SURFICIAL GEOLOGY OF THE BERLIN AND SHELBURNE QUADRANGLES, NORTHEASTERN WHITE MOUNTAINS, NEW HAMPSHIRE AND MAINE

Detailed surficial geologic mapping of the Berlin, New Hampshire 7.5-minute quadrangle was carried out by Thompson (2020-21), and the contiguous Shelburne quad to the east was mapped by Thompson & Keeley (2022-23). Both maps were funded by the New Hampshire Geological Survey’s STATEMAP cooperative with the USGS. The study area is mountainous with much exposed bedrock at high elevations and extensive till cover on hillsides. During recession of the Laurentide Ice Sheet, glaciofluvial sand and gravel formed in subglacial, ice marginal, and proglacial environments along the Androscoggin River valley. Sandy glaciolacustrine deltaic and lake-bottom sediments were deposited in small ice-dammed lakes along the proximal flank of the Mahoosuc Range. Holocene sediments include many alluvial fans deposited where mountain streams enter the Androscoggin Valley. Some of the fans have been active in historic times with resulting flood damage. Analysis of lidar imagery was important to map compilation, revealing terrain details such as moraines, glacially smoothed slopes indicating thick lodgement till, glacial meltwater channels, landslide scarps, and channel patterns on floodplains and alluvial fans.

Glacial retreat from the SE part of the Shelburne quad was interrupted by deposition of the large cross-valley Androscoggin Moraine Complex. Our mapping helped to answer a longstanding question of whether these moraines are coeval with the Older Dryas (~ 14 ka) White Mountain Moraine System (WMMS) that trends NE from Littleton to the Berlin Moraines near the NW corner of the Berlin quad. This correlation would have required an active ice tongue projecting far down the Androscoggin Valley from Berlin city to the Maine border. We found no moraines or other evidence to support this scenario. The 14 ka ice margin probably crossed the valley at Berlin and dammed the lakes NW of the Mahoosucs. At the same time, drainage from ice-dammed Lake Crescent stages (coeval with the oldest Berlin Moraines) eroded channels down the W side of the Androscoggin Valley. This drainage combined with meltwater coming from the ice margin to the north and from the Mahoosuc lakes spilling westward, resulting in a colossal water discharge that scoured much of the Berlin city area down to bedrock. Thus, the Androscoggin Moraines are inferred to predate the WMMS.