recession of the ice margin occurred in late T6 and T7 time and Fort Ann time and continuing in T7 time such that the ages of individual features vary with time, showing the progressive recession of the ice margin. All the Streaks identified in this Addendum study are at elevations which correspond with the late T6 and early T7 margin and Fort Ann strandline, again marking the lowering of Lake Vermont from the Coveville to the Fort Ann level. Fort Ann features as depicted on Chapman and Wagner profiles extend over a broad range of elevations. This likely reflects erosional lowering of the Fort Ann outlet and as well imprecision of elevations. Similarly, the occurrence of Streaks over a range of elevations may reflect this lowering but as well the progressive nature of the ice margin adjustment.
The following is a discussion of selected Streak examples, beginning in the northern portion of the basin and progressing southward. Interestingly, the nature of Streaks appears to vary in the Basin, with more southerly Streaks along the eastern lobe margin along the foothills showing evidence of more substantial meltwater, in contrast to Streaks further north and along nunataks which do not show such evidence. In fact, in the southernmost portion of the Champlain basin, again along the foothills, the late T6/early T7 margin is marked mostly by stagnant ice deposits and kame deltas as part of the step-down sequence, without the presence of Streaks. This observation is taken as part of the evidence of progressive recession and the southward accumulation of meltwater as part of the Glacial Dynamic.
a. Streak Examples in Missisquoi Basin
As discussed in my pre-Addendum report, including various Locales referred to in Appendix B, at a late glacial time the Missisquoi Basin was occupied by a discrete Champlain Basin sub-lobe, with multiple sub-sub-lobes associated with the micro-physiography. The Missisquoi sub-lobe was nourished by ice flow from the Laurentide ice sheet via multiple pathways extending into Vermont, again as depicted and discussed in my pre-Addendum report. The following map, taken from the Appendix B discussion of this Basin, shows the late T6 and early T7 ice margin and ice sheet cover in the Missisquoi Basin.
This map is intentionally schematic and imprecise. But it gives a sense of the complexity of the ice sheet pattern and ice supply flowlines. The blue-shaded pattern of ice margins was drawn using the Bath Tub Model to correlate ice margin features (mostly stagnant ice deposits, including kame deltas) with isostatically adjusted elevation as a guide. By this late time the ice sheet in this basin was relatively thin and showed a remarkable sensitivity to physiography. In general, the ice sheet extended up the Missisquoi Basin, including multiple feeders, with a main lobe tip near Richford, and as well smaller sub-lobes extending into headwater areas of tributary basins.
Streaks along the late T6 and T7 margin have been identified at four locations in the Missisquoi Basin, per the following map:
These locations were selected as more obvious features on LiDAR, but it is likely that further study will identify additional evidence of this ice margin
VCGI mapping for the Hardwood Hill location is described in Appendix B at Locale CV1. The map below, which identifies ice margin positions in T4 – T7 time, is reproduced from this Locale:
As can be seen, this portion of the Missisquoi Basin was occupied by step-down receding ice lobe margins in T4-T7 times. The Streaks at Hardwood Hill were recognized in my pre-Addendum report, but identified as “Steps,” marking the T6-T7 levels and times. The map below on the left is an unmarked close-up of the Lidar imagery of the northwest flank of Hardwood Hill, illustrating the LiDAR appearance of these features, and on the right is a markup tracing the T6-T7 linears:
The following is an unmarked larger close-up of these features:
This image clearly shows these Streaks as multiple distinct linear benches (or “Steps”) in the terrain. The LiDAR imagery gives no directional evidence for ice movement, whether parallel or perpendicular to the linears or otherwise, and no suggestion of the presence of running water. As can be seen, perpendicular elements are present, which are not unlike the LiDAR markings associated with Ribbed Lacustrine deposits. Of course some of these may represent post-glacial erosion. Based on the above VCGI map, these features lie along Hardwood Hill in conjunction with a T6 and T7 ice lobe margin which occupied the Missisquoi Basin floor.
The following is an enlarged screen shot taken from the above VCGI map:
Based on the evidence, again as discussed for Locale CV1, it is likely that the Missisquoi Basin was occupied by an ice lobe marked by a step-down pattern of recession in T4 – T7 times. Suggested flowlines are added to the above map. Obviously, the exact and specific pattern of flowlines is indeterminable based on this evidence, but the suggestion here is for a divergent, splaying pattern on the northwest flank of Hardwood Hill, with southeastward ice movement into the small drainage basins on the northeast and southwest sides of Hardwood Hill, accounting for stagnant ice deposits in the headwaters of these small basins (as discussed in detail for this Locale in Appendix B). The red arrow marks a Bedrock Groove formed by ponded meltwater drainage across a saddle in the terrain behind Hardwood Hill, leading to a deltaic deposit which at the time of this mapping was considered to represent either Coveville level and time or local ponding, but subsequently the evidence has indicated that Coveville Lake Vermont was restricted to the southern portion of the Champlain Basin, generally south of and at the furthest north possibly including the Winooski Basin.
It is believed that this ice movement pattern relates to formation of Streaks, with the ice sheet locally pressing against the northwest flank of Hardwood Hill in a compressive Glacial Dynamic condition. As discussed further below, such conditions are reported in the literature to be associated with the progressive imbricate thrusting of till as recession progressed, by which ground moraine terraces resulted, thus explaining these Streaks. This is a recurrent pattern found in many places, as discussed for other examples of Streaks. Streaks commonly are found at specific locations, which correspond with “pivot points” marking the impingement of the ice against upland terrain in locations of converging or diverging ice flow, or curvature of flowlines. Again, it is believed that this Dynamic was part of the destabilization triggered by the lowering of water levels in the main Champlain Basin from Coveville to Fort Ann levels, and other Dynamics.
Another Streak example in the Missisquoi Basin is southeast of the village of Enosburg Falls, per the following map, which is taken from Locale CV5 in Appendix B. This map also shows a Streak location near Lake Carmi, as discussed subsequently below:
Again, the Streaks southeast of Enosburg Falls are associated with the T6 (violet)and T7 (turquoise) times. The LiDAR image below shows the area of the Streaks:
And the following is an enlarged image of this area which gives a better sense of the linears:
As can be seen, these linears, although feint, are definitely and clearly observable. Field examination in this area in the 1970s indicated a till mantled hillside, with no observable suggestion of the presence of these linears. The Streaks are interpreted as having formed sequentially by a shear type Glacial Dynamic motion along the ice margin in late T6 and T7 time as the margin receded.
The Lake Carmi Streaks are located on a hillside east of Lake Carmi, again in close proximity to the T6 and T7 margins as shown on the above map, between the two brackets on the LiDAR image below. The Streaks appear as very feint and irregular NE-SW linears. The specific nature of the associated Glacial Dynamic for these Streaks is uncertain.
Streaks near the mouth of the Missisquoi Basin are depicted on the VCGI map below, taken from Appendix B Locale CV6.
The Streaks are located on the western flank of a small, un-named hill near Fairfield Pond. The receding T6 time margin is marked by the violet-colored line, T7 time by the turquoise green line, and T8 time by the yellow line.
As discussed for this Locale, the T6 and T7 ice margins show a remarkable sensitivity to the terrain, which is taken as indicating that ice sheet was relatively thin in this area at these times. A small T7 lobe of ice in the Fairfield Pond vicinity developed by flow from the main Champlain lobe across low cols in a foothill ridge, as marked by a significant, substantial stagnant ice deposit, indicated by the gray colored area on this map.
The following is a LiDAR image for this area:
The screen shot on the left is an un-marked LiDAR image on which curvilinear Streaks are evident, and on the right is a LiDAR image with the 540 foot contour highlighted, marking the linear and showing the pattern associated with the terrain. The enlarged LiDAR image below shows the Streaks in more detail:
As can be seen the Streaks are associated with benched terrain on the flank of a nob, not unlike the preceding case with Hardwood Hill. And again, the formation of the Streaks is interpreted as likely representing a compressional Glacial Dynamic, with a splayed pattern at a pivot point in T6-T7 times.
As an incidental note, the above VCGI map shows additional multiple linears in the northwestern portion of the map area which were identified on LiDAR in the pre-Addendum study. As discussed elsewhere, these may mark an ice margin, perhaps T9 (?), but their origin remains uncertain and they may instead represent some other explanation such as Mega-Scale Lineations. Detailed field study of these linears is needed.
b,. Streak Examples along foothills between Lamoille and Winooski Basins
Generally speaking, the foothills physiographic terrain on the western flank of the Green Mountains, east of the Champlain Basin floor, is penetrated by westward draining tributary basins. These small tributary basins were occupied by tributary sub-lobes of the Champlain Basin main lobe. Recession of ice in these sub-lobe basins is marked by step-down features beginning in T3-and T4 time continuing into T6 times, including local stagnant ice deposits and kame deltas, with evidence of a complex multiplicity of small local, upland proglacial lakes, with drainage of these local water bodies commonly marked by Bedrock Grooves. These features indicate that the Champlain lobe ice at these times generally was Cold ice, capable of impounding meltwater, with the stagnant ice margin features marking Warm ice locally along the outer fringe. In essence, these were Hybrid margins with a very complex pattern of step-down recession, leading to progressively larger, coalescing proglacial water bodies. As noted previously, it is believed that the eastern margin of the Champlain lobe in T6 and T7 time was associated with the progressive development of a narrow more or less open-water corridor, coalescing at the Fort Ann level, the head of which progressed northward along the Champlain lobe eastern margin in T7 time, whereby the eastern lobe margin transformed from a lateral toward a frontal margin.
The region between the Lamoille and Winooski Basins is physiographically different in that the foothills in this sector are comprised predominantly of north-south bedrock controlled linear terrain elements, with multiple, parallel, pronounced linear ridges and depressions, and not by typical westward-draining tributary basins. These linears include water bodies like Indian Brook Reservoir in Essex, Colchester Pond in Colchester, and Milton Pond in Milton, which historically served as man-made impoundments for development of regional water supplies beginning in the early 1900s and continuing through the 1950s and 1960s. During my pre-Addendum mapping I regarded these linears as reflecting bedrock fabric incised by meltwater erosion along lateral Champlain lobe ice marginal channels. However, field mapping in this area in search of fluvial deposits in the Addendum field study generally was unsuccessful, which previously was attributed to burial of such deposits under more recent peat and muck associated with the many beaver ponds in these linear depressions. Field mapping associated with the Addendum, however, identified exposures and erratics of the same peculiar till which has been identified in the Addendum field study of the LaPlatte Basin. This till was interpreted as deposited along a lateral shear margin, whereby gravel formed in basal tunnels beneath the ice margin became incorporated into till as part of the Dynamic associated with lateral shear margins. This hypothesis requires further study, but if correct would suggest that the recession of the step-down margin in the Lamoille-Winooski foothill part of the Basin was marked by lateral shear margins, fitting with the Streak evidence for both the Champlain lobe deglacial history and as well with the evidence for the importance of subglacial meltwater in the Glacial Dynamic.
The late T6 and early T7 ice margins in this part of the basin are marked by numerous Streaks as discussed below, per the following map:
This map was taken from VCGI. Owing to limitations of VCGI, the details depicted on this map are difficult to read, but are clarified in the following. Also, the many linear ravines likewise are not evident at this scale, but Indian Brook, Colchester Pond, and Milton Pond which are in low linear ravines, are identified. Also shown are the T6 (violet) and T7 (green) ice margins. Streaks are identified in Areas A, B, C, and D. Also shown is a Scarp (red line) , which while not part of this discussion, fits with the deglacial history presented in this Addendum. Two eskers are also identified in the Essex area. These mark the T7 margin and are graded southward to deltaic deposits associated with the Winooski Basin Fort Ann delta.
The following presents LiDAR imagery associated with Streaks A-D, as marked by the dashed black lines:
Streaks A are on the north flank of Georgia Mountain, along the foothills margin, as shown by the following enlarged LiDAR image:
The north flank apparently served as a pivot point for the Lamoille Basin ice lobe. The Streaks are marked by terraced steps, again interpreted as produced by imbricate thrusting of the ice and associated till ground moraine along the margin in a locally compressive setting. Small drainage gullies show incision of postglacial drainage across step risers.
Streaks B are shown on a VCGI topographic map below left and a LiDAR image below right:
The Streaks occur on a sloping hillside over a range of elevations, but no terracing is evident. The nature of the Glacial Dynamic is unclear.
Streaks C:
Streaks D:
As can be seen, these Streaks are associated with terraced terrain, possibly suggesting a compressive Glacial Dynamic, such as expected with a frontal margin.
c. Streak Examples at the Mouth of Winooski Basin
The following is a map presented previously, in my pre-Addendum report for Locale W1 at the mouth of the Winooski Basin:
As explained elsewhere, the deglacial history in this Locale is relatively complex and is not repeated here. Ice margins are:
- T4 – heavy blue line
- T5- heavy orange line
- T6 – heavy violet line
- T7 – green dashed line
The heavy yellow dashed line represented the T8 margin in my pre-Addendum report, which is now modified by this Addendum study to indicate that the T8 readvance was restricted to the Missisquoi Basin.
In my pre-Addendum study, I identified curvilinear features along the north side of Oak Hill which were then regarded as possibly marking the Coveville and Fort Ann strandlines, representing the entry of these water bodies into the Winooski Basin. The Addendum LiDAR study has instead identified these linears in this area as Streaks, and the associated follow-up field work showed that there is no evidence of strandlines along these linears. Instead, the Streaks are in till ground moraine with no soil or topographic evidence of strandlines.
The following is an Addendum map for this area showing the Streaks and a Scarp. The T6 and T7 margins are shown by the violet and green colored lines, with the blue line added to trace the lower and later T7 ice margin. As depicted, the Streaks again are on the north flank of Oak Hill and as well on the northwest flank of another hill to the north. The Scarp to the west lies close to and just above the elevation of a deltaic Champlain Sea deposit, and is on the same pivot point upland where further south in Shelburne is a set of curvilinear grooves in the terrain associated with a nearby Scarp, as discussed below. These lower elevation curvilinears are thought to be associated with the Onset Zone for ice streaming of the Champlain lobe.
The following is a LiDAR image for the area north of Oak Hill, showing the Streaks at the late T6 and early T7 margin which are relatively obvious.
The associated Glacial Dynamics associated with these particular Streaks is uncertain but is believed to be related to the lowering of the ice sheet and associated ice margin recession in late T6 and T7 times, and with the lowering from Coveville to Fort Ann levels, representing destabilization of the Champlain lobe.
d. Streaks at Snake and Buck Mountains
Snake and Buck Mountains are prominent upland nunatak nobs near the village of Vergennes in west-central Vermont and the central portion of the Champlain Basin. The following map is taken from my VCGI mapping, showing the Snake Mountain area:
Certain, specific physiographic “provinces,” representing major physiographic divisions of the Champlain Basin as defined previously above, are shown, specifically the “Foothills,” “Middlebury Bench,” and “Trough.” Snake Mountain and Buck Mountain nearby to the north, both stand as nobs along the western margin of the Middlebury Bench, and eastern margin of the Trough. Otter Creek, which represents one of the major drainage basins within the Champlain Basin, and as well a major re-entrant basin in the Middlebury Bench, passes between these upland nobs near Vergennes, continuing southward into the Vermont Valley near Rutland, beyond this map area.
Geologically, the receding Champlain lobe margins are marked, including:
- T6 margin (violet)
- early T7 margin (green)
- late T7 margin (blue)
The orange-colored line marks the Champlain Sea marine limit. The Lake Fort Ann strandline per se is not marked but is closely related to the early T7 margin. Likewise, the Coveville strandline is not marked but is closely related to the T6 margin. The village of Bristol is marked, which is a location of a major delta at the Coveville level, just west of which are lower deltaic deposits at the Fort Ann level, along with Ribbed Lacustrine deposits, Headless Deltaic deposits, and Thickened Bouldery Lacustrine deposits, all marking the beginning of calving of the “ice stream” in the New Haven River and Little Otter Creek re-entrant basins.
As stated previously, destabilization of the Champlain lobe was caused by and resulted from the lowering of Coveville Lake Vermont to the Fort Ann level. This destabilization is marked by Streaks at the late T6 and early T7 margins, levels, and times. Destabilization led to localized calving in the multiple re-entrant basins in the Middlebury Bench, with progressive downgradient recession in T7 time by calving. The calving ice masses in re-entrants were long, narrow tongue-like massed confined to these narrow re-entrant basins, with westward progressing calving ice margins in T7 time, which are thought of as like ice streams, but whether this involved true ice streaming is uncertain.
By later T7 time, the Champlain lobe eastern margin stood along the Trough margin, freeing the Champlain lobe from the Middlebury Bench as buttressing support. As discussed in the next section below, owing to this reduced buttressing support, and as well as other Glacial Dynamics, a second phase of instability was triggered, marked by Scarps which are likewise discussed in detail below.
On the west and north flanks of Snake Mountain are LiDAR linears identified as Streaks on the above map. As is obvious, Snake Mountain served as a nunatak in the ice sheet, with the Streaks on its north and west flanks formed along the late T6 and early T7 margin. As discussed previously in the pre-Addendum report, the eastern flank of Snake Mountain is marked by numerous linears which are too numerous to be shown on the above map at this scale. These linears are part of the previously discussed “Ice Tongue Grooves” which are interpreted as marking destabilization of the ice margin associated with the lowering of Lake Vermont from the Coveville to Fort Ann level.
Also shown on the above map, but discussed elsewhere in this Addendum, are Mega-Scale Lineations as marked by dotted black lines on the above map, just south of Vergennes, and as well Transverse Morainic Ridges, marked by nobs on this Ridge near Addison (as marked by turquois colored, more or less circular pattern lines delineating the tops of nobs on these Ridges).
In addition, as can be seen other linears which are identifiable on LiDAR, are also shown on the above map, as for example on the Trough floor in the Dead Creek vicinity. In general, LiDAR imagery is very good at depicting linears. However, whereas these particular linears likely are Champlain lobe markings, their origin is uncertain and not yet understood.
The following is an enlarged LiDAR image of the Snake Mountain area showing the Streaks in more detail:
Like the Streaks discussed previously, these Streaks are feint curvilinear marks identifiable on LiDAR imagery. The Streaks on the west flank of Snake Mountain are in till ground moraine with no visible evidence of their presence detectible by subsequent field work. The Streaks on north flank of Snake Mountain are in an area where follow-up field work suggests these Streaks may be associated with granular soils, perhaps associated with a stagnant ice margin and meltwater. The Streak location on this northern flank of Snake Mountain is a distinct tomographic bench.
These linears were identified in the pre-Addendum study, but at that time were not understood.
The features at Buck Mountain were not recognized in the pre-Addendum study. The following are VCGI maps for the Buck Mountain area. The first map below is a larger scale VCGI map of this area:
These linears were identified in the pre-Addendum study, but at that time were not understood.
The features at Buck Mountain were not recognized in the pre-Addendum study. The following are VCGI maps for the Buck Mountain area. The first map below is a larger scale VCGI map of this area:
As can be seen, once again, Streaks mark the late T6 and T7 margin, specifically at a pivot point associated with Buck Mountain as a nunatak.
e. Streaks on southern Champlain Basin Nunataks
The following VCGI map shows the southern Champlain Basin:
This area, which encompasses the southwestern portion of the Middlebury Bench, shows ice margins and times:
- The violet-colored line marks the late T6 ice margin along the foothills and is depicted here schematically and only approximately.
- The dark green line marks early T7 time.
- The dark blue line marks later T7 time when the Champlain lobe had receded to the Trough, still in Fort Ann time.
- The orange line represents the Champlain Sea strandline at the marine limit. Again, this relates to Scarps which are discussed in a subsequent section below.
The focus here is on Streaks at early T7 time. As can be seen Streaks are associated with nunatak “islands” in the Champlain lobe. Again, these nobs are part of the Middlebury Bench. The Champlain lobe tip at this time was bifurcated, with one branch extending southeastward into the Vermont Valley in the Otter Creek Basin. The other branch occupied the main floor of the Basin. This location is in the latter area.
The detailed appearance of individual streaks for Streaks M, N, O, P, and Q is shown on several maps below:
As stated previously, further closer study likely will identify more Streaks, the examples given here being only the “tip of the iceberg,” so to speak, or perhaps better to say “only scratching the surface.” These features again suggest their formation by compressive and shear type Glacial Dynamics along the Champlain margin in early T7 time at the time of the transition from Coveville to Fort Ann Lake Vermont. Thus, again, the Streaks represent destabilization of the Champlain lobe when the ice occupied the Middlebury Bench, triggered by the lowering of Lake Vermont from the Coveville to Fort Ann levels.