ON THE DE GEER MORAINES OF NEW YORK STATE

Detailed mapping of De Geer moraines is now possible with LiDAR. This allows for potential new insights into the configuration and pace of Laurentide Ice Sheet recession during the last deglaciation. De Geer moraines are small linear moraine ridges parallel (transverse) with an ice margin. Several processes have been proposed to form such small-scale transverse moraines such as basal crevasse fill, grounding line deposition during a standstill, or grounding line deposition during winter re-advances. De Geer moraine swarms we identified lie within previously described proglacial lakes and the majority of the swarms consist of repeated and regularly spaced parallel, long ridges (up to km in length), which we interpret as relating to annual (winter) re-advances superimposed on overall net ice-margin recession. We measured the spacing between De Geer moraines to estimate the annual rate of recession in various locations across the state ranging between the Finger Lakes region and the Saint Lawrence River Valley, areas that deglaciated between 16 - 13 ka. In the Finger Lakes, we notice a significant increase of recession rate (194 m/yr) north of the Waterloo Moraine versus south (86 m/yr), which is thought to date to ~15 ka. In the northern Adirondack Mountains, De Geer moraine swarms are common in the uplands above the extent of glacial Lake Iroquois and allow us to reconstruct and locate the formation of numerous unnamed short-lived ice-dammed lakes. These ice-dammed lakes would have extended across the uplands of the Saint Lawrence River Valley and the Northern Adirondacks as the Laurentide Ice Sheet was retreating during the Bølling-Allerød (14.7-12.9 ka). Meltwater originating from the ice sheet generally flowed eastward towards the Black River valley entering various lakes on its journey before draining into glacial Lake Iroquois. Sequences of such lakes constantly re-configured as the ice front retreated northward, down-elevation into the St. Lawrence valley illustrate the ice-margin configuration in the Adirondacks. The mean annual recession rates of the Laurentide Ice sheet obtained from De Geer moraine swarms give insight to the rate of deglaciation of the Laurentide Ice Sheet and the location of past ice-dammed lakes in New York State.