| Summary: | During the late Wisconsin glaciation (circa 26,000-13,000 carbon-14 yrBP) the Cordilleran glacier complex formed vast ice fields and largeglaciers along the crest of the Coast Mountains. As these glaciersflowed west to the Pacific Ocean, they were joined by local glaciersoriginating on the higher reaches of the Alexander Archipelago (Mannand Hamiltion, 1995). This extensive volume of ice was channeled intodeep troughs (present-day fiords) that formed major outlet glaciers,such as the glaciers that occupied Chatham Strait and Dixon Entrance.In several places along the coast, deep glacially scoured submarinetroughs indicate that glaciers reached to the edge of the continentalshelf. For instance, the glacier that extended into the Dixon Entrancetrough is known to have extended to the edge of the continental shelf.Its retreat began sometime after 16,000-15,000 carbon-14 yr BP (Barrieand Conway, 1999). The exact extent of late Wisconsin cordilleran ice in southeasternAlaska is poorly known. Small-scale maps and reports of the regioncommonly show or imply ice extending west to the edge of thecontinental shelf (Capps, 1931; Coulter and others, 1965; Flint, 1971;Pewe, 1975; Denton and Hughes, 1981; and Prest, 1984). These mapsrelied heavily on earlier work, much of it of a reconnaissance nature. The map shows our interpretation of the limit of the Cordilleran icesheet, which is more restricted than previous estimates, and possiblerefugium (an area that escaped the extensive glaciation of the lateWisconsin and so provided a suitable habitat for relict species) in thesouthern Alexander Archipelago during the late Wisconsin glaciation.In addition to the analysis of the bathymetric map, the limits of theCordilleran ice sheet and possible refugia were also identified byanalyses of aerial photographs, USGS topographic maps (1:63,360 and1:250,000 scales), NOAA bathymetric charts (1:20,000 and 1:40,000scales), previous literature, and reconnaissance fieldwork throughoutthe region. Ice-free areas that may have served as refugia include (1)high mountain nunataks (too small to show at map scale), (2)unglaciated ocean-facing slopes and forelands (Dahl, 1946), (3) theouter islands of the Alexander Archipelago (Worley, 1980), and (4)parts of the inner continental shelf exposed by the lowering of sealevel during the late Wisconsin by an estimated 125 m (Bard and others,1990).
|
|---|