Dependence of Gully Networks on Faults and Lineaments Networks, Case Study from Hronska Pahorkatina Hill Land

1 Introduction

The process of gully erosion in the area of Central Europe has ceased. There are numerous examples from the past indicating that the formation of gully network resulted in the abandonment of land by farmers. Nowadays, this happens rarely due to the ability of man to change the land surface with heavy machinery. At the moment, we have almost no evidence of the formation of new gullies or gully systems in the area of Central Europe. However, there are still numerous examples of the reactivation of this process in existing gullies. The process of gully formation activates secondary processes, which are often more dangerous than the erosion process itself. For example, the formation of muddy floods or flash food is often directly connected to areas with permanent gullies. This is one of the reasons why it is necessary to identify and study factors influencing formation and reactivation of gullies.

The exact set of drivers of gullies is still questionable although there is a wide range of hypotheses which help to identify the majority of these drivers. One unconfirmed hypothesis assumes the dependence of gully networks on networks of tectonic faults (dislocations). The verification or falsification of this hypothesis through the analysis of network of gullies, valleys and faults on Hronska pahorkatina Hill Land is the primary goal of this paper. This hypothesis was verified mainly through the analysis of distribution of density and directions of networks of gullies, valleys and faults.

2 State of art

The agent of the gully erosion process is the ephemeral flow of running water [1]. The main cause of gully formation is too high overland flow, which may be caused by either climatic conditions or alteration in land use. According to various criteria, it is possible to distinguish valley floor from valley side gullies, continuous and discontinuous gullies, ephemeral and permanent gullies, linear (parallel) and dendritic gullies (the last ones occurring mostly in valley heads) see Fig. 1.

Figure 1

Examples of types of gullies according to: a, position b, area of cross-section c, presence of accumulation form d, position of other gullies.

The main trigger of the gully erosion process is still unclear. However, it can be assumed that land use changes and climatic changes are the main triggers [2]. One group of authors prefers only one of these causes whereas the other group considers both causes as equivalent. Moreover, the influence of land use and climatic changes is changing in time and space. According to Bučko, Mazúrová [3] and Klimaszewski [4], a genesis and evolution of gullies was associated with extreme rainfall and sudden snowmelt. The effect of these events was accelerated by a deforestation and agricultural utilization of a land with deep and soft regolith. An extensive synopsis of literature on historical gully erosion is provided in articles by Vanwalleghem et al. [5] and Dotterweich [6, 7].

Several stages of evolution of gully networks can be delineated on the basis of land use changes or climatic fluctuations, such as frequently occurring heavy rainfall events during the Little Ice Age (LIA) (e.g. [6, 8–12]). There are huge sets of studies suggesting that many permanent gullies have been formed since the beginnings of agriculture (e.g. [8, 13–17]). According to Vanwalleghem et al. [5], it was confirmed that the majority of gullies studied in Central and north-western Europe were formed between the 14th and 20th centuriey, with most of them being formed in the 20th century, after LIA. However, examples of older gullies still exist, e. g. [18, 19]. The oldest gullies in Europe are related to the Neolithic. Schmidtchen and Bork [20] dated charcoal at the bottom of an infilled gully. Semmel[21] described a Neolithic fossilized gully, though without chronometric dating.

According to Poesen [22], current ephemeral gullies are formed not only in natural drainage lines (thalwegs of zero order basins or hollows), but also along linear landscape elements (e.g. drill lines, dead furrows, headlands, parcel borders, access roads, etc). Most of the mentioned determining factors (besides topographical ones, there are many artificial linear landscape features) were important also in the formation of historical permanent gullies, which could have originally started as ephemeral gullies. The study of permanent gullies in the Myjava Hill Land, Slovakia, revealed that the majority of gullies linked to linear artifcial landscape features typical for the pre-collectivization arable land pattern, mostly to access roads, trails and field boundaries. One of the most curious artifcial predispositions for gully formation is plough furrow which has been used as an indicator of cadastral boundaries. In several cases, gully position can be linked to pastures in the original land use mosaic [16].

The abovementioned overview would suggest that gullies have been formed only since the beginning of human pressure due to climatic fluctuations and that their spatial distribution is controlled by land use pattern and topography. However, there are hypotheses that many large gullies were formed under forest during the wetter Atlanticum (7800–5000 BP) [23] or were formed under periglacial conditions and were cut into the loess blanket long before a protective vegetation cover was established during the Holocene [24], as shown on gullies in the forested area of Central Belgium.

A different point of view was presented by Knox [25]. According to his work, at long-time scale, gully development was a complex response to a set of driving factors, including tectonic movements. In recent years, some efforts have been made to establish a long-term chronology of gully erosion events for the understanding of both paleoenvironmental dynamics and the present-day state of geomorphic systems under the influence of multiple natural and anthropogenic factors (e.g. [26]). Huang et al. [27] confirmed synchronous down-cutting of gullies and rivers over the Loess Plateau, China, which implies that neotectonic uplift of the land mass was a major dynamic force causing gully incision. According to this study, a gully incision occurred when large-scale monsoonal climatic shift met with coincided neotectonic uplift; it was dated to ca. 600 ka, 240 ka, 125 ka and 11.5 ka.

In literature coming from the former Czechoslovakia, we can find only a handful of works that mention a possible relationship between gully formation and tectonics, either directly or indirectly. According to Láznička [28], the creation of a dense network of gullies in the Jihlava River valley around Ivančice, Czech Republic, was caused by tectonic movements. He distinguishes younger (historical and recent) valley side gullies from older (prehistorical) valley floor gullies which have been formed in glacials and interglacials. Harčár [29] states on the basis of research in the area of NW part of the Low Beskids, Slovakia, that tectonics, together with further geological factors, created primary conditions leading to gully formation. Hrašna and Liščák [30] found out that in the framework of an assessment of geodynamic phenomena in the Pokoradzká tabul’a Plateau, Slovakia, the occurrence of gullies is related to the tectonically deteriorated rocks and the orientation of gullies coincides with main tectonic directions in this area.

In the area of Hronska pahorkatina Hill Land, several authors point to the relationship between tectonics and some topographical erosion linear features, though not gullies. It is the relationship between tectonics and delllike landforms formed between the Latest Pleistocene age and Holocene age [31] and the relationship among tectonics and deflation troughs [32].

This paper presents the first integral analysis of the relationship between gully networks and tectonics in the area of Hronska pahorkatina Hill Land.

3 Regional settings

The Hronská pahorkatina Hill Land lies in the southwestern part of Slovakia. It is the most extensive part of the Danube Hill Land, representing the higher part of Danube Lowland and a transition zone between the Western Carpathians and the West-Pannonian Basin [33] or border area of Western Carpatians [34]. The location and division of the study area into units of lower hierarchical levels is described in Fig. 2.

Figure 2

Location of Hronska pahorkatina Hill Land in Slovakia and its division into smaller units.

Basic information about characteristics of the study area is contained in Table 1.

From among the five hilly subunits of the Danube Hill Land, Hronská pahorkatina Hill Land together with Ipel’ská pahorkatina Hill Land were uplifted more often and earlier [12] and tilted less [42] during the Quaternary than the remaining three parts.

The bedrock of the area consists of continual Late Miocene succession represented by lacustrine Ivanka Formation, deltaic Beladice formation and alluvial Volkovce formation [43]. The bedrock in the dominant part of the study area was formed by the Volkovce Formation with the upper age limit around 6.0–7.0 Ma and consist of major food plain facies (clays and silts) and minor meandering channel belt facies (sands). In the east, south and southeast parts of the study area, extensive areas of Neogene formations in the bedrock are covered by river terrace gravels blanketed by the loess of variable thickness or locally by eolian sands. Quaternary strata are deposited discordantly on older sequences due to the Pliocene basin margins inversion (sensu[44]) and uplift, which was active during the Pleistocene.

Elementary information about geomorphometric settings are contained in Fig. 3.

Figure 3

Statistical distribution of morphometric variables calculated for the whole area of Hronská pahorkatina Hill Land. The resolution of DEM is 10 m. Z – altitude, m. a. s. l.; GA – slope steepness, degree; aspect – aspect of the land surface, degree.

Two local extremes of altitude can be found in Fig. 3 (135 and 160 m. a. s. l.). These two peaks represent flat areas in the western, eastern and southern margins of the study area. The large area of flatlands also influences the statistical distribution of slope steepness. Thus, global maximum around zero is caused by the existence of large areas of fluvial flatlands (see below). Two local maxima in the bar chart of aspect are linked to south-western and north-eastern slopes.

Two types of land surface can be distinguished in the area. The first category is represented by hilly land surface, which is prevalent. Altitudes for this type of land surface reach usually 200–320 m. a. s. l. and vertical dissection is between 31 and 100 m. In comparison with surrounding plains, it represents a system of moderately uplifted and vertically differentiated blocks. In general, the land surface is monotonous with broad, rounded to flat ridges, bearing traces levelled in the Upper Pliocene [45, 46]. Ridges are elongated mostly in NW-SE, N-S direction, less in NE-SW and W-E direction. They are separated by widely opened valleys. The shape and direction of valleys is controlled by tectonics and reflects evolution under periglacial conditions. A typical feature of valleys is principally their distinct slope asymmetry and right-angled texture. Steep slopes are usually free of the Quaternary cover, while gentle slopes are covered by thick layers of loess. Dells, gullies and cart-tracks are often situated on steep slopes (e. g. [46]). According to Králiková et al. [31], just dells with an almost uniform NW-SE direction represent the youngest (Würm-Holocene) tectonically controlled landforms in the area.

The second category is represented by gently inclined plain land surfaces of river terraces, covered by loess, which is also visible in Fig. 3. Altitudes are ranging from 115 to 200 m. a. s. l. and vertical dissection is up to 30 m.

Characteristic landforms for hilly and plain land surfaces are marked by deflation troughs with NW-SE directions [47, 48]. According to [49], their formation was influenced besides deflation by tectonics.

4 Methods

Directions of networks of valleys, gullies and tectonic faults for the Hronska pahorkatina Hill Land area are analyzed in this work. For the purpose of this work, a valley is defined as a depression form of a linear shape with or without a permanent river. A network of gullies was constructed based on Military topographic maps on a scale of 1: 10 000 [50]. For purposes of advanced analyses, a simple gully was represented by an axis and systems of gullies (complex gully) were split into sets of simple gullies and each simple gully was again represented by an axis as explained in Fig. 4.

Figure 4

Representations of simple and complex gullies used in the database for analysis. a,– simple gully b, – simple gully represented by axis of gully c, – complex gully split into sets of simple gullies d, – complex gully represented by axes of partial simple gullies.

A network of valleys was created based on a digital elevation model of this area derived from the Topographic map of Slovakia 1: 10 000 [51]. Firstly, isolines of altitude were digitalized using ArcGIS 10.0. Secondly, dataset of isolines with additional information about altitude was imported into GRASS GIS 6.3.2. Lastly, imported dataset was used for the interpolation of new digital elevation model (DEM) using v.surf.rst (tension = 40, smoothing = 0.1) interpolation algorithm. For the verification of results, axes of valleys were delineated in two ways:

1. Axes of valleys were manually digitalized by operator on the basis of DEM. An example of the process of valleys axes delineation is shown in Fig. 5.

2. Axes of valleys were delineated on basis of DEM using semiautomatic algorithm developed by Šilhavý et al. [52]. Firstly, the DEM is resampled from source raster in a specific resolution. The resolution of the DEM influences the level of detail of the extracted lines. Secondly, the set of variously illuminated and rotated hillshade rasters is computed from the input DEM. Lastly, the set of vector lines is generated from each hillshade using PCI Geomatica software. These sets are processed by hierarchical cluster line algorithm to one final representative set of lineaments which are classified as positive or negative in the final step of computation.

Figure 5

Dataset of axes of valleys used for analyses.

Dataset of faults was adapted from the Geological map of Slovak Republic 1:50 000. Fault lines in these datasets consist of three geological maps on a scale of 1:50 000 [53– 56]. Fault lines were delineated on the basis of datasets of boreholes and on the basis of land surface, mainly on shape of valleys in the area. Although normal faults were observed only on outcrop rupturing Late Miocene deposits [31], a widely accepted concept of tectonically predisposed asymmetric valleys (e.g. [31, 46, 57]) allows us to expect the activity of these faults during the Pleistocene. However, this approach means that distributions of valleys and faults are partially supported by the same data and their informative capability is similar. Higher density of faults is expected to occur in the highest lying areas, where Late Miocene deposits are exposed.

The density of networks was computed using Line density algorithm (search radius 4 000 km²) incorporated into ArcGIS 10.0.

One of the crucial questions in case of using networks is sufficient density of these networks. Where is the border between sufficient and insufficient density of all three types of networks? This border depends not only on value of density of network but also on the number of features described by dataset. Therefore, an area with one long fault line can have comparable density with an area with several shorter faults. However, it can be assumed that information value of second dataset is higher. Since there is no exact threshold between significant and insignifcant networks threshold values were only estimated.

Datasets of axes of valleys gullies and faults were divided into ten meters long segments (see Fig. 6). This approach has two advantages. First of all, concerning the length of axes, short axes are not as important as long axes; and secondly, the approximate length of axes in each direction can be computed by multiplying the total number of elements by ten. The directions of networks were presented by rose diagrams constructed using Golden Grapher 9. Constructed diagrams were bidirectional; hence each axis was computed twice in both directions. It is thus irrelevant, whether the axis of the gully is constructed in the upslope or downslope direction.

Figure 6

Example of splitting axes in gullies network. a,– representation of axes in dataset before split b, – representation of dataset after split.

5 Results

Dataset of valleys was delineated in two ways (see Methodology). In Fig. 7 we can find a comparison of results between manual delineation and semiautomatic delineation of axes of valleys. Results of manual delineation are more precise and were used for further analyses.

Figure 7

Comparison of methods for manual and semiautomatic delineation of axes of valleys in two areas. On the left are the results of manual delineation and on the right, the results of delineation by algorithm.

A representativeness of all three networks was analyzed using their densities in the first stage of research. It could be assumed that only areas with the sufficient density of networks can be taken into account in the next steps of research; however, it is problematic to set the exact value of threshold. The threshold has to be set on the basis of density of network in a combination with a total number of features described by this network (see Methods). Maps of densities of networks are shown in Fig. 8.

Figure 8

Density of valley, gully and fault networks in the Hronska pahorkatina Hill Land.

Density of all three net works is uncomparable because of their different average density. It is obvious that density of all three networks is varies in space. On one hand, there is a high density of valley network in the whole area, on the other, density of gully and fault networks reaches zero mainly in the southern part of the key site. Density of faults in Hurbanovské terasy, Chrbát, Strekovské terasy and Belianske kopce can be assumed as unrepresentative because we can find only one or two fault lines in every part, which cannot be considered as a sufficient occurrence. This fact has to be taken into account during the next steps of analysis. A most interesting phenomenon is the presence of areas with higher density of all three networks, which are situated in the northern and western part of Bešianska pahorkatina, as well as in Belianske kopce.

Basic information about average densities of networks for each part of Hronska pahorkatina Hill Land is given in Table 2.

The highest density of all three networks is located in some areas of Belianske kopce Hills, which is situated in the southern margin of the key site. This is one of the previously mentioned areas with higher density of all three networks. The lowest density of all three networks is found in Búčske terasy part, next to Belianske kopce. While interpreting the information about density of networks, it is necessary to remember that parts of Hronska pahorkatina Hill Lands were delimited on the basis of topography and geological attributes of the area and density of networks is not homogenous in the whole area. Average densities of all three networks are distributed logically because we are able to delineate majority of valleys and summarized lengths of their axes is the longest from all three types of networks. It can be assumed that the total density of faults is underestimated, due to the restricted ways for their delineation and mapping. All the mentioned information about density of networks has to be taken into account for the verification of the representativeness of any further experiments.

The main directions of all three types of networks were analyzed using non-weighted rose diagrams on basis of split (see Methodology) datasets of all three networks. The results for all three networks are showed in figures 9, 10 and 11.

Figure 9

Orientation of axes of valleys in Hronska pahorkatina Hill Land. Absolute length of axes in meters can be computed by multiplication of values by 10.

Figure 10

Orientation of axes of gullies on Hronska pahorkatina Hill Land. Absolute length of axes in meters can be computed by multiplying values by 10.

Figure 11

Orientation of axes of faults on Hronska pahorkatina Hill Land. Absolute length of axes in meters can be computed by multiplying values by 10.

It has to be taken into account that there exists one major and several minor directions on rose diagrams. An analysis has revealed identical major direction of networks of gullies and faults only in the subunit of Strekovské terasy. However, the representativeness of fault network in this subunit is questionable. In case of subunits Chrbát and Belianske kopce, the differences between major and minor directions of all three systems are minimal. In case of these two networks the major direction of one network and the first minor direction of another network (e.g. major direction of valleys and faults and minor direction of gullies) coincide. The specific situation occurs in case of Bešianska pahorkatina where direction of network of valleys and gullies is almost equal. In case of faults it is possible to distinguish two major directions from one minor direction. The minor direction of faults is perpendicular to the major directions of valleys and gullies. In case of Búčske terasy only network of valleys and faults can be taken into account because network of gullies is unrepresentative. The major directions of both networks coincide. An absence of gullies is conditioned by topography of area because the slope steepness is not sufficient to create gully network. There are several possible explanations of these coincidences (see Discussion).

In several parts of Hronska pahorkatina Hill Land, the major direction of gully network was either perpendicular to the major direction of faults (e.g. Chrbát, Strekovské terasy) or perpendicular to the major direction of valleys (e.g. Strekovské terasy, Chrbát). Perpendicularity of networks can be explained by the indirect correlation between network of gullies and network of faults (see Discussion). On basis of the results, we are not able to define the general direction of networks for Hronska pahorkatina Hill Land as one unit.

In fact, three main situations can occur:

1. A coincidence of all three networks, namely a coincidence of network of valleys, faults and gullies (e.g. Belianske kopce). This case is connected with the areas with the highest densities of all three networks.

2. A coincidence of two networks (e.g. Bešianska pahorkatina). A main direction of the last network can be situated perpendicularly (e.g. Chrbát), without any coincidence or to be unrepresentative (e.g. Hronská tabul’a).

3. No coincidence of networks. This case is typical mainly for the areas without sufficient density of all three networks (e.g. Hrušovské terasy).

6 Discussion

Higher preciseness of manual delineation of valleys axes is caused by an ability of the operator to change the hierarchy level of observation during the process of delineation. The semiautomatic method, however, has this hierarchy level defined by hard during computation of algorithm. The ability of the operator to change the hierarchy level of the analysis does not mean that operator changed the scale of the base map. He only has the ability to recognize several hierarchical levels at the same time.

While interpreting the results, it has to be taken into account that endogenous processes can explain only a minor set of gullies, the majority is still explainable by combination of erosion processes and suitable predisposition.

It cannot be assumed that the direct relationship between tectonic processes and gully networks is often. Not every gully or gully system is conditioned by a fault below it. These cases are rare, however, possible. It can be assumed that under certain conditions, the presence of a strike-slip fault line can accelerate the interception of ground water. This, in combination with the disruption of material due to fault movement, produces conditions that are favourable for the development of gully (see Fig. 12). In fact, the strike-slip fault line only increases the erodibility of the material and decreases its persistence. It can be assumed that such a case is rare and is probable under laboratory conditions. In this case, there is no coincidence between gully, fault and valleys networks on larger scales. Moreover, no neotectonic strike-slip faults were ever observed in the area.

Figure 12

An example of development of gully on a strike-slip fault line. a, presence of fault line b, disrupted material due to movement of blocks and acceleration of interception of ground water c, development of gully on attenuated material.

It can be assumed that an indirect dependence between tectonic processes and gully networks is more common in the landscape. In this case, mentioned tectonic processes are just influencing the land surface, in terms of changing its shape and creating conditions, which are suitable for formation of gullies. There are mainly two alternatives to such a situation.

It was proven in many cases that the presence of fault lines conditions the development of steep linear slopes.

The direction of slope lines of these slopes is perpendicular to direction of fault lines. Slopes are also characteristic by high slope values. A linearity of slopes is changed to a set of convex spurs and shallow concave valleys. A transformation from linear slope is mainly caused by a set of exogenic processes. Thalwegs of these valleys generally create linear network. The presence of valleys increases values of a catchment area. The combination of high values of catchment area with high values of slope steepness creates conditions suitable for gully formation. These gullies are in a fact situated on topographic predispositions (see Fig. 13). The less often case is a formation of dendritic fault network at the end of the fault valley. The frequency of linear gullies is usually incomparably higher than the frequency of dendritic gullies because of the presence of waste areas with slopes and relatively small area where valleys end. It can be assumed that gully network in this scenario is perpendicular to network of faults and valleys.

Figure 13

Example of development of gully on linear slope which is determined by the presence of fault line. a, initial block disrupted by fault b, presence of valley conditioned by fault with developed parallel gully network.

The second alternative is rarer. The main driver is tectonic regime of river valley mostly without presence of a fault line. The sedimentary cascade is disrupted by tectonic up-lift or down-lift. The disruption causes the development of river terraces (erosion or accumulation). The presence of gullies on these terraces is conditioned by low resistivity of terrace material against erosion. Thus not all river terraces have to be covered by dense network of gullies. A good example of this indirect dependence is the situation in the Chinese loess plateau around Yellow river [27]. Gully networks were formed on river terraces which had been produced by episodic incision of the Yellow river (see Fig. 14). Such scenario is highly improbable in the study area due to insufficient thickness of erodible material on these terraces. In this case, it can be assumed that there is no dependence between direction of faults and gullies or valleys. However, it can be assumed, that direction of axes of valleys and gullies is perpendicular. During our research we are not considering this scenario.

Figure 14

Example of development of gully on slopes produced on risers of fluvial terraces. a, initial incision of river caused by tectonic uplift b, developed system of river terraces with system of gullies developed on risers of fluvial terraces.

An interpretation of dependence of gully network on network of valleys is comparably complex. Under specific conditions a difference between direction of gully and valley systems can be used for analysis of type of triggers of gully formation. Coincidence of direction between networks could signalize, that gullies are strictly topographically conditioned. Thus a direction of thalweg (valley) coincides with a direction of gully. This presumption works just in case of shallow valleys because in case of deep valleys there are permanent flows. However, in case of deep incised valleys a situation often occurs that gullies are located on valley slopes. Thus the direction of thalweg (valley)is perpendicular to a direction of gully The other differences between networks increase possibility that gully formation is triggered by set of different drivers, than only by topography (e.g. artifcial structures, footpaths etc.). This relation will appear only in case of datasets with sufficient accuracy (grid spacing, inaccuracy of elevation etc.)

Since the existence of general direction for the three networks in Hronska pahorkatina Hill Land cannot be assumed, the area at the level of lower hierarchical units has to be analyzed [33]. It has to be taken into account that borders of these geomorphologic units have been delineated only on the basis of geomorphological and geological attributes. On the basis of achieved results, we are able to suppose that mainly indirect dependence exist between gully networks and tectonic processes. The most probable scenario in this area is a development of linear gully networks on fault slopes, which are covered by loess with different thickness. This scenario is proven by:

1. The perpendicular direction of faults (valleys) networks and networks of gullies in some geomorphological units in Hronska pahorkatina Hill Land. Not only same direction of major directions, however, identical direction of major and minor directions of networks.

2. The higher density of faults in all three networks in several areas could be explained by higher dissection of land surface, which is connected with higher ratio of erosion. Exposure of older - Late Miocene deposits instead of Quaternary accumulations is the result of erosion processes. These strata of the Volkovce Formation exhibit much higher rupture by faults compared to Quaternary deposits by means of outcrop study [31]. The observed distribution allows us to assume that higher density of gullies occur in areas consisting of older sediments with much higher density of fault rupture. It may also be connected directly to higher position of the areas causing higher intensity of erosion, without a direct relationship to fault orientation and rupture.

3. Study of Quaternary river terraces morphometry [58] documented that these fluvial accumulations show no apparent tectonic displacement in the study area with an exception of the Búčske terasy area, where a normal fault pattern oriented in the NW-SE direction was observed and with offsets at 10–15 m scale. These faults are most probably continuous to the Chrbát subunit and predisposed valleys, where a perpendicular orientation of gullies appears. This could be therefore an example of gullies caused by differential vertical tectonic movements of blocky patterned area, as described in the study by Huang et al. [27].

Mentioned arguments are only indirect thus the presented scenario is only the most probable, not confirmed. However, it is highly improbable that the majority of gullies is situated on faults and there is no evidence (direct or indirect) that there is an increased spatial variability of uplift in the Hronska pahorkatina Hill Land area and its surrounding.

The theory of formation of valley networks by eolian processes in the Panonian basin has a long tradition beginning in early 20th century [59, 60]. Recently, it was demonstrated that an extensive valley-ridge system in the region was formed by deflation and eolian erosion [61– 63]. Therefore eolian processes have to be taken into account while analyzing land surface in the southern part of the Hronska pahorkatina Hill Land. According to the mentioned theory of valleys and ridges (yardangs) formation, the area of the Panonian basin is characterized by prevailing winds which blow from north-north-west direction. This theory is in discordance with the commonly accepted theory of development of valleys in the area of the Danubian basin on the basis of tectonic predispositions. The theory about eolian formation of land surface was proven in the area of Hungary mainly by geophysical measurements, structural analysis of the area and orientation of ventifacts located in the northern and western parts of Hungary. It can be assumed that similar situation can be described also in southern part of Hronska Pahorkatina Hill Land. This assumption can be proven by:

1. Similar directions of valley networks in Chrbát, Hurbanovské terasy, Strekovské terasy and Búčske terasy with supposed directions of prevailing winds.

2. Similar directions of valley networks in the southern part of Hronska pahorkatina Hill Land and yardangs situated in the north-western part of Hungary.

3. Asymmetry of distribution of loess deposits mainly on the north-western side of valleys, which is expected to be leeward. This could be an alternative explanation for valley asymmetry described by Králiková et al. [31].

Only sufficiently extensive dataset of ventifact from the area of Hronska pahorkatina Hill Land can be considered as direct evidence which can be used for verification or falsification of this assumption. However, there is no occurrence of sufficiently resistant rocks in the study area, which may prevent ventifacts from being eroded.

Several limitations and restrictions of the used methodology were indentified during the experiments. The quality of presented results is decreased by any of these limitations; however, using information and datasets currently available, we are not able to achieve better results. The majority of the limitations is connected with the availability of dataset of faults for the studied area. Limitations are:

1. Map of fault lines was mainly in the northern part of the study area derived from the shape of valleys. Therefore, it can be assumed that the information from dataset of axes of valleys and dataset of fault lines are similar.

2. As stated before, faults observed on outcrops rupture only Late Miocene deposits were probably active mainly during the Pliocene. Due to moderate topography, there is a lack of outcrops and their informative value may be overestimated. It should be considered that tectonic rupture is much higher in pre-Quaternary sedimentary units and distribution of fault rupture is dependent on the stratigraphic position (and age) of the unit forming the immediate substrate.

In this text we are present only one possible explanation of relationships between all three networks. However due to used methodology, we have to take into account several restrictions and consequently several other scenarios. The main restriction lies in the fact that we do not analyze direction of each segment of network against the closest element, only major directions of both networks. Thus the set of gullies can be parallel to the set of valleys of zero order and perpendicular to the set of larger valleys at the same time. The methodology is mostly restricted by the fact that we do not take into account artifcial predispositions as one of limiting factors. On the basis of works (e.g. [16]) we are assume that linear predispositions in the area of Central Europe are crucial drivers for gully formation.

All presented experiments were done as best as possible on the basis of best available information, however, the level of uncertainty in the case of network of faults is high.

7 Conclusion

The main goal of the proposed work was to find the possible relation between networks of valleys, gullies and faults in the area of Hronska pahorkatina Hill land. This hypothesis was tested using methodology of analysis of directions of networks. The coincidence of all three networks was rare. It was proved only in a part of Belianske kopce. More often scenario was the coincidence of network of valleys and gullies. Directions of network of faults were different or perpendicular. In many areas a density and number of features were insufficient for the other types of analyses (e.g. gullies on Hronská tabul’a, faults on Búčske terasy). One of the most surprising facts is that of perpendicular direction of gullies and valleys. There are two possible explanations for this fact. The first one is that gullies are situated on slopes of large valleys, which are almost perpendicular against direction of the main thalweg. Presence of these valleys can be conditioned by fault network in many cases. The second explanation is that gully formation was driven by another set of factors (e.g. foot paths or borders of fields). The most probable scenario is the development of linear gully networks on fault slopes, which are covered by loess with different thickness. This fact was proven by results of analysis of networks in combination with geological findings. All interpretations have to take into account a high level of uncertainty.