LATERAL RECESSIONAL MORAINES IN THE GREEN MOUNTAINS OF NORTHERN VERMONT

Among the myriad of detailed landforms visible on newly-available LiDAR imagery are flights of asymmetric ridges along the steep hill slopes of the Green Mountains in northern Vermont. These ridges are composed of till and are best developed in areas where the till cover is aerially extensive and sufficiently thick to cover bedrock. Individual ridges frequently occur as short segments cut by stream channels, but can sometimes be traced for over 3 km. Generally they contour across hill slopes and can be traced around the headwalls of many of the tributary valleys. Some ridges merge with one another and others appear to cross-cut upslope ridges. None have the lobate form of solifluction lobes. Spacing between adjacent ridges is typically between 50 and 150 meters. In profile these ridges sometimes have positive relief, but more commonly form a step-like topography where the top surface of the step is level or slopes gently before pitching steeply downhill. The elevation drop between adjacent ridges is between 9 and 23 meters.

The most reasonable interpretation of these landforms is that they are push moraines produced along the margin of the retreating Laurentide Ice Sheet. Specifically, the process responsible for this step-like topography is the winter advance/thickening of the ice sheet margin pushing/bulldozing till exposed during the previous summer’s retreat. Net retreat/thinning during the following summer sets up the ice sheet margin to form another push moraine during the following winter downslope of the previously formed moraine. While this interpretation implies that these moraines are annual, it’s possible that no discernible moraines form if the ice margin is at a standstill during a warm winter or that moraines produced during different years cannot be distinguished if the ice sheet margin advances/thickens to the same position during two or more consecutive winters. If these are annual recessional moraines, the ice sheet was thinning between 9 and 23 m/a., i.e. the 1,000 m relief of the mountain range was deglaciated within 43 to 111 years. This is consistent with the rapid ice sheet thinning inferred from identical exposure age dates of 13.9 +/- 0.6 ka across a 800 m elevation range on Mount Mansfield, the highest peak in the northern Green Mountains.