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The preliminary results of research of accumulation glacial forms in the surroundings of Laka „lake“

Pavel Mentlík

pment@kge.zcu.cz Department of geography of University of West Bohemia in Pilsen, Veleslavínova 42,

306 19 Plze�

Introduction Laka „lake“ is one of the glacial lakes in the Šumava Mts. (KUNSKÝ 1933).

Although the previously glaciated areas are very important for paleogeomorphological reconstruction of the development of the Šumava Mts., the state of geographical studies is not satisfactory in the surroundings of this lake. Despite some limnological studies being carried out there (for example JANSKÝ & ŠOBR 1999), the known geomorphology of the area is very poor.

The aim of this paper is to reconstruct the occurrence of glacial landforms and also to investigate their probable genesis and their connections with the surrounding georelief in the Laka „lake“ area. This article is the first published study which has been carried out in this area and presents the preliminary results of comprehensive geomorphological research. This contribution focuses mainly on construction glacial forms in the area of interest.

Area of interest The Laka „lake“ is not a lake genetically but, in fact, is a reservoir dammed

by a low, man-made dam. This reservoir was used for trout farming and also as a source of water for the glass industry (JANSKÝ & ŠOBR 1999). The “lake” lies at 1,096 m a.s.l. and two mountains higher than 1,300 m a.s.l. form the catchment of the lake – Plesná Mt. (1,336 m a.s.l.) and Ždánidla Mt. (1,308 m a.s.l.). The Jezerní Potok brook which drains the lake firstly flows to the N and than to the NE. The brook changes its direction very sharply behind the Stará H�rka area and the character of the valley is completely different there. Glacial landforms have not been investigated there and the character of the valley is markedly fluvial.

Conversely, part of the catchment in the surroundings of the lake is strictly connected with glacial genesis and also some fluvial landforms such as a river terrace were investigated there. The genesis of this form, however, is markedly influenced by glacial activity.

Miscellanea geographica 11 KGE, Z�U v Plzni, 2005 Contributions from geomorphological seminary Šumava ´05

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Because of these facts, the part of the catchment of Jezerní Potok brook above the Stará H�rka area with adjacent ridges has been chosen as the area of interest for the comprehensive research of glaciation in the Laka „lake“ area (Fig. 1).

The previously glaciated areas in the Šumava Mts. can divide according to dominant fossil glacial processes to construction and destruction segments of georelief (cf MENTLÍK 2003). The ‘glacial construction segment’ is a part of georelief where we can find unconsolidated glacial sediment (Fig. 1). Conversely, the ‘destruction glacial segment’ is mainly created by destruction glacial forms and it is presented mainly by the corrie of Laka „lake“ (Fig. 1) in the area of interest.

Fig. 1. Geographical position of the area of interest

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Geological settings Two main bedrocks can be found in the area of interest (VEJNAR et al.

1990, PELC & ŠEBESTA 1994). Firstly, the western part of the area mainly consists of biotitic and sillimanite-biotitic para-gneiss which is here and there migmatized and also some sites of granodiorite can be found. The position at the headwall of the corrie is particularly important (VEJNAR et al. 1990), because the blocks of granodiorite create very distinctive features of glacial landforms in the surroundings of the lake.

Secondly, the eastern part of the area of interest is mainly created by cordierite-biotite migmatite. Some belts of medium-grained to coarse-grained porphyric biotitic granite (the Weinsberg type) can be found particularly on the ridge which runs from Ždánidla Mt. to D�ev�nná H�l Mt. The top plateau of Ždánidla Mt. is created by two kinds of granite (fine-grained to medium-grained biotitic granite and muscovite-biotitic granite, the Weinsberg typ of granite – from west to east) and the west part of the summit consists of migmatite and anatexite (PELC & ŠEBESTA 1994).

The lower part of the area under concern (particularly in the surroundings of the Stará H�rka area) consists of granite (the Weinsberg type) (PELC & ŠEBESTA 1994).

Three marked belts of quartzite create the ridge plateau, westerly from the locality Zlatý Stole�ek. Their presence could be very important, because they are very resistant rocks from a geomorphological point of view. The highest part of the area of interest (the summit plateau of Plesná Mt.), however, is created from “common” para-gneiss (PELC & ŠEBESTA 1994).

One presumed fault covered by younger sediments crosses the area under concern. The fault runs from the Nová H�rka area and is oriented to the NNE. The fault direction changes approximately 400 m in front of the dam of the lake to the N and the fault is oriented to the NNW (VEJNAR et al. 1990, PELC & ŠEBESTA 1994). It is necessary to say that the general direction of the line of the fault is oriented to the N as well as in the surroundings of Prášilské Jezero „lake“ (MENTLÍK 2002, 2005). This fact might be important for the development of the glacial lakes in the Šumava Mts. (HOUSAROVÁ & MENTLÍK 2004).

State of geomorphological research Generally, the knowledge about the geomorphology of the surroundings of

Laka „lake“ is very poor. The destruction glacial forms in the surroundings of Laka „lake“ were mentioned by CHÁBERA et al. (1987), who said that the lake lies in a shallow corrie.

The construction forms are mentioned by JANSKÝ & ŠOBR (1999). They were particularly engaged with a limnological research of the lake, however, they also tried to find remnants of the frontal moraine wall damming the lake.

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They were not successful due to an anthropogenetic impact and also considerably dense vegetation in the area (JANSKÝ & ŠOBR 1999).

Methods of research Detailed geomorphological mapping of elementary forms of relief

(cf. MINÁR 1996) and consequent geomorphological analysis in Geomorphological information system (GmIS) (cf. MENTLÍK et al. 2006) have been carried out in the area of interest.

The geomorphological analysis has three main steps: identification, differentiation (which has three special phases) and systematization (URBÁNEK 2000a,b and MENTLÍK et al. 2006).

The outline of the main steps of geomorphological analysis in the area of interest can be summarized as follows:

Identification – the area under concern was delimitated during this phase (see part ‘Area of interest’).

Differentiation – this step is composed of three particular phases. During the first phase the elementary forms of relief were delimitated (phase of delimitation). The process of delimitation can be summarized as follows:

o A digital elevation model (DEM) was created and elementary forms were delimited. The contour lines from ZABAGED were used as the main source of information about altitude, although rivers and ridge lines were also used for the creation of DEM. However, the parts of the surface of the earth, which were remodelled by glacial activity, were discovered to be highly rough, so that a lot of elementary forms in the ‘glacial segments’ were not significant due to the low accuracy of the DEM. Because of this, detailed geomorphological mapping was carried out.

o Detailed geomorphological mapping with GPS was performed in the area, when the boundaries of elementary forms were verified or mapped, respectively. The GPS Pathfinder GEO–Explorer was used.

o Cross profiles (Fig. 2, 3 and 4) were made across the area with glacial accumulation landforms to increase the accuracy of the delimitation of the boundaries of the elementary forms. Simultaneously, a good idea about the morphology of the whole ‘glacial construction segment’ was obtained.

o The map of the spatial distribution of elementary forms was obtained by compiling the received data in GmIS (cf. MINÁR et al. 2005 and cf. MENTLÍK et al. 2006).

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Fig. 2. Cross profile through the ‘glacial accumulation segment’ (for the position of the profile see Fig. 4 – Profile 1)

Fig. 3. Longitudinal profile through the ‘glacial accumulation segment’ (for the position of the profile see Fig. 4 – Profile 2)

Just the most obvious glacial forms (primary geomorphological forms) were identified during the second phase of geomorphological analysis. Particularly moraine walls, steep steps delimitating the ‘glacial construction segment’ against the surrounding area as well as depressions filled with peat were identified in the ‘glacial construction segment’.

Systematization – the analysis of spatial connections of the glacial landforms was carried out during this step. The position of significant remnants of the lateral moraine walls were compared with the position of the steep steps delimitating the ‘glacial construction segment’ and depression filled by peat, also the position of the construction forms was compared with the spatial distribution of the destruction forms such as the headwall of the corrie, the bottom of the corrie, glacial furrow (see below) etc. Consequently, the general pattern of distribution of the glacial forms in the area of concern was postulated (Fig. 1 and 4).

The connections between the spatial position of the destruction glacial forms (the bottom of the corrie and the headwall of the corrie etc.) as well as the location of the flat parts of georelief on the ridges and also glaciofluvial or fluvial landforms at the bottom of the valley, was highly important in this phase of geomorphological analysis. However, the complete results of this analysis exceed the aim of this study and will be presented in future papers.

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For analysis of morphochronology of the parts of the ‘glacial construction segment’ the Schmidt hammer method (cf. HUBBARD & GLASSER 2005) was used. The method was particularly used for the analysis of intact rock strength (IRS), or precisely, for relative dating of the surfaces of the granodiorite blocks which are diffused in the ‘glacial construction segment’. The comparison of the relative age of particular localities in the area of interest was carried out to determine the potential number of glacial phases there (Table 1).

The method according to EVANS et al. (1999) was used. The R-values were calculated from a sum of five blows per boulder on between five or seven boulders per an investigated locality. An average of the five highest R-values for the whole locality was then used as a representative value for the weathering of the boulders at the particular locality (Table 1). The six rules recommended by HUBBARD & GLASSER (2005) (p. 352–353) were used for collection of the data.

Some sedimentological methods were used for specification and verification of the results of the geomorphological analysis. It was necessary to investigate mainly the genesis of the sediments. Two methods were used: investigation of particles morphology (analysis of clast shape and roundness) and analysis of clasts macrofabrics (cf. HUBBARD & GLASSER 2005, BENN & EVANS 1998).

The direction and dip as well as the size of a-, b-, c-axis were measured for each clast (minimally 50 clasts were investigated for each sample) (Fig. 5 and 6). The roundness was classified visually according to the scale postulated by (POWERS 1953 in HUBBARD & GLASSER 2005) and commonly used in glaciological research (HUBBARD & GLASSER 2005, BENN & EVANS 1998).

For the final analysis of the clast shape, the data was plotted on shape triangles, following the method of SNEED & FOLK (1958) and BENN & BALLANTYNE (1994). The ternary plots were produced using the TRI-PLOT spreadsheet of GRAHAM & MIDGLEY (2000).

The final analysis of clast macrofabrics were investigated by 2D rose diagram and 3D contour data diagram (Fig 5 and 6). For the creation of the diagrams and consequent statistical analysis of the data, the software GEOrient 9.2 by HOLCOMBE (2005) was used.

Results The whole area where we can find unconsolidated, probably glacial,

sediments (the ‘glacial construction segment’) has a perimeter of 1.1 km and a maximal width of 0.362 km. The main part of the segment is the glacial lobe lying in the front part of the segment. Although the whole length of the segment is 0.497 km, it is not a symmetrical form, but starts higher on the ridge above the lake (the ridge lying to the NW of the lake) and finishes in front of the lake (Fig. 4).

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The lake is not a typical ‘lake dammed by a moraine’ but the ridge of the lateral moraine starts 0.102 km in front of the frontal part of the contemporary lake. It is probable that the material deposited by the glacier dammed the whole head of the valley and created good conditions for the accumulation of water.

Fig. 4. The map of elementary forms of relief of the ‘glacial segment’ in front of Laka „lake“ (the canal flowing along the contour was built for requirements of the glass industry in the surroundings of the Stará H�rka area)

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The whole lobe is clearly delimited against the surrounding relief by steep slopes, however, the remnants of lateral moraine walls lies at both sides of the lobe (Fig. 4). The other significant moraine wall runs parallel with the outer moraine wall, exactly at the front part of the lobe (see Fig. 2 and 3 for morphological characteristics of these forms). The front side of the lobe is created by the steep step cutting by the brook which flows from the lake. A marked erosion furrow is created by the brook at this place, where the brook leaves the lobe (Fig. 4).

The last significant moraine wall lies to the N of the dam of the lake and it is elongate in the same direction as the axis of the lake or, more precisely, as the ridge bordering the lake. The height of the wall is about 10 metres and it is possible that the material from the part of the wall exactly in the surroundings of the lake was used to build the dam and other constructions at this place.

The asymmetrical position (Fig. 1 and 4) of the whole lobe towards to the lake basin, or respectively towards the head of the valley, is very peculiar. It is obvious that the glacier did not flow directly from the valley (or through the lake basin), but the glacier had to overlap the ridge adjoining the lake basin to the NW.

We can find evidence for this fact at the ridge, particularly at the place where the glacial lobe wanes (Fig. 4). Two steep steps elongating in the direction of the probable glacial flow lie there and the flat surface covered with granodiorite blocks occurs below these steps. This form might be created by the erosion activity of the glacier at the place where the glacier overlapped the ridge. It has been not possible to find any evidence for similar geomorphological forms in geomorphological literature, so that the form has been given a working name, ‘glacial furrow’. It was necessary to test the hypothesis that the glacier overlapped the ridge and accumulated the material in the frontier of the present-day lake.

For that reason the relative age of the material at the ridge plateau below the ‘glacial furrow’ (Fig. 4 – L1) was compared with the relative age of the blocks of the glacial lobe (Fig. 4 – L2). If the sediments were accumulated at the same time, it might be evidence for mutual genesis at both places. The results of the Schmidt hammer test show that the level of weathering of the blocks is very similar at both localities (Table 1).

The sedimentological research was carried out at two localities (Fig. 4). The first locality (Locality 1 – Fig. 4) lies at the front side of the lobe, exactly at the front of the steep slope delimiting the glacial lobe. According to the geomorphological evidence a frontal moraine is supposed to have existed here. The second locality (Locality 2 – Fig. 4) was at the place where the track cuts the ridge of the lateral moraine (in front of the field station belonging to rangers of the Šumava National Park) (Fig. 4).

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Table 1. Comparison of R-values of granodiorite blocks at the ridge and at the area of moraines

Locality The highest rebound values (R) R-values average

Ridge plateau

(L1) 52 48 46 42 42 46

Area of moraines

(L2) 46 46 44 42 50 45,6

By analysing the 2D rose diagrams, the main direction of clasts was found

to be the NW at both samples. A particularly high concentration of clasts in this orientation was investigated at the second locality (Locality 2 – Fig. 6; the ridge of lateral moraine). The concentration of clasts of this orientation was also investigated by analysis of 3D contour data (Fig. 5 and 6). This orientation is the same as the direction of the axis of the main lateral moraine wall.

The clast shape and roundness were very similar at both localities, however, the numerical characteristics (RA, C40) as well as the graphical representations (Fig. 5 and 6) show, that the clast roundness was generally lower at the Locality 1, despite the fact that some rounded (r) clasts were investigated there (Fig. 5).

Discussion The presented results can be summarized as follows:

o Remnants of two parallel moraine walls, which were constructed by a large glaciation when the glacier was covering the ridge above the Laka „lake“, have been found.

o The moraine wall running parallel with the axis of the lake, which was probably made during the glaciation when the glacier developed just at the lake basin, has been investigated.

For a more serious interpretation of presented results of geomorphological

research it is necessary to prove the glacial origin of the sediments. A lot of types of sediments with different characteristics can be found in

glacial environments (HAMBREY & EHRMANN 2004), therefore the analysis of potentially glacial sediments can be very complex. The characteristics of clast depend on the type of the bedrock, type of transportation processes and finally post-depositional processes, which can change the general characteristics of the sediments.

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n = 70 Sector angle = 11°

Scale: tick interval = 2% [1,4 data] Maximum = 18,6% [13 data]

Mean Resultant dir'n = 117–297 [95% Confidence interval = ±36°]

Circ.Median = 112–292 Circ. Variance = 0.74

Circular Std.Dev. = 1.6 Circ. Dispersion = 6.44 Circ.Std Error = 0.3034

(95% konfidence arc for mean direction) 3D contour data (n = 70)

Clast shape

Laka 1

c : a b : a

(a - b) / (a - c)

Clast roundness

0

5

10

15

20

25

30

35

40

45

50

va a sa sr r

Fig. 5. Graphical representation of sedimentological analysis: Locality 1

n = 70 C40 = 56.9 %

n = 70 RA = 62.1 % y axis [%] va=very angular, a=angular, sa=sub-angular, sr=sub-rounded, r=rounded

Graphical representation of clast macrofabric data 2D rose diagram

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n = 81 Sector angle = 10°

Scale: tick interval = 5% [4,1 data] Maximum = 23,5% [19 data]

Mean Resultant dir'n = 145-325 [95% Confidence interval = ±18°]

Circ.Median = 145-325 Circ. Variance = 0,57

Circular Std.Dev. = 1,3 Circ. Dispersion = 2,05 Circ.Std Error = 0,1592

(95% konfidence arc for mean direction)

Clast shape

Laka 2

c : a b : a

(a - b) / (a - c)

Clast roundness

0

10

20

30

40

50

60

va a sa sr

Fig. 6. Graphical representation of sedimentological analysis: Locality 2

3D contour data (n = 81)

n = 81 C40 = 41.9 %

n = 81 RA = 58.7 % y axis [%] va = very angular, a = angular, sa = sub-angular, sr = sub-rounded, r = rounded

Graphical representation of clast macrofabric data 2D rose diagram

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BENN & Evans (1998) argued that clast shape is depended on both – process and lithology: “In mountain environment, actively transported clasts of massive, coarse-grained rocks such as granite and gabro tend to have compact, blocky shapes, in contrast with the more elongate and slabby shapes of periglacially weathered clasts of similar lithologies” (BENN & EVANS 1998, p. 207). For fissile rocks (the crystalline shale which dominates the area of interest) more elongated shapes, indicating preferential fracture along bedding planes, are typical (BENN & EVANS 1998).

Despite the high complexity of glacial sediments we can find some clues, which are used to distinguish glacial environments. BENN & EVANS (1998) assume that the till material has lower values of C40 (from 6 to 24 approximately) and also very low values of RA (from 0 to 2). The values typical for scree, however, are much higher: C40 from 50 to 78 and RA from 68 to 90 (BENN & BALENTYNE 1994 in BENN & EVANS 1998 p. 206).

Despite of these facts, it is obvious that it is very difficult to compare results of sedimentological studies from different localities (or even mountainous areas) due to dissimilarities of bedrocks, differences of geomorphological conditions such as various elevations of the slopes of the valleys, dimension of valleys etc., and also the intensity of the processes, which mainly means the duration of the glacial transport in this case. However, RA/C40 plots, firstly used by BENN & BALLANTYNE (1994), have been used for discrimination of different glacigenetic facies and environments and even for comparison of different glaciological studies (see HAMBREY & EHRMANN 2004 p. 305).

Therefore, the comparison of the samples from the area of interest with the samples from the Prášilské Jezero „lake“ area was used for the determination of the sedimentological environments. The samples from potential moraine walls, toes of debris flows and material from the block field (probably frost weathered material from a glacial age) were compared (Fig. 7).

Two significant clusters presenting two different environments can be distinguished on Fig. 7. The first cluster (cluster 1 – Fig. 7) represents potential input to the glacier – material produced by weathering and material transported for short distances mainly by debris flows or, precisely, by specific gravitation processes (debris slides plus debris flows – MENTLÍK 2005).

Both samples from the surroundings of Laka „lake“ include the second cluster (cluster 2 – Fig. 7), thus the material creating the lobe at the frontier of the lake is probably created by the material developing in a very similar environment to the glacial accumulations in front of Prášilské Jezero „lake“. According to relative dating (Schmidt hammer test) and the geomorphological position of the landforms in the surroundings of Prášilské Jezero „lake“, they present the middle and the youngest glacial phases which were probably connected to the same cold period.

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This might mean that the lobe in front of Laka „lake“ was also created by a small glacier, however, it is necessary to investigate the facies more specifically because the glacier rock glacier development (cf. BENN & EVANS 1998) is possible in the surroundings of Prášilské Jezero „lake“.

Fig. 7. RA/C40 plot of samples from the surroundings of the Prášilseké Jezero „lake“ and Laka „lake“ areas

The results of the Schmidt hammer test (Table 1) show that the surfaces of the blocks have a very similar level of weathering, which confirms the hypothesis that the whole accumulation was created during the same glaciation event.

According to the presented facts it is possible to postulate the following hypothesis of development of the glacial landform in the surroundings of Laka „lake“:

1st phase – the largest glaciation which created the lobe in the frontier of the lake was developed. The glacier overlapped the ridge above the lake and created one significant retreat moraine.

2nd phase – the glacier was localized just at the area which was nearly similar to the current lake basin. The moraine wall elongated according to the axis of the lake was accumulated as a lateral moraine where the glacier was leaving the lake basin and spreading.

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Conclusion The lobe created by unconsolidated material, which was probably

accumulated by a small mountain glacier, has been identified in the frontier of Laka „lake“. The lobe has an asymmetrical position compared to the present lake basin, which means that the glacier overlapped the ridge to the NE from the lake. This asymmetrical position needs detailed investigation of the whole fossil geomorphological system, particularly research of the position of destruction glacial forms and potential deflation areas lying at the ridges as well as the connections of morphology of the corrie and geological conditions.

The preliminary hypothesis of development of georelief was postulated: Two phases of glaciation have been recognized in the surroundings of Laka

„lake“. The occurrence of the glacier was much bigger during the first phase (similar to the spreading of the lobe of probably glacial sediments), whereas the glacier was lying approximately just at the area similar to the present-day basin of the lake during the second phase.

It is not possible, however, to ignore a hypothesis that some older glaciations had existed in the area of interest and composed the lobe before the glaciation which created the significant moraine walls of the first glacial phase.

It is necessary to stress that the presented results are just preliminary and the research of the surroundings of Laka „lake“ as well as Prášilské Jezero „lake“ is being carried out at present. Mainly the analysis of the whole geosystem – the position of glacial destruction forms compared to the construction forms as well as the position of deflation areas and other forms (river terraces) might be very useful for verification or falsification of the presented hypothesis. SEM analysis of the sediments as well as pollen analysis and numerical dating have also been carried out in both areas (MENTLÍK et al. 2005).

This paper has been created in the framework of the Czech-Slovak

Intergovernmental Scientific-technical cooperation project: ‘Geomorphological information system as a base of environmental applications’ number 116. Research has been supported by the grant of the Czech Academy of Science of the Czech Republic number KJB300460501 and by the grant of the Scientific Grant Agency of the Ministry of Education of the Slovak Republic and the Slovak Academy of Science (1/1037/04).

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