Glacial Erosion/Sedimentation of the Baltic Region and the Effect on the Postglacial Uplift - The Baltic Sea Basin - page 56

ice streams, the annual erosion rate for ice streams is most likely 2 0.5 mm/year

(Nesje et al. 1992 , Nesje and Sulebak 1994 ) . Comparable mean rates were reported
for Isfjorden region of Svalbard over the glacial cycle (Elverhøi et al. 1995 ) . In the
Antarctic, average erosion rates are considered to be three times higher beneath ice-
stream tributaries which are underlain by deep subglacial troughs (0.6 mm/year)
than beneath ice-stream trunks (0.2 mm/year) (Bougamont and Tulaczyk 2003 ) .
Remnants of marine Quaternary sedimentary sequences indicating high
glacial/fluvioglacial erosion rates in the Baltic-White Sea lowland are a cornerstone
in the validation of our erosion-accumulation modeling. The sporadic distribution
of the youngest marine interglacial strata in the form of remnants around the Gulf
of Finland attests to strong erosion during even the last glaciations (which was the
smallest of all in this area). Sediments from previous glacial cycles are very rare in
the axial part of the lowland, but in rare isolated locations remnants can be 40 m
thick (Malakhovsky and Amantov 1991 ) . Surface reconstruction suggests that, in
addition to thick marine strata, at least 10-20 m of the underlying sediments were
removed. In ice-stream zones like Lake Ladoga, remnants of older Quaternary beds
survived the deep erosion in protected positions, indicating more than 60-70 m
of erosion during the last glaciation. This suggests that in zones of active erosion
the present cover belongs nearly entirely to the last glaciations (moraine cover and
late-postglacial sediments).
Where soft sedimentary sequences have been glaciated, buried channels and hol-
lows of several generations suggest local linear erosion of 100-200 m (Amantov
1992 ) . Rarely, older channels can be seen to be entrenched at shallower depths than
the younger channels that cross them (Amantov 1992 ) . The nature of these channels
depends on whether they are radial or parallel to the glacial front, affected by sed-
imentary infilling, deformed by ice or melting waters, etc. Lithology and structure
are also dominant factors. The channels may often have nearly parallel orientation,
sometimes with arc shape that roughly coincides with the boundaries of retreated
glacial tongues. The depth of the channels decreases in the direction to the mod-
ern shield, so that the base of the channels tends to parallel the relief of the basal
platform sediments, mostly entrenching only into the weathered top of the resistant
crystalline basement. A similar rapid decrease in channel depth occurs toward resis-
tant lithologies such as carbonate rocks forming prominent scarp-like features on
the bedrock topography.
The depth of both glacial and fluvioglacial erosions strongly depends on lithology
(Amantov 1992 , 1995 ) . In the Baltic-White Sea, depressions in bedrock topography
suggesting maximum long-term erosion are evident in zones with pliable sedi-
ments. Here, glacial erosion rates inferred geologically and in our analysis reached
2 mm/year, with local short-term rates up to 8 mm/year. The thickness of erodable
sediments should be taken into account. The rate of erosion should decrease if a
pliable sedimentary unit is completely removed in an area with exhumation of resis-
tant surface. The Landsort Deep illustrates how removal of a thickness of pliable
sediment can create a strongly overdeepened ice-proximal negative form.
Another key factor controlling glacial erosion is the ice sliding velocity at the
ice-bed contact (Humphrey and Raymond 1994 ) . Our analysis addresses ice-stream

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