DEFORMED TREES IN GLACIAL SUBSTRATE, BERING GLACIER, ALASKA

The Bering Glacier foreland, central coastal Alaska, contains till sheets bound by decimeter thick outwash of stratified sand and gravel. Rooted at the base of multiple, sub-meter sand sheets are dozens of fossil trees (alder and spruce) ranging in diameter up to 19 cm, and typically with fewer than 25 growth rings. The trees remain in the living position, some extending several meters upward into overlying pebble gravel. Trunks demonstrate forms of deformation that suggest a common directional strain parallel to that of overriding ice flow.

A typically encountered style of deformation is the centimeter to decimeter offset of tree trunks along horizontal shears confined to thin, strata-bound, clay-rich zones at the base of sand sheets. Additionally, tree trunks are warped and kinked coincident with shear zones. Primary stratification within the lower portion of some sand sheets display deformed laminar bedding and cross-bedding. Fine to medium sand is chaotically deformed with greatest intensity immediately above shears and with diminished severity upward.

Such forms of deformation may be attributed to directional stress capable of penetrating several meters of substrate. In this setting, the most logical source of deforming stress is overriding ice.