Title:

Using glacial striations on bedrock to determine ice flow behavior during the last deglaciation in the New Hampshire Seacoast region

Approximately 15,000 years ago, southern New Hampshire became ice-free as the Laurentide Ice Sheet (LIS) margin retreated northward. While the timing of glacial retreat is documented in regions both north and south of the New Hampshire Seacoast, detailed knowledge of ice recession in the Seacoast is limited. Here, we reconstruct former ice flow patterns by mapping glacial striations in bedrock at sites distributed across the Seacoast region. We test the hypothesis that striation directions registered across a range of altitudes are indicative of ice thickness and activity during retreat, and we consider two main schools of thought: (1) parallel striations at all elevations would suggest that glacial erosion in the Seacoast occurred only when the ice sheet was very thick (>1 km) and its flow was unaffected by topographic irregularities, and that this was followed by ice stagnation and downwasting; (2) striation orientations that vary by elevation (with more divergence at lower elevations) would suggest that glacial erosion was still actively occurring after the glacier had thinned and was flowing around topographic highs. We collected 63 striation measurements from four mountains in the Seacoast region using a compass and a hand-held GPS. Striations were typically measured on quartz veins in bedrock spanning various elevations on each mountain. We found that striation directions from high elevations near mountain summits vary less than those from low elevations. These data reflect flow divergence at low elevations and suggest that the LIS was still actively eroding the landscape as it thinned and became topographically confined. Combined with other field evidence of glacial activity including roches moutonnées and glacial smoothing, these striation data offer insight into a controversy about how the LIS behaved as it retreated from New Hampshire, and are not consistent with earlier models of widespread ice stagnation across the region during deglaciation. Understanding how past glaciers behaved is critical to understanding how our present-day ice sheets will respond to our growing climate crisis, thus helping to inform future predictions.

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