_{Geometry, chronology and dynamics of the last Pleistocene glaciation of the Black Forest}

In the first half of the 19^th century Karl Schimper reported glacial features in the southern Black Forest. Further descriptions of glacial landforms, as terminal moraines, roches moutonnées or glacial cirques, were described in the subsequent decades. After becoming professor at the university of Freiburg, Gustav Steinmann (see the photo) published a groundbreaking article on the last glaciation of the southern Black Forest in 1902. In his map of the area around the Feldberg, Steinmann subdivided the last glaciation into three phases. Despite later modificiations, this subdivision is still valid.

Further studies were published in the interwar period. Most of the research on the last glaciation was published in the golden period of the 1960s. During the late 1980s, interest declined markedly and most descriptions of glacial landforms appeared in the commentaries to the geological maps at the 1: 25,000 scale published by the State Geological Survey of Baden-Württemberg (LGRB) in Freiburg. Several years ago the LGRB stopped publishing geological maps at the 1:25,000 scale.

Altogether several dozens of studies have been published since the 19^th century. They mainly focus on the location and the correlation of glacial landforms. Few landforms were subjected to detailed sedimentological investigations. The only conclusions on the chronology of the last glaciations were drawn by analogy with data from sediment cores from lakes and mires inside terminal moraines in Seebachtal north-west of the Feldberg. This data only allows for a vague reconstruction of the chronology and should be interpreted with caution. Modern techniques for dating terminal moraines, such as dating with cosmogenic nuclides, have hitherto not been applied to the southern Black Forest.

We use high-resolution remote sensing data for the first time to map glacial landforms in the southern Black Forest. Thanks to the high spatial resolution of the data, even small-scale structures are visible. The analysis of the remote sensing data enables glacial features to be detected before field mapping and field mapping is thus less time-consuming. As the remote sensing data used in this study does not reveal information on the internal structure of landforms (sediment or bedrock), possible glacial features are thoroughly inspected in the field.

It is commonly accepted that sedimentological studies allow former environments during the formation of glacial landforms to be reconstructed, as well as the transport history of sediments to be deciphered. For example, the analysis of clast fabric in subglacial sediment provides information on the direction of former ice streams. Deformation structures in terminal moraines indicate that they formed during a glacial re-advance that led to the deformation of sediment in the glacier forefield.

Exposed bedrock surfaces and the bedrock below the surface is exposed to cosmic radiation. Cosmic radiation induces reactions in the minerals, thereby leading to the formation of radioactive isotopes. For example, Beryllium-10 forms in quartz-rich rock. As the production rates of cosmogenic nuclides are nowadays well known, measuring concentrations of cosmogenic nuclides in bedrock make possible the calculation of the exposure age of a rock surface. This method can be applied to terminal moraines: Rock samples are taken from the surface of large and well preserved moraine boulders; their exposure ages reflect the time when the terminal moraine started stabilising.

During the exposure of sediment to sunlight, a signal in minerals is set to zero. When sediments are buried, the signal starts increasing due to radioactive decay and the effects of cosmic radiation. This signal can be measured by stimulating samples with visible light. If the increment of the signal in the minerals is known, the duration of burial can be inferred. This requires that the sediments are sufficiently bleached prior to the burial. We intend to use this dating method to determine the age of terminal moraines.

The thickness of former glaciers is reconstructed with data from current glaciers. For the first step, the glacier thickness at the main flowlines is calculated. The glacier thickness at points along the flowlines is linked, through hand-drawn contour lines, with today’s terrain surface. According to observations from current glaciers, the contour lines are convex and concave in shape within the ablation and accumulation areas of a glacier, respectively. As terminal moraines reflect ice-marginal positions, they are used for the verification of the results.

The equilibrium line altitude of the reconstructed glaciers is subsequently calculated. It corresponds to the lower limit of the accumulation area of a glacier where the snow cover does not disappear during summer. Aside from a few exceptions, the equilibrium line altitude is largely controlled by the climate. If the age of the equilibrium line altitude is known, summer temperature and/or precipitation can be inferred.