Late Wisconsin ice-flow reversal and deglaciation, northwestern Maine

Reconnaissance surficial mapping in the upper St. John River area of northwestern Maine has shown that the latest Wisconsin events in the region were ice-flow reversal and southeastward ice marginal retreat. Widespread erratics derived from the Canadian Shield substantiate ice movement from the west or northwest into Maine prior to these events. Striations, rattails, and small-scale stoss-and-lee forms, all indicating northward and subsequently northwestward ice flow, are superimposed over earlier evidence showing eastward ice movement. Recessional ice-margin positions, delineated by lakes, drainage routes, and moraine segments, demonstrate that the ice margin retreated to the southeast following the emergence of the Notre Dame Mountains as nunataks near the present International Border. Marginal drainage was generally northeastward—perpendicular to topographic slope—via short-lived lakes and spillways. One major stillstand of the ice margin interrupted overall recession in northernmost Maine; however, no conclusive evidence suggests a north or northwestward readvance during this phase. Evidence for north and northwestward ice flow requires a late-glacial dome located over north-central Maine. One explanation for such dome formation is that a late-glacial ice stream developed in the St. Lawrence Valley and drained most of the ice from central Maine before marine calving isolated an independent ice mass. Rapid areal deglaciation ensued with only rare stillstands interrupting extensive disintegration.