CONNECTING RIVER TERRACES TO REGIONAL GLACIAL LAKE IROQUOIS PHASES, COLTON, NY

During the retreat of the Laurentide Ice Sheet following the last glacial maximum the St. Lawrence lowlands, north of the Adirondack Mountains, were occupied by a series of pro-glacial lakes. The main stage of Lake Iroquois occupied the St. Lawrence lowlands approximately 11.7 ka. A series of terraces are preserved in Stone Valley, along the Raquette River, near Colton, NY. These terraces, which exist at the transition from the Adirondack highlands in the south to the St. Lawrence lowlands in the north, indicate rapid lowering during the transition from the Main Lake Iroquois and Frontenac phases to the Upper Fort Ann (Belleville) phase. Interpretation of strandline data by Rayburn (2003) set approximate elevation for Main Lake Iroquois phase at 280 m, Frontenac phase at 250 m, Upper Fort Ann (Belleville) phase at 180 m, and Lower Fort Ann (Trenton) phase at 150-160 m in the Stone Valley vicinity. We constructed a slope map of Stone Valley, using 10 m DEM rasters, to minimize the effects of post-formational erosion while identifying individual terraces. The terraces were extracted from the slope map based on continuous areas with low slope values. A longitudinal profile was constructed using upstream and downstream elevations of each terrace that was extracted using the slope map. Several terraces were preserved between the Main Lake Iroquois stage and the Frontenac stage, representing 30 m of lowering. The regional lake level then dropped about 80 m between the Frontenac stage and the Upper Fort Ann stage, preserving multiple terrace sequences. This high rate of preservation likely represents rapid local lake level lowering.