# New age constraints of the LGM onset in the Bohemian Forest – Central Europe

Václav Duffek, Zbyněk Engel, Régis Braucher, Pavel Mentlík, Aster Team

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Václav Duffek, Zbyněk Engel, Régis Braucher, Pavel Mentlík, Aster Team. New age constraints of the LGM onset in the Bohemian Forest – Central Europe. Open Geosciences, 2025, 17 (1), 10.1515/geo-2025-0846. http://hal-05232146

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<figure><img src="image_1.png" /></figure>
Research Article

Václav Duffek*, Zbyněk Engel, Régis Braucher, Pavel Mentlík, and Aster Team

# New age constraints of the LGM onset in the Bohemian Forest – Central Europe

https://doi.org/10.1515/geo-2025-0846

received October 21, 2024; accepted June 20, 2025

**Abstract:** During the Last Glacial Maximum (LGM), valley glaciers in the Bavarian/Bohemian Forest mountains (Central Europe) reached their maximum extent, forming prominent terminal moraines. In this study, we provide the timing for the terminal moraine complex in the south-facing Großer Schwarzbach Valley using *in situ* produced cosmogenic $^{10}$Be dating. The moraine complex was deposited during the onset of the LGM, as indicated by the weighted mean exposure age of 25.7 ± 1.1 ka. This age represents the oldest cosmogenic data obtained for terminal moraine ridges in the Bavarian/ Bohemian Forest. It is also older than the dated LGM mor- aines in other Central European Variscan ranges. In the Bavarian/Bohemian Forest, the age suggests an asynchronous stabilization of LGM moraines across the mountain range, which we attribute to differences in insolation of north- and south-facing sites. Additionally, the youngest preserved moraine, located at the mouth of the cirque, was deposited around 16.2 ± 0.6 ka.

**Keywords:** glaciation, LGM, cosmogenic exposure dating, Bavarian/Bohemian Forest, Central Europe

1 Introduction

During the Last Glacial Maximum (LGM; 27.5–23.3 ka; [1]), defined as an event coeval with the Greenland Stadial (GS) 3, Central Europe was situated (Figure 1) between the southern margin of the Fennoscandian Ice Sheet and the glaciated Alps [2]. Periglacial conditions prevailed in this region, and only high-elevated parts of the European Var- iscan ranges (Black Forest, Vosges, Harz, Giant Mountains, and Bavarian/Bohemian Forest – see Figure 1) hosted mountain glaciation [3]. Despite the terrestrial records (such as lake sediments) at high elevations in the Variscan ranges that could provide information about climatic and environmental changes during the Late Pleistocene [4,5], investigating moraines as a terrestrial record of climate change is one of the few solutions to obtain paleoclimatic proxies for high elevations in the Variscan ranges. If we know the Equilibrium Line Altitude (ELA) of ice masses during moraine formation, these proxies allow us to recon- struct paleotemperature and roughly precipitation [6].

Such proxy data are more useful if the period of mor-
aine formation can be constrained with chronological data.
In the Central European Variscan ranges (Figure 1b), local
glaciation chronologies have been established for (i) the
Vosges [12,13], (ii) the Black Forest [14–16], (iii) the Giant
Mountains [17 and references therein], and (iv) the
Bavarian/Bohemian Forest [8,9,18,19]. However, these
reconstructions are not sufficient yet. Based on the existing
reconstructions (see [14]), the moraines stabilized in GS-1
(11.7–12.9 ka), GS-2.1 a (14.7–17.5 ka), GS-2.1b (17.5–20.9 ka),
and GS-2.1c (20.9–22.9 ka) in the Central European Variscan
ranges (the timing of stadials by [20]). In these ranges,
there is only limited evidence for moraine stabilization
before the LGM (before the global LGM, i.e., >27.5 ka)
([21] and references therein), and the available data indi-
cate that LGM could have been asynchronous [14]. Recent
studies from the European mountains have highlighted the
influence of relief configuration and local climate on gla-
cier fluctuations that could have occurred earlier in the
south-facing valleys, e.g., [22–24]. Our results provide
new information on both the stabilization of moraines in

<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

<footnote>* Corresponding author: Václav Duffek, Department of Physical Geography and Geocology, University of Ostrava, Chittussiho 10, 710 00, Ostrava, Czech Republic; Centre of Biology, Geoscience and Environmental Education, University of West Bohemia, Klatovská 51, 30619, Plzeň, Czech Republic, e-mail: duffekv@fpe.zcu.cz, tel: +420-728681210</footnote>

<footnote>Zbyněk Engel: Department of Physical Geography and Geocology, Charles University in Prague, Albertov 6, 12800 Praha, Czech Republic</footnote>

<footnote>**Régis Braucher, Aster Team:** Centre Européen de Recherche et d'Enseignement en Géosciences de l'Environnement, Aix-Marseille Univ., CNRS-IRD-INRAE UM 34, 13545, Aix-en-Provence, Cedex 4, France</footnote>

<footnote>**Pavel Mentlík:** Centre of Biology, Geoscience and Environmental Education, University of West Bohemia, Klatovská 51, 30619, Plzeň, Czech Republic</footnote>
<figure><img src="image_4.png" /></figure>

**Figure 1:** Location of the study site in Europe (a), Central Europe (b), and (as a red triangle) in the Bavarian/Bohemian Forest (c). Transparent blue areas in panels A and B show the LGM maximum extent of the Fennoscandian Ice Sheet and Alpine glaciation (after [3]). Elevation data for all three panels from [11]. Panel B: black dots indicate the glaciated mountain ranges in Central Europe. Panel C: black triangles indicate the location and the orientation of Pleistocene glaciers, and the arrows in them indicate the orientation of cirques (after [7]). Circled triangles show the locations where the ages of moraines are determined by the cosmogenic $^{10}$Be exposure dating method [9].

the LGM and the stabilization of moraines in the south-
facing valley.

The Bavarian/Bohemian Forest is the Central European Variscan range where the terrestrial records of Pleistocene glaciations are found in the south- and north-oriented valleys. Based on these studies [8,9,19], the Bavarian/Bohemian Forest moraines were stabilized in GS-2.1a, GS-2.1b, GS-2.1c, and GS-3 [9]. However, all the geomorphological studies supported by moraine dating in the Bavarian/Bohemian Forest have been conducted in north-facing sites (N and NNE) so far. Geomorphological research [7,25] mapping landforms and describing the glacial segment of the landscape has been carried out in one south-facing valley (Großer Schwarzbach Valley, Figure 1c).

This article aims to (i) determine the extent of the Last
Glaciation and the corresponding ELA for glacial phases
(documented by moraines) in the south-facing Großer
Schwarzbach Valley and (ii) to establish chronology of pre-
served moraines using *in situ* produced cosmogenic $^{10}$Be
dating. The results are interpreted in a regional and
Central European context and refine our understanding
of the last glacial paleoenvironmental evolution of
Central Europe.

## 2 Regional setting

The Bavarian/Bohemian Forest (Bayerischer Wald/
Šumava) is one of the Central European Variscan ranges
that has traces of Pleistocene glaciations (Figure 1b). The
mountain range is situated on the border of the Czech
Republic, Germany, and Austria and extends from the
northwest to the southeast (Figure 1a). The maximum ele-
vation exceeds 1,400 m.a.s.l. (main summit is Großer Arber
– 1,456 m.a.s.l.), and large plateaus at 1,000–1,200 m.a.s.l.
are typical for the mountain range. The range is mainly
built of metamorphic rock and granite [26]. The present
climate is described as the transition between a temperate
and a continental climate. The mean annual temperature
ranges from 6.0°C at 750 m.a.s.l. to 1.3°C at 1,300 m.a.s.l. [27],
and the mean annual precipitation ranges from 800 mm in
the lowlands to 1,800 mm at the high elevations [28]. The
annual precipitation also differs on windward (more than
1,500 mm) vs leeward (less than 1,000 mm) slopes [28,29].
These are determined by prevailing westerly winds [28],
which probably prevailed in the Last Glaciation [30,31].

The dominant glacial landforms in the Bavarian/
Bohemian Forest are cirques and preserved moraines
near them. The cirques developed at the lee sides of defla-
tion plateaus (which served as a source of snow for gla-
ciers) in the north- or south-facing [8,9] and east-facing [10]
valleys. Lakes or infilled lakes occur in some cirque
floors [32].

Despite almost 150-year long history of glacial research
(probably the first glacial research was performed by
Partsch [33]), the extent of glaciation in the Bavarian/Bohe-
mian Forest is still a matter of debate. The hypothesis of
extensive glaciation in the Bavarian/Bohemian Forest is
still assumed by some authors [34–36], in which long valley
glaciers (up to 7 km) are considered. On the other hand,
shorter valley glaciers and cirque glaciers are described by
[8–10,18]. These authors analyzed the preserved moraines
with spatial and morphological connections to the cirques.
Only the research of short valley glaciers and cirque gla-
ciers is supported by exposure dating of moraines [8–10].

Reuther [19] reported the ages of four phases of mor-
aine stabilization (WIa 19.1 ± 2.0 ka; WIb 18.7 ± 1.9 ka; WII
17.3 ± 1.6 ka; 15.5 ± 1.7 ka) near the Kleiner Arbersee Lake.
Mentlík et al. [9] presented exposure ages of moraines near
the Prášilské Lake and Laka Lake. All three sites reported
in these studies are north-facing, and no dating has been
done in south-facing valleys. Well-developed glacial land-
forms are preserved in four south-facing sites (Figure 1c) in
the Bavarian/Bohemian Forest: Schwellbach Valley, Gruft-
bach Valley, Lake Rachelsee, and Großer Schwarzbach
Valley [7]. Pronounced moraine ridges and overdeepened
cirque floors are observed at the latter two. In this study,
attention was turned to the Großer Schwarzbach Valley
marked in red in Figure 1c).

The Großer Schwarzbach Valley is located on the
German side of the Bavarian/Bohemian Forest about 15 km
northeast of Grafenau. The Großer Schwarzbach Valley is a
unique Bavarian/Bohemian Forest site. It is one of the few
valleys facing south, with a well-preserved system of ice-mar-
ginal moraines [7,25]. In addition, a lake existed at the bottom
of the strongly overdeepened cirque [25]. The valley faces
south, but the cirque faces southeast (Figure 1c). The locality
(where coniferous trees predominate) benefits from Bavarian
National Park protection regulations. Its bedrock is composed
of gneiss alternating with granite [37]. This valley has been
geomorphologically mapped and analyzed [7,25,35,38]. The
suggested maximum ice-marginal position (black dashed
line in Figure 2) has been assigned to the Last [34] or Penulti-
mate [7,35] Glaciation. However, this ice-marginal position is
not documented by ice-marginal landforms. The present
study, therefore, focuses on moraines that have a morpholo-
gical connection to the cirque.

Duffek et al. [25] presented the results of detailed geo-
morphological mapping (Figure 2). They described three

moraines (Sch2, Sch4, and Sch8) with more than three large
granite boulders suitable for ¹⁰Be exposure dating (Figure 2).
Since moraines Sch1–Sch5 represent one complex landform
(the distance between particular ridges is max. 30 m, and
they are not significantly separated), the results will be inter-
preted for the whole landform (see Sch1–5 in Figure 2).
This Sch1–5 complex landform probably corresponds to
the LGM ice-marginal position. The moraine Sch8 is prob-
ably related to one of the Late Glacial ice-marginal posi-
tion [25]. Thus, the present study focuses only on these
two ice margins.

3 Methods

## 3.1 Determination of the glacier size and ELA calculation

The glacier size (area and volume) and ELA were calcu-
lated for the LGM (defined by Sch1–5 complex landform)
and Sch8 ice-marginal position (Figure 2). A similar meth-
odological approach was used for comparison with other
areas in the Bavarian/Bohemian Forest [8–10]. The glacier
size and ELA were determined based on the Digital Eleva-
tion Model (DEM) with 1 m resolution [39], field mapping
[25], and paleoglacial modeling guidelines e.g., [40–42]. We
created DEMs of glacier surfaces (the contour lines were
drawn by hand) for the LGM and Sch8 ice-marginal posi-
tion. According to the study by Pearce et al. [42], the upper
ice limit for the minimum extent follows the highest, most
continuous contour of the cirque headwall. For the max-
imum extent, the upper ice limit was drawn ~30 m below
the top of the headwall [42]. The former glacier surface was
reconstructed as concave in the upper part, convex in the
lower part, and straight in the middle parts of the former
glaciers [40,43]. The reconstructed glacier surface was
superimposed over the current topography to approximate
ice volume determination. The maximum thickness of the
glacier was determined as the maximum elevation differ-
ence between the valley bottom and the reconstructed ice
surface.

The accumulation area ratio (AAR) approach was
selected for the ELA reconstruction [42] using the ELA_CALC-
culation_toolbox [44]. AAR indexes (0.54 and 0.44) were
determined based on the glacier area following Kern and
László [45]. An AAR 0.50 was also used to determine ELA
values that allow direct comparison with the published
data [9,10,19]. In the last step, we calculated the area of
deflation plateau, which could have influenced the glacier
extent and its ELA. The deflation plateau linked with the Großer Schwarzbach cirque was delimited by the area with an inclination between 0 and 12° (same as [46]).

## 3.2 Exposure dating

The sampling focused on boulders located on the moraine Sch8 (three sampled boulders), which delimit one of the Late Glacial ice-marginal positions, and on the complex landform Sch1-5 (6 sampled boulders) at the mouth of the Kleiner Schwarzbach Valley. Sampling for exposure dating was carried out according to the recommendations by Gosse and Phillips [47]. Samples were taken by hammer and chisel from the top surface of >1 m high upright granite boulders of the Finsterau series [37] located on the crests of the moraine ridges. The sampled boulders

were not covered by moss or other vegetation. The location and description of the sampled surfaces are given in Table 2, and boulder locations are shown in Figure 2.

The samples were crushed, sieved, and cleaned with a mixture of HCl and H₂SiF₆. The extraction method for ¹⁰Be ($T_{1/2}$ = 1.387 ± 0.017 Ma; [48,49]) involved the isolation and purification of quartz and elimination of atmospheric ¹⁰Be. A weighed amount (~0.15 g) of a 3,025 + 9 ppm [50] ⁹Be carrier solution was added to the quartz decontaminated in HF (48%). Beryllium was subsequently separated from the solution by successive anionic and cationic resin extractions and precipitations. The final precipitates were dried and heated at 800°C to obtain BeO and finally mixed with niobium powder prior to the measurements, which were performed at the French Accelerator Mass Spectrometry (AMS) National Facility ASTER (CEREGE, Aix-en-Provence).

The beryllium data were calibrated directly against the STD-11 beryllium standard using a ¹⁰Be/⁹Be ratio of 1.191 ±

<figure><img src="image_5.png" /><figcaption>**Figure 2:** Moraines in the Großer Schwarzbach Valley with the position of sampled boulders. Dashed lines represent the limits of glaciers during the LGM and Late Glacial. Note that the moraines Sch1-Sch5 represent one complex landform. The underlying DEM was provided by the National Park Authority.</figcaption></figure>
0.013 × 10⁻¹¹ [51]. Age uncertainties include an external AMS uncertainty of 0.5%, blank correction, and 1σ statistical error on counted ¹⁰Be events [52]. The ¹⁰Be/⁹Be measured blank ratio associated with the samples is 2.889 × 10⁻¹⁵. A rock density of 2.65 g cm⁻³ and an attenuation length of 160 g cm⁻² were assumed for the thickness correction. A sea-level, high-latitude spallogenic production rate of 4.01 ± 0.18 at g⁻¹ a⁻¹ [53] was used and scaled for latitude and elevation using the Stone [54] scaling scheme. The surface production rates were also corrected for the shielding induced by dipping sampling surfaces and surrounding terrain [55]. Topographic shielding for sample sites was calculated from the DEM [39] following the methods described by Li [56]. Shielding from the snow was estimated according to Gosse and Phillips [47] using an average snow density of 0.3 g cm⁻³ and an estimated mean thickness and duration of snow cover (5 months) at the sample sites. The snow cover thickness and duration were calculated at each site (specific values are in Table 2) using a linear regression model based on the data collected by the Czech Hydrometeorological Institute and Deutscher Wetterdienst at seven weather stations located up to 35 km from the study sites at an elevation range of 615–1,437 m.a.s.l.

Snow cover can significantly affect terrestrial cosmogenic nuclide production rates and, as a result, underestimate exposure ages [57,58]. The effect of snow cover has often been considered small and neglected, but recent studies indicate its strong influence at particular sample sites [59,60]. Therefore, this correction should always be considered within past glacial environments, where significant snow cover occurred for most of the Late Glacial period.

Individual ages are reported with an external error of 1σ, which accounts for both measurement uncertainties, including uncertainties associated with AMS counting statistics, chemical blank measurements, and AMS internal error (0.5%), as well as uncertainties in the reference nuclide production rate for spallation. To demonstrate the sensitivity of our ages to erosion, we calculated the ages assuming zero erosion and an erosion rate of 5 mm ka⁻¹ [8]. The chi-squared (χ²) test was used to identify exposure ages at each moraine. If the 95% critical value for χ² with n-1 degrees of freedom was higher than the theoretical value (Table 3), the age with the largest calculated χ² value was excluded from the dataset until the distribution passed the χ² test [61]. The reduced χ² statistic and the standard deviation to the arithmetic mean exposure age ratio were used to approximate the scatter in the data following Blomdin et al. [62]. Weighted and arithmetic means of exposure ages were then calculated for the moraine ridges.

# 4 Results

## 4.1 ELA calculation and determination of the glacier size

In the LGM position (delimited by Sch1-5 moraine, Figure 2), the ELA ranged (Table 1) between 1,039 ± 18 m.a.s.l. (AAR 0.54) and about 1,055 m.a.s.l. (AAR 0.50) and a short valley glacier reached a length of at least 1,750 m. During the investigated Late Glacial ice-marginal position (delimited by Sch8 moraine, Figure 2), the ELA ranged between 1,126 ± 20 m.a.s.l. (AAR 0.44) and about 1,115 m.a.s.l. (AAR 0.50) and glacier front of at least 630 m long glacier was situated at the entrance to the cirque (Figure 3). The similar ELA values for each phase (Table 1) determined by different methods (indexes according to [45] and AAR 0.50) show similar results of ELA determination. In the investigated Late Glacial ice-marginal position phase, the glacier decreased in area and volume by about 70 and 60%, respectively, compared to the LGM position phase (Table 1). The maximum glacier thickness was determined to be about 85 m in the LGM position. The area of the deflation plateau (Figure 2) is 0.54 km².

## 4.2 Exposure dating

The six exposure ages from the terminal moraine Sch1-5 are poorly clustered and range from 14.6 ± 3.3 to 29.0 ± 2.2 ka (Table 2). Within this sample group, samples PM6, PM7, and PM11 have been identified as an outlier based on the results of the χ² test (marked by a star in Figure 3 and Table 2). The remaining three exposure ages are moderately clustered, with a weighted mean exposure age of 25.7 ± 1.1 ka (Table 3). The three exposure ages obtained from the investigated Late Glacial ice-marginal moraine Sch8 are well clustered between 15.8 ± 0.7 and 17.7 ± 1.5 ka (Table 2), with a weighted mean age of 16.2 ± 0.6 ka (Table 3). The timing of the moraine sequence is in line with the geomorphologically based stratigraphy (Figure 3).

# 5 Discussion

## 5.1 Age correlation within the Bavarian/Bohemian Forest

The Sch1-5 terminal moraine is older than the oldest dated moraines in the area of the Prášilské, Laka, and Kleiner Arbersee Lakes (Figure 4). The difference in the timing
<table><caption>**Table 1:** Spatial characteristics of the glaciation and ELA in the Großer Schwarzbach Valley for the LGM and investigated Late glacial ice-marginal phases</caption><thead><tr><th>Glacial phase</th><th>Lowermost point (m a.s.l.)</th><th>Length of glacier (m)</th><th>Glacier surface (km²)</th><th>Volume (km³)</th><th>ELA (m a.s.l.)</th></tr></thead><tbody><tr><td>LGM position</td><td>902</td><td>1,750</td><td>1.00-1.08</td><td>0.0276-0.0279</td><td>1,039 ± 18 (AAR 0.54)*; 1,055 (AAR 0.50)</td></tr><tr><td>Late glacial ice-marginal position</td><td>1,018</td><td>630</td><td>0.28-0.34</td><td>0.0109-0.0113</td><td>1,126 ± 20 (AAR 0.44)*; 1,115 (AAR 0.50)</td></tr></tbody></table>

*AAR index according to [45].

<table><caption>**Table 2:** Description of the sample sites in the Großer Schwarzbach Valley and the surface exposure ages for the collected samples</caption><thead><tr><th rowspan="2">Sample</th><th rowspan="2">Longitude (°E)</th><th rowspan="2">Latitude (°N)</th><th rowspan="2">Altitude (m a.s.l.)</th><th rowspan="2">Boulder height (m)</th><th rowspan="2">Surface aspect/ dip (°)</th><th rowspan="2">Sample thickness (cm)</th><th rowspan="2">Snow cover depth (cm)</th><th rowspan="2">Snow shielding factor</th><th rowspan="2">Topographic shielding factor</th><th rowspan="2">Total shielding factor</th><th rowspan="2">Production rate (at<sup>-1</sup> g<sup>-1</sup> yr<sup>-1</sup>)</th><th rowspan="2"><sup>10</sup>Be concentration (10<sup>5</sup> at<sup>-1</sup> g<sup>-1</sup>)</th><th colspan="2"><sup>10</sup>Be exposure age (kyr)</th></tr><tr><th>No erosion rate</th><th>Erosion rate</th></tr></thead><tbody><tr><td>PM3</td><td>13.53917</td><td>48.95401</td><td>1,035</td><td>2.0</td><td>Horizontal</td><td>3</td><td>50</td><td>0.9641</td><td>0.9879</td><td>0.952</td><td>9.85</td><td>1.538 ± 0.68</td><td>15.8 ± 0.7</td><td>17.0 ± 0.8</td></tr><tr><td>PM4</td><td>13.53898</td><td>48.95380</td><td>1,029</td><td>1.5</td><td>Horizontal</td><td>2</td><td>49</td><td>0.9646</td><td>0.9875</td><td>0.953</td><td>9.81</td><td>1.69 ± 0.14</td><td>17.7 ± 1.5</td><td>19.2 ± 1.6</td></tr><tr><td>PM5</td><td>13.53908</td><td>48.95369</td><td>1,035</td><td>1.6</td><td>250/12</td><td>2</td><td>50</td><td>0.9641</td><td>0.9874</td><td>0.952</td><td>9.85</td><td>1.62 ± 0.15</td><td>16.7 ± 1.6</td><td>18.0 ± 1.7</td></tr><tr><td>PM6*</td><td>13.54608</td><td>48.94839</td><td>988</td><td>1.2</td><td>Horizontal</td><td>2</td><td>47</td><td>0.9679</td><td>0.9940</td><td>0.962</td><td>9.52</td><td>1.38 ± 0.31</td><td>14.6 ± 3.3</td><td>15.6 ± 3.5</td></tr><tr><td>PM7*</td><td>13.54634</td><td>48.94706</td><td>971</td><td>2.3</td><td>Horizontal</td><td>2</td><td>45</td><td>0.9687</td><td>0.9955</td><td>0.964</td><td>9.37</td><td>1.61 ± 0.14</td><td>17.3 ± 1.5</td><td>18.6 ± 1.6</td></tr><tr><td>PM8</td><td>13.54610</td><td>48.94727</td><td>972</td><td>2.0</td><td>90/7</td><td>3</td><td>45</td><td>0.9687</td><td>0.9947</td><td>0.963</td><td>9.40</td><td>2.19 ± 0.15</td><td>23.5 ± 1.6</td><td>26.1 ± 1.8</td></tr><tr><td>PM9</td><td>13.54506</td><td>48.94839</td><td>986</td><td>1.6</td><td>Horizontal</td><td>3</td><td>46</td><td>0.9681</td><td>0.9940</td><td>0.962</td><td>9.50</td><td>2.66 ± 0.20</td><td>29.0 ± 2.2</td><td>33.1 ± 2.5</td></tr><tr><td>PM10</td><td>13.54492</td><td>48.94744</td><td>978</td><td>1.4</td><td>100/4</td><td>3</td><td>46</td><td>0.9685</td><td>0.9967</td><td>0.965</td><td>9.44</td><td>2.42 ± 0.18</td><td>26.4 ± 1.9</td><td>29.8 ± 2.2</td></tr><tr><td>PM11*</td><td>13.54484</td><td>48.94776</td><td>979</td><td>1.9</td><td>110/6</td><td>4</td><td>46</td><td>0.9683</td><td>0.9945</td><td>0.963</td><td>9.45</td><td>2.03 ± 0.10</td><td>22.4 ± 1.2</td><td>24.8 ± 1.3</td></tr></tbody></table>

The star shows which age has been identified as an outlier.
<table><caption>Table 3: Grouping of $^{10}$Be surface exposure ages for the sampled moraines</caption><thead><tr><th>Moraine</th><th>Number of samples/ sample code</th><th>Theoretical χ<sup>2</sup></th><th>χ<sup>2</sup></th><th>χ<sup>2</sup><sub>R</sub></th><th>SD</th><th>SD to arithmetic mean age (%)</th><th>Degree of scatter</th><th>Arithmetic mean age (kyr)</th><th>Uncertainty-weighted mean age (kyr)</th></tr></thead><tbody><tr><td>Sch1-5</td><td>6/PM6 to PM11</td><td>11.07</td><td>31.16</td><td>6.2</td><td>5,423</td><td>24</td><td>Poorly clustered</td><td>22.2 ± 2.2</td><td>22.4 ± 0.7</td></tr><tr><td></td><td>3/PM8 to PM10</td><td>5.99</td><td>4.68</td><td>2.3</td><td>2,756</td><td>10</td><td>Moderately clustered</td><td>26.3 ± 1.6</td><td>25.7 ± 1.1</td></tr><tr><td>Sch8</td><td>3/PM3 to PM5</td><td>5.99</td><td>2.19</td><td>1.1</td><td>959</td><td>6</td><td>Well clustered</td><td>16.8 ± 0.6</td><td>16.2 ± 0.6</td></tr></tbody></table>

Note that three outliers were eliminated from the Sch1-5 landform dataset.

suggests asynchronous moraine stabilization during the LGM, which could be tentatively attributed to the different slope aspects of the sites. The glacier in the south-facing valley likely retreated rapidly to the cirque after depositing the LGM moraines at the onset of the LGM. In contrast, glaciers in the north-facing valleys remained at the LGM position longer due to colder conditions [63]. Similar effects of the slope aspect on the timing of the local LGM have been observed in the Alps [22], where LGM moraines in south-facing sites were found to be older than those in north-facing ones.

The exposure age obtained for the Sch8 moraine corresponds (Figure 4) to the ice-marginal moraine in the Prášilské Lake and Laka Lake areas of the Bavarian/Bohemian Forest [9]. In the cirques of Prášilské and Laka Lakes, other ice-marginal moraines have been dated to around 14 ka BP. No such moraine developed in the Großer Schwarzbach Valley, which can tentatively be attributed to the generally warmer conditions of the south-facing valley.

Several studies (e.g., [64–67]) point out that disregarding individual boulders with exposure ages younger or older than the overall mean is not always justified. When considering inheritance in sampled boulders, the age of the oldest dated boulder can be regarded as the closest approximation to moraine stabilization [9,65]. Following this approach, it can be suggested that the complex landform Sch1–5 stabilized around 29.0 ± 2.2 ka, while the moraine Sch8 stabilized around 17.7 ± 1.5 ka. These ages shift the timing of moraine stabilization in the Großer Schwarzbach Valley and suggest an even greater asynchrony in moraine stabilization within the Bavarian/Bohemian Forest.

## 5.2 Age correlation within the Central Europe

The weighted mean exposure age of 25.7 ± 1.1 ka for the terminal moraine Sch1–5 in the Großer Schwarzbach Valley is consistent with the global LGM period (27.5–23.3 ka, [1]) as defined in the NGRIP ice-core stratigraphy [20]. This moraine is older than the dated LGM moraines in other Central European Variscan ranges (Figure 5). The apparent lack of age-correlated terminal moraines is primarily due to the decreasing availability of boulders suitable for exposure dating as moraine age increases. Nonetheless, a strong temperature anomaly gradient and decreased precipitation from west to east across Central Europe [6] may have created varying conditions for glacier advances and retreats, leading to asynchronous timing and differing regional and local LGM extents.
<figure><img src="image_6.png" /><figcaption>**Figure 3:** (a) shows the error-weighted mean exposure ages for moraine ridges (in bold) and exposure ages for sampled moraine boulders and reconstructed glacier surface and ELA for minimum ice-marginal position. Individual ages are reported with an external error, including the internal error. The star shows which age has been identified as an outlier. The underlying DEM was provided by the National Park Authority; (b) shows the environment in the cirque and some of the sampled boulders.</figcaption></figure>

The oldest ages reported from the Vosges predate the global LGM (Figure 5), indicating an ice cap glaciation of around 38 ka [13]. Ice caps have also been reported from other European Variscan ranges high-elevation regions [16 and references therein], but their timing remains unresolved. The same applies to the LGM moraines in the Vosges, Black Forest, and Giant Mountains, where terminal moraines suitable for dating have not been preserved or the retrieved ages are underestimated [13,17]. The Kleiner

Arbersee (24 ka) and Großer Schwarzbach (26 ka) Valleys in the Bavarian/Bohemian Forest remain the only sites where the timing of the local LGM has been constrained with exposure ages.

The dated ice-marginal moraine Sch8 is consistent in age (16.2 ± 0.6 ka) with the Late Glacial GS-2.1a [20] and Heinrich Event 1 [69,70]. Moraines in several areas of Central Europe were also deposited in the GS-2.1, a cold period (Figure 5). The age of moraine Sch8 overlaps with the mean
<figure><img src="image_7.png" /></figure>

**Figure 4:** Correlation of exposure ages of moraine in the Bavarian/Bohemian Forest. GS and Greenland Interstadial (GI) from the study by Rasmussen et al. [20]. LGM after Hughes and Gibbard [1]. Data for Kleiner Arbersee Valley (Wla-WIV), Prášilské Valley (Pras1-Pras3), and Laka Valley (Laka1-Laka2) from Mentlík et al. [9].

exposure ages of the moraines at the Lac de la Maix and Trou du Cuveau sites in the Vosges [13] and in the main Zastler Tal Valley in the Black Forest [15]. It coincides with exposure ages determined for individual moraine boulders at the mouth of the Wielki Śnieżny Kociół and Mały Staw Lake cirques in the Giant Mountains [17] and in the north-western part of Sankt Wilhelmer Tal in the Black Forest [14]. The age of the Late Glacial ice-marginal moraine Sch8 also coincides with moraine stabilization in the Alps [22,71].

## 5.3 ELA of Bavarian/Bohemian Forest glaciers

The ELA values determined for the two dated phases in the Großer Schwarzbach Valley are consistent with results published from other areas (Table 4) in the Bavarian/Bohemian Forest. The ELA for the LGM position in Großer Schwarzbach was similar to the ELA for the LGM position at Prášilské Lake [9] and Kleiner Arbersee Lake [19]. The ELA is primarily controlled by regional climate, but local topography may be significant due to its shading effect and snow redistribution by wind or avalanches [72]. The snow drift from exposed surfaces to leeward topographic depressions is particularly effective in low-relief mountains of the British Isles [73], Scandinavia [74], and Central Europe [75], where large gentle summit areas favor drifting. As a result of enhanced snow drifting, the ELA of cirque glaciers may occur

below the regional ELA of plateau glaciers [76]. The similar ELAs for the Großer Schwarzbach Valley and north-facing sites could be tentatively attributed to the bigger deflation plateau connected with the Großer Schwarzbach. The area of the deflation plateau connected with the Großer Schwarzbach is approximately 3 times bigger than the deflation plateau connected with the Prášilské Lake [46] and approximately 5 times bigger than the deflation plateau connected (lying south of the cirque) with the Kleiner Arbersee Lake.

It is also possible that the ELA in the Großer Schwarzbach Valley was higher in the LGM, which could be explained by the different glacier reconstructions considered, e.g., by Hauner et al. [7] because he assumes a much bigger glacier in the Großer Schwarzbach Valley, which is supposed to have an accumulation area higher up in the valley.

The GS-2.1a ELA in Großer Schwarzbach was very similar (Table 4) to the ELAs at the Prášilské and Laka Lake sites [9]. During this phase, the Bavarian/Bohemian Forest glaciers were confined to the cirques, and their formation was influenced by the deflation of snow from plateaus above the cirques [26]. The region's susceptibility to snow drift introduces uncertainty in assessing snow cover conditions in leeward depressions. This also increases uncertainty in surface exposure dating, as evidenced by differences between our cosmogenic nuclide ages calculated with and without snow cover corrections. These ages are 3.2 to 3.7% higher when the correction is applied. Although the observed increase is smaller than alpine regions, where snow-corrected ages have been reported
<figure><img src="image_8.png" /><figcaption>**Figure 5:** Kernel density estimates (blue shaded areas), individual exposure ages (black ticks), and age estimates reported for the LGM (blue shaded ellipses) and readvance (blue shaded circles) moraines in the central European mountains. Data for the Bohemian Forest [8,9], Giant Mountains [17], Black Forest [14,15], and Vosges [13,79] are recalculated using the calculator formerly known as the CRONUS-Earth online exposure age calculator [68]. Ages were calculated using a rock density of 2.65 g/cm³, assuming no erosion and no snow shielding. Age estimates for the LGM and readvance moraines after previous studies [13,16,17].</figcaption></figure>

to be 10–15% older [59,77,78], it should still be accounted for to avoid underestimating exposure age. Given that the current climate in Central Europe is one of the warmest since the onset of the Holocene [80], we suggest that the present-day snow cover correction likely underestimates the true shielding effect of snow over the postglacial period and should be considered a minimum.

<table><caption>**Table 4:** Comparison of available ELA (AAR 0.50) for different time periods at different Bohemian/Bavarian Forest locations</caption><thead><tr><td>Period/locality</td><td>Großer Schwarzbach</td><td>Prášilské Lake</td><td>Kleiner Arbersee Lake</td></tr></thead><tbody><tr><td>LGM</td><td>1,055 m.a.s.l.</td><td>1,066 m.a.s.l.</td><td>1,067 m.a.s.l.</td></tr><tr><td>GS-2.1a</td><td>1,115 m.a.s.l.</td><td>1,140 m.a.s.l.</td><td></td></tr></tbody></table>

Prášilské Lake from [9] and Kleiner Arbersee Lake from [19].

# 6 Conclusions

Großer Schwarzbach Valley glacier covered an area of around 1 km² during the LGM, storing 0.028 km³ of ice. ELA of 1,039 ± 18 m.a.s.l. determined for this period is consistent with ELA values reported for north-facing glaciers at the Prášilské Lake (1,066 m.a.s.l.) and Kleiner Arbersee Lake (1,067 m.a.s.l.). During the GS-2.1a period, the area and volume of the cirque glacier were estimated to be 0.3 km² (70% of the LGM extent) and 0.011 km³ (60% of the LGM ice volume), respectively. Again, there were no significant differences between the ELA values determined for this period in the Großer Schwarzbach Valley (1,126 ± 20 m.a.s.l.) and other sites in the Bavarian/Bohemian Forest.

The oldest moraine complex in the Großer Schwarzbach Valley was deposited around 25.7 ± 1.1 ka, coinciding with the LGM onset in Europe. The weighted mean exposure age and the
oldest individual age of 29.0 ± 2.2 ka provide the oldest available cosmogenic data for LGM moraines in the Central European Variscan ranges and suggest asynchronous LGM moraine stabilization in the north- and south-facing valleys of the Bavarian/Bohemian Forest. The Sch8 moraine was deposited around 16.2 ± 0.6 ka, corresponding to the cold climate conditions of the GS-2.1a period. This age is consistent with the reported timing of moraines in the Bavarian/Bohemian Forest, the Central European Variscan ranges, and the Alpine valleys.

**Acknowledgments:** The authors would like to acknowledge the Bavarian Forest National Park Administration for the loan of the digital elevation model and the research permission. We also thank the Czech Hydrometeorological Institute (CHMI) and Deutscher Wetterdienst (DWD) for providing snow cover data. We thank J. M. King for the translation and language corrections.

**Funding information:** This study was supported by the Faculty of Education, University of West Bohemia (projects SGS-2016-055 and GRAK25_25_16) and the Czech Science Foundation (project 25-17976S). ASTER AMS national facility (CEREGE, Aix-en-Provence, France) was supported by the INSU/CNRS and IRD.

**Author contributions:** All authors have accepted responsibility for the entire content of this manuscript and consented to its submission to the journal, reviewing all the results, and approving the final version of the manuscript. VD participated in the geomorphological mapping, sampling for dating in the field, and preparing the manuscript. ZE participated in the field sampling for dating, preparing the samples for dating, and preparing the manuscript. RB participated in the ¹⁰Be dating. PM participated in the geomorphological mapping, sampling for dating in the field, and preparing the manuscript. Aster Team participated in the ¹⁰Be dating.

**Conflict of interest:** The authors state no conflict of interest.

**Data availability statement:** All data generated or analyzed during this study are included in this published article.

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