Holocene Evolution of the Southern Baltic Sea Coast and Interplay of Sea-Level Variation, Isostasy, Accommodation and Sediment Supply - The Baltic Sea Basin - page 245

barriers, but relict estuarine mud are not found there, with the exception of the off-
shore Zingst and Hiddensee areas. That means that no barriers developed during the
early transgression phase and only narrow, flat, exiguous beach ridges transgressed
rapidly over the very gently dipping surface (0.05 0.15 ◦ , inherited from Late

Glacial lakes) driven by the rising sea. Some few overstepped beach ridges occur
only where coarse gravels crop out (Gromoll 1994 ) . Although much sand has been
available for sediment transport, the shape and character of the surface left by the
Late Glacial drainage system and the modest current and wave energy of the Baltic,
further reduced by bottom friction effects, prevented barrier formation. Hence, we
can confirm the results of barrier translation models (Roy et al. 1994 , Stolper et al.
2005 ) that on a flat substrate, increasing friction and decreasing wave power led to
a reduction in barrier size.
The transgressive beach ridges finally stranded at the toes of the Pleistocene ele-
vations interspersed mostly seawards between the Late Glacial lake sediments. Due
to the steeper substrate slope, the shoreline recession decelerated and erosion possi-
bly started. At that point the ratio between sediment supply and accumulation space
became critical and determined whether the volume of the beach ridges grew and
developed into spits and barriers or the elevations were finally eroded and drowned
and the embryonic spits were eroded. Under this perspective it becomes clear that
small elevations may play a special role in the process of beach ridge stranding.
They fix the migrating beach ridges/barriers but can maintain them for a longer
time only if (i) they will not get drowned by the rising sea and (ii) the sediment
supply is big enough to fill the still growing accommodation space. Therefore, the
present barriers are all connected to viable feeder cliffs and in all of them, cores from
Pleistocene sediment can be found. All lower elevations located farther seawards
became eroded, drowned and today, build shoals and reefs.
After the sea-level rise ceased - and hence the accommodation space started to
shrink - the stranded spits grew faster. Where the distance between spit anchors
(i.e. the accommodation space) was adequately small in relation to sand supply, spit
ends grew together and progradation started. This process led to the development
of bay barriers with wide beach ridge plains and dune fields (Fig. 12.7 ) . Where the
distance between the spit anchors was large compared to the sediment supply, the
bays were not completely cut off from the open sea. In fact, these spits are still
growing, thereby receding landwards and traversing lagoon sediments (Hiddensee,
Zingst, Bug, Rustwerder, Kieler Ort, see Fig. 12.1 ) .
Large areas behind the beach ridges or dune belts on the barriers developed as
wind flats, whose surface was levelled due to frequent floodings occurring through
shallow inlets. Vertical sand accumulation in the flats kept pace with the moderate
sea-level rise until the inlets became truncated by a beach ridge and peat accumula-
tion started (Fig. 12.8 ; Hoffmann et al. 2005 ) . The preservation of wide flats behind
the beach ridges or dune belts is generally considered to be evidence of shoreline sta-
bility or only slight retreat which is in accordance with the finding that the lagoonal
sediments usually do not crop out at the sea shoreface.
Where the headlands or islands which provided anchor or hinge points to the bar-
riers were not high or voluminous enough to survive, they drowned or were abraded,

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