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compilations always have some uncertainties due to gaps in confirmation of seis-
mic stratigraphy, different estimations of drainage provinces, and possible input
of eroded material from irrelevant provinces to depocenters, etc. We estimate that
the amount of material eroded in the Baltic region during Plio-Pleistocene is about
90,000 km 3 (Amantov 1995 ) .
We estimate both the erosion and the sedimentation over specific intervals of
time and require that erosion equal sediment accumulation over these periods. We
use 1 ka timesteps over the last 50,000 years and longer 5-10 ka steps for early
Weichselian stadials and across earlier glacial cycles. For the early Weichselian we
assume two interstadials with ice-free conditions following Lundqvist ( 1992 ) and
Lokrantz and Sohlenius ( 2006 ) as corrected by Svendsen et al. ( 2004 ) and Sarala's
( 2005 ) interpretation for southern Finnish Lapland.
The margins of the glacial ice sheets are the starting point for our analysis. The
ice margins at the LGM are shown in Fig. 3.1 . We use a number of tools to simulate
erosion under the ice cover and sedimentation under, at the margin, and outside
the ice. The tools are computation modules that allow useful geological analysis
procedures to be repeated easily. The procedures might include sampling of gridded
data (sub-ice lithology, for example), connecting sparse kinds of data with a best
fitting surface, inferring velocity fields from the distance to an ice depocenter and
topography, subtracting surfaces to determine the material removed, visualizing the
geology in particular ways, etc.
Erosion under the ice sheets is estimated using such tools by requiring that the
long-term glacial erosion rates are reasonable and the pattern of erosion conforms to
the concentric (radial) changes in erosion observed as well as the “spider's web” pat-
tern of grounded ice sheet's movement (ice streams). This is illustrated in Fig. 3.2 .
Figure 3.2a shows the erosion and sedimentation that might occur if only the ice
velocity were considered. The concentric pattern results from the low ice velocity
under the center of the continental glaciers and the more rapid basal ice velocity
near the margins. Figure 3.2b shows how this simple pattern is modified if the likely
effect of the spider-web pattern of ice flow with the enhanced erosional capacity of
ice streams is taken into account. Figure 3.2c illustrates the effect of different erod-
ability of sedimentary bedrock and basement lithologies. The glacial erosion module
contains adjustable parameters that allow the sediment redistribution it “predicts” to
be controlled by only concentric factors (Fig. 3.2a ) or increasingly influenced by
lithology, dendritic ice flow, and ice streams (Fig. 3.2b , c).
An important control is sub-ice topography which helps control the spider-
web flow with “topographic” ice streams and erosion paths. The drainage pattern
is determined from the paths raindrops runoff would follow in reaching the sea.
Submodules refine the interpretation. For example, overdeepening of bedrock sur-
face is imposed where slopes are >10-20 and oriented such as to cause rotational
ice flow that could locally maximize basal sliding (Lewis 1949 ) . The modules create
grids that capture erosion surfaces over time and show the exhumation of sedimen-
tary rocks, the boundaries of the sedimentary cover, expansion of the crystalline
shield exposure, etc.
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