Common
origin for De Geer moraines of variable composition in Raudvassdalen, northern
Norway
Kevin P. Blake, Department of Geography and Geology, Eastern
Michigan University, Ypsilanti, MI 48197 USA
The study of De Geer moraines in Raudvassdalen shows
that most De Geer moraines are likely to have a common origin at the grounding
line of glaciers despite variability in composition of the ridges. Pebble fabric, grain-size analysis and
structures within exposures of De Geer moraines in the Raudvassdalen area, with
compositions ranging from mostly till to mostly sorted sediment, indicate that
the ridges all formed at the grounding line of a tidewater glacier by common
processes: deposition of sorted sediments
beyond the grounding line followed by deformation of pre-existing sediments and
deposition of a till as the glacier overrode the ridges. The compositional variation of the ridges is
probably related to the position of the section studied relative to the
location of the outlet of subglacial streams, and ridges with a component of
sorted sediments form in closer proximity to these streams. This unifying theory of De Geer moraine
formation, along with theoretical and geological evidence showing that there
are limited physical conditions where basal crevasses can form, suggests that
the number of De Geer moraines interpreted to have formed in basal crevasses is
probably unrealistic.