Subglacial deformation of trees within overridden foreland strata, Bering Glacier, Alaska P. Jay Fleisher a,e, * , Matthew S. Lachniet b,e , Ernest H. Muller c,e , Palmer K. Bailey d,e a Earth Sciences Department, SUNY-Oneonta, Oneonta, NY, 13820-4015, USA b Department of Geoscience University of Nevada, Las Vegas, NV 89154-4010, USA c Earth Sciences Department, Syracuse University, Syracuse, NY, 13210-2936, USA d CRREL (retired), Anchor Point, AK, 99556-9702, USA e Prince William Sound Science Center, Cordova, AK, 99574, USA Received 20 September 2003; received in revised form 26 January 2005; accepted 26 January 2005 Available online 20 October 2005 Abstract The foreland stratigraphy overridden during recent Bering Glacier surges bears evidence of subglacial deformation. The pre- existing, fine textured substrate (till and diamicton) experienced diminished strength because of saturation, thus resulting in shallow mobilization and the formation of a new till of limited thickness. Glacial coupling with well drained sediment resulted in ploughing that generated a diamicton that retains vestiges of outwash sorting and stratification. The outwash sequence extending decimeters beneath the surface till contains four prominent sub-meter sand beds. Each sand bed holds multiple small, fossil trees still rooted in underlying layers of gravel. Virtually all trees in the upper two sand beds are deformed. Several are offset by centimeter to decimeter horizontal shears confined to thin, silt, and clay-rich zones at the base of each sand bed. Trees that escaped shearing are warped and kinked. Deformed trees are present at depths that range from 15.76 to 5.31 m. beneath potential ice/substrate interface surfaces. The most likely source of deforming stress in this foreland setting is related to glacial advance. The style and orientation of tree deformation are consistent with the direction of ice movement. Therefore, the occurrence of deformed trees is attributed to stress applied by overriding ice. D 2005 Elsevier B.V. All rights reserved. Keywords: Substrate deformation; Overriding glacial ice 1. Introduction Bering Glacier, Alaska is known to have surged re- peatedly in historic time (Post, 1972; Muller and Fle- isher, 1995; Fleisher et al., 1995; Molnia and Post, 1995). Each of the past two surges (1965–67 and 1993–95) were punctuated by jokulhlaups along the eastern sector ice front. Observations during the 1993–95 surge, coupled with stratigraphy exposed because of flood erosion of bluffs indicate that pre-existing sediment was deformed when overridden by ice. Although recognition of sub- strate deformation is not new (Alley, 1989; Alley, 1993; Benn, 1995; Boulton, 1979; Boulton and Hindmarsh, 1987; Clarke, 1987; Clarke et al., 1984, Hart et al., 1990), seldom can the cause and effect be so clearly and directly linked. 0169-555X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2005.01.013 * Corresponding author. Earth Sciences Department, SUNY- Oneonta, Oneonta, NY, 13820-4015, USA. Tel.: +1 607 436 3375; fax: +1 607 436 3547. E-mail address: fleishpj@oneonta.edu (P.J. Fleisher). Geomorphology 75 (2006) 201 – 211 www.elsevier.com/locate/geomorph