SURFICIAL GEOLOGIC MAP OF THE NEWPORT AND NEWPORT CENTER QUADRANGLES, NORTHEASTERN VERMONT

The Newport and Newport Center Quadrangles were recently mapped using funds secured by the Vermont Geological Survey through the USGS StateMap program. Field mapping utilized the Fulcrum Mapping App customized for collecting surficial geological observations. A downloaded LiDAR hill-shade base map (mbtiles format) allowed mapping to occur without relying on data accessibility over the cell phone network in this rural and hilly region. Almost 4,000 separate field observations provide the basis for the surficial geological map compilation.

The mapped quadrangles lie along Vermont’s northern border with Québec and mark the beginning of an effort to provide modern detailed geological mapping to the northeastern part of Vermont, an area that has seen little geological work since the 1960’s. The maps are centered on Lake Memphremagog, a large lake that extends some distance north into Québec. Several large north-flowing rivers all empty into this lake. Relatively rare glacial striations parallel grooved till and indicate the ice sheet was flowing to the south as it thinned and retreated to the north. Several mapped moraines may mark standstills or limited readvances that may be correlative with the Littleton-Bethlehem moraines of New Hampshire. Several eskers were mapped and are likely the northern extension of an extensive and long-lived subglacial drainage system.

During ice retreat glacial lakes formed in tributary valleys. These eventually merged to form Glacial Lake Memphremagog, a large glacial lake that grew to a considerable size as the ice sheet retreated into Québec. Well-preserved sequences of large-scale channels document the drainage of high-elevation lakes into adjacent lower-elevation lakes during ice retreat. Other well preserved channels mark extensive water flow along the margin of the ice sheet. Several mapped deltas allow the isostatic tilt of the region to be calculated as 1.2 m/km towards N35W. Further mapping will refine this calculation. A fining-up sequence of glaciolacustrine sediments was deposited in this lake. Ice-proximal sand and gravel deposits occur adjacent to the known eskers whereas extensive deposits silt were deposited well away from the ice sheet margin. These fine-grained sediments underlie large parts of the lake basin including many wetland areas.