The Mouth of the Gamperdonatal

In Vorarlberg, the  Quaternary landforms and sediments of the southern Walgau region are a unique scientific archive, which can be used for the study of landscape development in the Alpine environment as a function of climate. The working of the glaciers of the last ice age (Würm, Weichsel), the restoration of a drainage pattern in response to the melting of the ice, the reshaping of the glacial topography by mass movement and periglacial activity – these processes have left their footprints within a framework of a complex geological substratum. A key factor was the interaction of the trunk glacier in the large Illtal, with origin in the high parts of the Montafon region and Silvretta mountains in southeast Vorarlberg, and the local glaciers in the tributary valleys of the northern Rätikon mountains where the accumulation areas of snow and ice are at lower altitudes (see The Walgau Glaciation Model). The occurrence of abundant crystalline erratics in the lower sections of these side valleys unequivocally documents the penetration of the Ill Glacier. Crystalline bedrock does not occur in the tributary valleys. It crops out in the Silvretta mountains, i.e. the highest accumulation areas of the Ill Glacier. The local glaciers apparently did not fill the lower valley sections during the periods of major climate change, leaving space for the trunk glacier to penetrate. The differences in altitude of the accumulation areas account for this pattern: the local glaciers appeared later and disappeared earlier than the trunk glacier.

The Ill Glacier, for example, was able to penetrate the ice-free lower reaches of the Gamperdonatal approximately 7 km at the onset of the last major glaciation before coalescing with the local Gamperdona Glacier. Conversely, the Ill Glacier still occupied the lower parts of the Gamperdonatal during deglaciation when the local glacier had already retreated into the upper valley reaches. As a consequence, the drainage of the river Meng was blocked at these times and ice-dammed lakes formed in the valley. Water level and spatial extent of the lake formed during the last deglaciation period in the Gamperdonatal can be reconstructed from the landforms and sediments which are preserved in the lower and central parts. Older lakes are documented in the stacked and lithified deltaic deposits preserved in the flank of the gorge of the Meng at Büderhöhe – a sequence unmatched in the eastern Alps.

The southern flank of the Ill Valley between the villages of Gurtis and Nenzing is covered by an intricate pattern of ice-marginal landforms, both accumulative and erosional. They formed during the last period of deglaciation and document the stepwise retreat of the Ill Glacier from the area. A drainage network was developing in the Gamperdonatal (and Galinatal) at this time, with local base levels being determined by the height of the Ill Glacier, and sediments were deposited by the streams at the mouth of the valleys against this glacier predominantly in a downstream direction. At times, the running water eroded deposits and created gullies which are now present in the landscape as dry valleys. The reworking of morainic material from the Ill Glacier is evident from the considerable amount of large debris of crystalline composition. The irregular topography of a number of these landforms is ascribed to the presence of dead ice: the sediments were deposited on top of ice masses which were disconnected from the actively moving ice body; these blocks melted under the cover of sediments slowly, thus creating a hummocky and occasionally kettled topography.

Information on former drainage networks, ice-margin positions and glacier-surface gradients can be deduced from ice-marginal landforms such as those between Gurtis and Nenzing. These and similar techniques make it possible to reconstruct how the glaciers gradually, i.e. step by step, disappeared from the area at the end of the last major Pleistocene – i.e. Upper Würmian – glaciation. All information contributes to a better understanding of the pattern of climate change during the transition from the last glacial period to the present interglacial. The scientific relevance of the ice-marginal landforms and deposits as well as their high rank in the assessment of geoconservation potential is, thus, evident.

Three sub-areas  are presented here. Bazulwald and surroundings are situated at the mouth of the Gamperdonatal. Corresponding landforms occur at Gadon and Latz, at the mouth of the Galinatal, and at Gurtis and surroundings.

The above figure shows the geomorphological map (left) and potential geoconservation map (right) of Bazulwald and surroundings. The digital polygonal geomorphological map (legend: level Tier 3 of hierarchical Geomorphological mapping) is displayed as a semi-transparent overlay on a mosaic of 2 classical geomorphological maps (legend: classical Geomorphological mapping). The contour interval of the potential geoconservation map is 25 m.

The above figure shows the geomorphological map (top) and potential geoconservation map (bottom) of the Gadon/Latz area. The digital polygonal geomorphological map (legend: level Tier 3 of hierarchical Geomorphological mapping) is displayed as a semi-transparent overlay on a classical geomorphological map (legend: classical Geomorphological mapping). The contour interval of the potential geoconservation map is 25 m.