TRACES OF ANCIENT GLACIATIONS IN KARST ENVIRONMENT 
- A CASE STUDY IN THE VENETIAN PREALPS 
SLEDOVI STARIH POLEDENITEV V KRAŠKEM OKOLJU – 
ŠTUDIJA PRIMERA V BENEŠKIH PREDALPAH
Ugo SAURO 1,2
Izvleček UDK 551.324.8:551.44(234.323.4)  
Ugo Sauro: Sledovi starih poledenitev v kraškem okolju – štu-
dija primera v Beneških predalpah
Za rekonstrukcijo starodavnih poledenitev na gorskem 
območju je treba v njihovem topografskem kontekstu skupaj 
obravnavati oblike erozije in akumulacije, da bi lahko modeli-
rali relativne ledeniške sisteme, ki jih je mogoče razumeti na 
podlagi medsebojne povezanosti med raznimi oblikami. Na 
kraških območjih so oblike ledeniške erozije pogosto zlahka 
prepoznavne, saj so se zaradi geomorfološke dinamike dobro 
ohranile. Ob tem pa so zelo stare oblike odlaganja komaj za-
znavne ali jih sploh ni, saj se je tudi zaradi kemičnega prepe-
revanja zmanjšala prostornina moren, pri čemer so nekatere 
popolnoma izginile. Kot vzorčni primer so prikazane reliktne 
oblike starodavnega lokalnega ledeniškega sistema Valon del 
Malera, razvitega v jurskem apnencu in glavnem dolomitu, 
ki se je delno ohranil v Lesinskih Alpah (Beneške predalpe, 
SV Italija), delno pa v bližnjem masivu Carega (Mali Dolo-
miti). Iz oblike erozije je razviden razpad segmenta ledeniške 
doline zaradi pospešene erozije, ki jo je mogoče pripisati ak-
tivni tektoniki vzdolž pomembnega sistema prelomov. Dru-
gi deli ledeniške doline, ki so v obliki korita, so zelo dobro 
ohranjeni. Poleg tega je mogoče zaznati oblike, ki so posledica 
čezmernega odtisa nekaterih starih morenskih grebenov, saj 
se je v ugodnih primerih morenski pokrov na stiku pokrova 
in kamnine osnovnega apnenca počasneje zniževal v primer-
javi z okoliškimi skalnatimi površinami, ki jih ne prekrivajo 
nanosi meljevca, kar je povzročilo nastanek skalnatih greben-
ov. Vendar je začasni vodonosnik, ki je visel znotraj morene, 
spodbudil razvoj pokritih škrapelj v apnencu, z izoliranimi 
skalnimi vrhovi ali stebri. Opisani so grebeni, ki jih večinoma 
ali v celoti oblikujejo kamnine na mestu, za katere se glede na 
vse dokaze zdi, da so posledica čezmernega odtisa starih mo-
renskih grebenov, ki so delno ali v celoti erodirani.
Ključne besede: reliktne ledeniške oblike na kraških območjih, 
morenski grebeni, čezmerni odtis, kraško raztapljanje, Beneške 
predalpe.
ACTA CARSOLOGICA 53/2-3, 115-126, POSTOJNA 2024
1 Lecturer of the Graduate Study Karstology, University of Nova Gorica
2 Honorary member of AIQUA (Italian Association for the study of Quaternary), e-mail: ugo.sauro@gmail.com
Prejeto/Received: 19. 7. 2023
DOI: https://doi.org/10.3986/ac.v53i2-3.13214 
Abstract UDC 551.324.8:551.44(234.323.4) 
Ugo Sauro: Traces of ancient glaciations in karst environment 
- a case study in the Venetian Prealps
To reconstruct the ancient glaciations of a mountainous area, it 
is necessary to consider together, in their topographic contexts, 
the forms of erosion and those of accumulation, aiming to 
model the relative glacial systems, which can be understood on 
the basis of the interrelationships between the various forms. In 
the karst areas, the forms of glacial erosion are often easily rec-
ognizable, given that the geomorphological dynamic favours 
their conservation. The very ancient deposition forms, on the 
other hand, are hardly detectable or absent given that chemical 
erosion contributes to the decrease in volume of the moraines 
until their total disappearance. As an exemplary case, the relict 
forms of the ancient local glacial system of Valon del Malera, 
developed in jurassic limestone and Main Dolomite (Dolomia 
Principale), partly preserved in the Lessini Mountains (Prealpi 
Venete, NE Italy) and partly in the nearby Carega massif (Pic-
cole Dolomiti), are illustrated. The forms of erosion document 
the dismantling of a segment of a glacial valley due to accelerat-
ed erosion phenomena attributable to active tectonics along an 
important fault system. Other segments of the trough-shaped 
glacial valley are very well preserved. On the other hand, forms 
resulting from the “over-imprinting” of some ancient morainic 
ridges are detectable, due to the fact that in favourable cases the 
morainic cover has reduced the rate of lowering at the cover-
rock interface of the underlying limestone, in comparison with 
the surrounding rocky surfaces not covered by till deposits, 
causing the formation of rocky ridges. However, the temporary 
aquifer hanging within the moraine has favoured the develop-
ment of grike type covered Karren in the limestone, with iso-
lated spikes or pillars of rock. Ridges mainly or totally formed 
by rock in place are described, which, according to all evidence, 
seem to be the result of the “over-imprinting” by ancient mo-
raine ridges partly or totally eroded.
Keywords: relict glacial forms in karst areas, morainic ridges, 
over-imprinting, karst solution, Venetian Prealps.
1. INTRODUCTION
Some relicts elements of a local glacial system in the Les-
sini Mountains (Venetian Pre-Alps) are investigated in 
this paper (Figure 1). Such system is indicated with the 
toponym of its most typical glacial form (Valon del Mal-
era) (Figure 2, Figure 3), a glacial trough present in the 
NE sector of the main plateau. This system shows forms 
of medium and small size, given that the relative glacier 
has never reached 10 km in length.
At this point it is important to underline that in re-
search on ancient glaciations it is essential not to limit 
oneself to the analysis of single forms of erosion or de-
position, but must consider as a whole the complex of 
UGO SAURO
Figure 1: Location of the studied 
area.
Figure 2: The glacial trough of Valon del Malera, on the right, and the Carega Group (maximum elevation of 2259 m asl), on the left.
116 ACTA CARSOLOGICA 53/2-3 – 2024
TRACES OF ANCIENT GLACIATIONS IN KARST ENVIRONMENT - A CASE STUDY IN THE VENETIAN PREALPS
forms, aiming at the reconstruction of the “ancient gla-
cial systems”. In other words, it is necessary to conceive 
models, albeit approximate, of the ancient glaciers, which 
are compatible with the relative complex of forms of ero-
sion and deposition, considered in their topographic and 
morpho-dynamic contexts.
The primary purpose of this note, however, is not 
simply to analyse some relict forms and to identify the 
related ‘system’, investigating aspects that seem to be of 
predominantly local or regional interest. Instead, we 
try to demonstrate how the integrated analysis of some 
problematic forms, considered in their context, can lead 
to the definition of morphogenetic models in which the 
interferences between different processes, such as the 
glacial and the karst ones, are considered. The application 
and verification of models of this type in researches on 
similar glacial systems of the Pleistocene, recognizable in 
selected orographic karst units of the Alps and of the Di-
naric Mountains, as well as in other mountain groups in 
soluble rocks, could, in perspective, provide further data 
useful for understanding the paleogeography and chro-
nology of old local glaciations.
Fundamental premise for the study and ‘reconstruc-
tion’ of a glacial system is the fact that such system can 
never be considered the ‘photograph’ of a precise moment 
of the past. In fact, the ‘current photo’ of the traces and 
components of the system is the result of the overlapping 
of an undefined number of phases of formation, growth, 
retreat and extinction of a glacier that occupied the upper 
part of a given mountain, which has been changing over 
time. Therefore, we have to treat with a ‘poly-chronolog-
ical’ and complex glacial system, within which it is pos-
sible to try to recognize a ‘stratigraphy of forms’.
Another important consideration is that the history 
of Pleistocene glacialism is decidedly more problematic 
in the mountainous areas formed by carbonate rocks, 
such as limestone and dolomite, than in mountains made 
up of crystalline and metamorphic rocks. In fact, if the 
karst process can favour the conservation of large and 
medium-sized forms of erosion, the processes of erosion 
and dissolution can determine variations in the volume 
and shape of the moraines. A form of deposition, such 
as, for example, a morainic ridge made up mostly of cal-
careous diamicton, is subject to changes of volume and 
shape due to mechanical erosion and karst dissolution. 
Therefore, these depositional bodies tend to lose volume 
not only by mechanical erosion, but also by ‘chemical 
erosion’, becoming thinner, until they are completely re-
Figure 3: The furrow of the Valon, 
below, and the cirque of Malga Pos-
ta, above. Between the two is the 
deep incision of the Val dei Ronchi.
ACTA CARSOLOGICA 53/2-3 – 2024 117
moved. This explains, at least in part, the difficulty of re-
constructing the ancient glacial history within the karst 
mountains.
In any case, it should be borne in mind that the re-
construction of the ‘paleogeography of the glaciations’ 
is particularly problematic also for the surfaces where 
crystalline rocks prevail, including the mountainous ar-
eas that hosted the great Alpine glacial systems, as can be 
seen from a review of studies of the past. For example: 
a) the fundamental work on the glaciation of the Alps 
(Penck & Brückner, 1909), has been the subject of count-
less discussions and ‘corrections’ by different authors, 
without ever reaching a solution deemed definitive; b) 
the Quaternary scholar Sergio Venzo, after having ana-
lysed the morainic amphitheatre of Garda Lake (Venzo, 
1965, 1969), formulated, after a few years, different and 
contrasting reconstruction. This difficulty is explained by 
the fact that, during the Pleistocene, in a complex sedi-
mentary building, such as the above mentioned Garda 
morainic amphitheatre, many phases of retreat and ad-
vance of the glaciers have occurred, with modification 
and reworking of the glacial deposits of the previous gla-
ciations. Other more recent papers underline the com-
plexity of the glacial history of the large glacial systems 
of the Alps (Ravazzi et al., 2014; Monegato et al., 2017). 
On the problems of the paleogeographic reconstruction 
of the evolution of Garda morainic amphitheatre, see 
also the works: Habbe, 1969; Chardon, 1975; Cremaschi, 
1987, 1990; Castiglioni, 2004; Sauro, 2005.
2. RELICT FRAGMENTS OF AN OLD LANDFORM OF GLACIAL EROSION
In the high Lessini Mountains, understood in the strict 
sense (plateau between the Val d’Adige and the Valle 
d’Illasi), there is clear evidence of forms of glacial erosion 
and deposition referable to the Upper Pleistocene and 
in particular to the last phase (LGM) of glacial expan-
sion known as Würm III, dating back to about 30-18 ky 
(Sauro, 1973; Ragnolini & Sauro, 1982). Here we do not 
resume the descriptions of the mentioned papers, which 
remain fundamentally valid with regard to the last glacial 
expansion. Instead, we will try to clarify some aspects re-
lating to landforms and deposits that could be the result 
of more ancient events. In fact, as we shall see, some of 
the forms of glacial erosion and deposition certainly have 
a lifespan longer as a few tens of ky, of the order of mag-
nitude of hundreds of ky, and in some cases, perhaps, of 
over one million years (1 My).
For simplicity, we will limit ourselves to consider a 
complex of forms as referable to a single ancient glacial 
system, starting from the analysis of the main forms of 
glacial erosion, and then delving into the problems of 
identifying and studying the traces of the related depo-
sitional forms.
In fact, in the Lessini Plateau the only truly typical 
form of glacial erosion is the Valon del Malera, which 
looks like a ‘glacial trough’, that is a valley with a U-
shaped transversal profile, over 2 km long, about 500 
m wide, extended between 1500 and 1865 m a.s.l., and 
relatively deep (Figure 2). At first sight, this glacial val-
ley appears as oversized, if compared to the surround-
ing glacial forms. It also lacks a head in form of a ‘gla-
cial cirque’, so much so that it seems that its upper part 
was dismantled following the retreat of the head of the 
Val dei Ronchi, a tributary of the Adige Valley. Also the 
morainic cover that occupies the high Vajo di Squaranto 
(the morphonym “vajo” means deep valley with canyon 
like characteristics), which constitutes the continuation 
of the Valon towards south, is of considerable extension.
The hypothesis that in ancient times the glacial 
trough of Valon extended further north, when the Lessi-
ni plateau was still connected with the Carega dolomitic 
group, has been formulated by several authors and is well 
presented and discussed by Corrà (1970a, 1970b, 1976). 
It is certainly plausible, as it justifies the relatively large 
size of this U-shaped valley, and, in part, also the charac-
teristics of the high Vajo di Squaranto, which is particu-
larly wide in its upper part.
If we look at the Carega group we can hypothesize 
that the ancient head of the Valon was located right at 
the glacial cirque of Malga Posta (maximum elevation of 
2259 m asl), whose valley threshold is about 2.5 km far 
from the upper edge of the Valon (Figure 3). Between the 
two forms of glacial origin there is now the head of the 
spectacular Val dei Ronchi, which is set along a bundle of 
active faults and is subject to accelerated erosion, as well 
as the entire Carega dolomitic massif. This is demon-
strated by the very frequent seismic crackling, detected 
by the seismic network of the province of Trento, which 
makes it one of the most active tectonic structures in the 
southern Alps (Sauro & Ferrarese, 2016).
So, the connection between the two glacial forms 
(Cima Posta cirque and Valon del Malera) must have 
been interrupted since the lower-middle Pleistocene (?) 
by the piracy action of the receding head of Val dei Ron-
chi, causing important geomorphological changes. But, 
UGO SAURO
118 ACTA CARSOLOGICA 53/2-3 – 2024
we must consider that glacial currents are often able to 
overcome vertical obstacles. It is therefore possible that, 
in ancient glacial expansions (pre-upper Pleistocene), 
subsequent to the ‘dismantling by piracy’ of a long seg-
ment of the ancient Cima Posta-Valon trough, part of the 
glacial flow of Val dei Ronchi was transfluent in the Va-
lon, helping to feed the relative glacial tongue.
In favour of this latter hypothesis, there are some 
forms, such as massive, overhanging and rounded lime-
stone frames, which characterize the upper band of the 
right slope of the Valon trough, and particular glacial 
contact potholes present on the walls of the right side of 
the upper Vajo di Squaranto. Such evidences would sug-
gest that, in not very ancient phases, the glacial tongue 
of Valon himself had a considerable thickness, almost to 
the point of filling the relative valley (Ragnolini & Sauro, 
1982).
In favour of this interpretation, a wide ‘rock-cut-
terrace’ like surface is also recognized, indicated with the 
name of ‘surface of Malga Bosco’, which extends down-
stream of the first segment of the upper Vajo di Squar-
anto, which can be interpreted as the relict of the ancient 
glacial valley bottom, hanging by about 70-110 m above 
the narrow fluvial incision that marks its western edge. 
Therefore the relict segments of the ancient glacial forms 
seem to document a glacial system with a total length of 
about 9 km and an altitudinal range of about 900 m.
3. FORMS AND CHARACTERISTICS OF THE GLACIAL DEPOSITS
The morainic deposits of the Valon glacier are present in 
the Valon segment located upstream of the confluence in 
the upper Vajo di Squaranto and, especially, in the upper 
part of the vajo itself, in the form of an almost continuous 
cover, characterized by large undulations and irregulari-
ties. It could be the result of several depositional phases, 
referable to the middle and upper Pleistocene (Figure 4). 
The surface of the deposits is dotted with medium and 
large boulders, which are affected by forms of karst solu-
tion. If man had not partially removed the stones to im-
prove the grazing area, it would have been interesting to 
analyse the morphometry and distribution of the boul-
ders, which on the oldest surfaces should have smaller 
dimensions and minor density.
Among the many forms worthy of attention, a relief 
located NW of Malga Porcarina deserves to be reported: 
TRACES OF ANCIENT GLACIATIONS IN KARST ENVIRONMENT - A CASE STUDY IN THE VENETIAN PREALPS
Figure 4: Overview of the terminal segment of Valon del Malera (bottom left) and the upper Vaio di Squaranto with its extensive morainic 
cover. Highlighted are the morainic covers of upper Vajo Squaranto (1); the “over-imprinted ridge” of Porcarina (2); the “over-imprinted 
ridge” of Malga Bosco (3); the “over-imprinted ridge” of Spiazzoi (4); the rock cut terrace of Malga Bosco (5); and the incision of the Vajo di 
Squaranto (6).
ACTA CARSOLOGICA 53/2-3 – 2024 119
it is a low ridge that extends for a length of about 300 
meters and a width of 60 meter (Figure 5, Figure 6), 
parallel to the asphalted road that descends from San 
Giorgio towards the Parpari locality. This could be the 
remains of a lateral morainic ridge deposited by a glacial 
tongue larger than that of Würm III. Curiously, this ridge 
is mainly formed by rock in place and only locally has a 
thin and discontinuous cover of weathered till material, 
as evidenced by some scattered boulders.
This shape could be explained by the fact that at first 
the morainic ridge was several meters thick, but later the 
karst solution processes consumed the moraine. The lat-
ter, on the other hand, ‘protected’ the underlying rock, 
in which the solution processes led to an average lower-
ing of the ‘moraine / bedrock interface’ lesser than that 
which would have occurred if the moraine cover had not 
existed.
From a chronological point of view, on the ridge is 
UGO SAURO
Figure 5: The “over-imprinted ridge” of Malga Porcarina (immediately upstream of the paved road and with some cows grazing above it), 
resulting from an ancient lateral moraine now almost completely removed by chemical erosion.
Figure 6: Forms of the ‘covered 
karst’ within the “over-imprinted 
ridge” of Malga Porcarina, which 
are explained by the action of the 
ancient temporary aquifer hanging 
within the moraine.
120 ACTA CARSOLOGICA 53/2-3 – 2024
significant the presence, as well as weathered till deposits 
of modest thickness, of paleosols of the ‘terra rossa’ type, 
which are not found on the more recent moraines, on 
which loess-like sediments are instead observed, charac-
terized by less evolved soils. In the paleosols, highlighted 
in correspondence with the excavation of a drinking 
ponds for cattle, flint tools from the Middle Palaeolithic 
were collected, referable to Levallois typologies. In the 
more recent moraines, flints of the Epi-Paleolithic (Epi-
gravettian) have been collected (Chelidonio et al., 1990, 
1992).
Returning to the shape of the ridge, it is obvious 
that it cannot be defined as a morainic ridge, since it is 
mainly made up of massive limestone in place. The only 
possible interpretation is that this shape is the result of 
a particular process of “over-imprinting” of a moraine 
on the underlying rock. It is therefore proposed to give 
this shape the definition: “rocky ridge derived from the 
over-imprinting of a morainic ridge”, or the shorter one of 
“over-imprinted ridge”.
About 1 km south of Malga Porcarina, on the left 
side of the Vajo di Squaranto, there is a wide rock cut 
terrace on which the buildings of Malga Bosco (eleva-
tion of 1363 m a.s.l.) are located. This is the terrace like 
rock surface, hanging of about 70-110 m above the cur-
rent valley incision, previously described and interpreted 
TRACES OF ANCIENT GLACIATIONS IN KARST ENVIRONMENT - A CASE STUDY IN THE VENETIAN PREALPS
Figure 7: Lidar DEM of part of the 
studied area. The three broken lines 
show the ridges that could have de-
rived from the over-imprinting of 
morainic ridges.
ACTA CARSOLOGICA 53/2-3 – 2024 121
as a relict of the bottom of a wide glacial valley. It is the 
result of an ancient modelling, probably carried out in 
several advances of glacial tongues that favoured the en-
largement of the valley floor. In the context of this ‘ter-
race’ we have tried to recognize others “over-imprinted 
ridges”. In fact on the DEM-Lidar it is possible to distin-
guish two slightly arched ridges (Figure 7). One is SW 
of Malga Bosco and is about 300 m long and 100 m in 
width; its high range above the surrounding surfaces is 
of about 10-15 m; the other ridge is between the pasture 
areas of Malga Bosco to the north and those of Spiazzoi 
to the south, and presents about the same dimensions of 
the previous one. Both are covered by woods and, due to 
their planimetry, they could be interpreted as segments 
of arches with the concavity upstream. These are difficult 
landforms to explain, also due to their remarkable topo-
graphical evidence. Probably they are “over-imprinted 
ridges” corresponding to segments of terminal moraines 
of considerable size and thickness (Figure 7, Figure 8), 
deposited when the connection between the Malga Posta 
cirque and the Valon del Malera trough still existed.
These shapes, being entirely covered by the forest, 
are not well perceptible in their characteristics. Visiting 
them, it is possible to observe a high degree of karstifica-
tion with scattered blocks of limestone and toppling phe-
nomena affecting the slopes.
4. ASPECTS OF THE INTERFERENCE BETWEEN THE GLACIAL AND THE KARST 
PROCESSES
In the areas in soluble rocks, which have been affected by 
the expansion of glaciers, it is interesting to consider the 
inter-relationships between the glacial processes (erosion 
and deposition) and the processes of karst dissolution, 
which we prefer to call ‘chemical erosion.’ In fact, in some 
glacialized areas, chemical deposition phenomena at the 
ice/rock interface (known as ‘sub-glacial calcite precipi-
tates’) can sometimes be observed, as on certain moun-
tain in the Alps and in the Rocky Mountains (Muir & 
Ford, 1985 ). In general, however, chemical deposition is 
clearly subordinated, in terms of entity, to chemical ero-
sion.
The best known forms, deriving from the combina-
tion of the glacial and the karst processes, are the cirques 
and the closed glacio-karstic depressions in the bottom 
of the glacial cirques and valleys, and in the plateaux 
areas. These forms are the result of the cooperation be-
tween the ‘over-excavation’ operated by the glaciers and 
the chemical erosion of the rocks, which occurs mainly 
on the bottom of the depressions. However, while the 
first process operates when the glaciers are present, the 
second prevails after their extinction, favouring, among 
other things, the drainage of temporary lakes eventually 
present in the depressions. Thus, in the Dolomitic groups 
of Italy there are known both closed depressions com-
pletely devoid of bodies of water, and depressions within 
which there are lakes subject to strong variations in level 
and, in some cases, ephemeral, as they are present only 
UGO SAURO
Figure 8: Details of the three “over-
imprinted ridges” in a Lidar DEM. 
The more southern could derive 
from terminal morainic arches 
deposited when the connection 
between the Malga Posta cirque 
(Carega Group) and Valon del Mal-
era still existed.
122 ACTA CARSOLOGICA 53/2-3 – 2024
for a few days in concomitance with the snow thawing 
(Bini et al., 1997; Sauro, 2007).
Less studied is the issue of the role that the karst 
process may have played in modifying the forms of gla-
cial deposition. Obviously, chemical erosion causes a de-
crease in the volume of the moraines, which are, howev-
er, also affected by mechanical erosion, especially in the 
context of the steepest slopes.
Some methods make it possible to calculate chemi-
cal erosion at an areal scale, based on the hydrological 
balance of a basin and the mineralization of the runoff 
waters, in particular those of the karst base level springs 
(Meneghel et al., 1986; Pulina & Sauro, 1993). Thus, as an 
example, in the basin to which the studied area belongs 
(basin of the Montorio springs, near Verona), the aver-
age chemical erosion corresponds to a rock thickness of 
the order of 3,9 cm/1000 years (about 39 m per million 
years) and therefore, of about 1170 m, if extrapolated 
to about 30 million years, the estimated age from when 
this territory emerged from the sea. This is certainly a 
value higher than that of the thickness of the rock actu-
ally removed, which could be of the order of 300-500 m, 
about a third of the value mentioned above. There are 
numerous variables which make it difficult to calculate 
the average lowering of the topographic surface, includ-
ing the fact that chemical erosion does not only affect 
the surface but also occurs in depth. Furthermore, in 
certain climatic phases the winds have deposited con-
siderable thicknesses of loess, consisting mainly of silts 
deriving from carbonate rocks (Cremaschi, 1990); and 
again, within the carbonate sequences there could also 
be volcanic rocks.
For these considerations, on a morphometric basis 
it is almost impossible to establish the age of the over-
printed ridges. The only chronological indication that at 
the moment can be provided for the ridge of Malga Por-
carina is given by the presence of flint stone tools of the 
middle Palaeolithic within a ‘terra-rossa’ type of soil. But 
the two most southern ridges are much more weathered 
than the Malga Porcarina ridge.
5. DISCUSSION AND CONCLUSIONS
There is no doubt that the glacial valley of which 
we have described relict erosional forms is referable to a 
very ancient glacial system, even if at the present state of 
knowledge it is practically impossible to attribute a well 
defined age range to this system. The state of preserva-
tion of the relict elements denotes how the conservative 
forms are in contact with strongly modified forms result-
ing from accelerated erosion processes. In fact, a long 
segment of the great glacial valley, corresponding to the 
Cima Posta threshold - head of the Val dei Ronchi - up-
per margin of the Valon trough, has been completely dis-
mantled, while the segments further downstream, within 
the Lessini mountains, corresponding to Valon del Mal-
era - hight Vajo di Squaranto, are well preserved. In cor-
respondence of the dismantled segment the maximum 
amount of vertical deepening reaches the impressive 
value of about 700 m (Figure 9).
In any case, the most problematic forms are those of 
TRACES OF ANCIENT GLACIATIONS IN KARST ENVIRONMENT - A CASE STUDY IN THE VENETIAN PREALPS
Figure 9: Approximate longitudinal profile along the axis of the old glacial valley (broken line), as it was before its capture, compared with 
the present day profile (solid black line). The maximum value of deepening, due to accelerated erosion, is of the order of 700 m.
ACTA CARSOLOGICA 53/2-3 – 2024 123
deposition and among these worthy of attention are the 
ridges in limestone, which are actually erosional forms 
deriving from depositional forms, which, according to all 
evidences, are explainable as the result of the “over-im-
printing” by a morainic ridge. These rock ridges may have 
formed only where morainic ridges of moderate or con-
siderable thickness rested on sub-horizontal or slightly 
inclined erosional surface on limestone. On limestone 
surfaces of moderate and stronger inclinations, forms of 
this type are not present as the slope processes favour a 
relatively rapid dismantling of the morainic ridges. Also 
the lithology is important: these landforms develop espe-
cially in massive limestone; if the rock is very fractured 
these forms are not present or have less evidence.
In June 2010, at the invitation of the Institute of 
Karst Studies of Postojna, I took part in a fieldtrip in karst 
areas of the Dinaric karst, and I was able to observe from 
afar rock shapes that could derive from ‘over imprinting’ 
on rock by ancient morainic ridges. These are forms that 
arouse curiosity and are worthy of further study on the 
relative glacial systems. I hope that someone will accept 
the implicit invitation to study the glacial morphology of 
the Dinaric Karst mountains, keeping in mind the model 
proposed in this paper. Obviously, these forms can be 
identified only in areas with a modest slope, where the 
chemical erosion of the morainic ridges clearly prevails 
over the mechanical one.
Based on the characteristics of the few forms iden-
tified, it is possible to propose a model on the evolu-
tion of these forms (Figure 10): after the deposition of 
a morainic ridge and the deglaciation of the area, in 
humid periods a hanging aquifer forms inside the tills, 
which determines temporary conditions of water satu-
ration at the moraine-rock interfaces. These conditions 
favour the penetration of water into the fissures in the 
rock (in this case limestone of the ‘Calcari Grigi Group’ 
of lower Jurassic age), resulting in the development of 
covered Karren, consisting of networks of grikes, that 
in time isolate rock pillars (Figure 6). Overall, the ex-
tent of chemical erosion of the rock in place under the 
moraine is lower than that which affected the surround-
ing rocky surfaces not covered by till. In fact, part of the 
solvent action of the infiltration water is exerted on the 
calcareous fragments of the diamicton of the till. In any 
case, it should be borne in mind that the rock located 
below the moraine is protected from other weathering 
UGO SAURO
Figure 10: Sketch of the “over-im-
printing” of a morainic ridge on 
the underlying calcareous rock: A) 
cross-section of a morainic ridge 
resting on a sub-horizontal calcare-
ous surface (the moraine-rock inter-
face is highlighted); B) the moraine 
is mainly removed by chemical ero-
sion and covered karren develop at 
the moraine-limestone interface; C) 
in place of the moraine, which has 
been completely removed, there is 
a strongly karstified ridge hosting 
infillings of paleosols and loess like 
sediments.
124 ACTA CARSOLOGICA 53/2-3 – 2024
processes, including cryoclastic processes. In this way, 
the surface of the rock below the ridge is subjected to 
an inferior rate of ‘karst lowering’ compared to that of 
the surrounding surfaces, not covered by the moraine. 
When the cover is almost completely removed, the rock 
in place began to emerge. The surface of these outcrops 
outlines a ridge in limestone that reproduces the form 
of glacial deposition, sometime hosting relict patches of 
the ancient covers trapped in karst pockets. However, 
the deep and strong karstification of a ridge of this type, 
mainly or totally in rock, can favour gravitational and 
mechanical erosion processes especially at the margins, 
due to stress release phenomena consisting of detach-
ment and ‘toppling’ of rocky blocks deriving from the 
‘pillars’, and consequent deconstruction of the rocky 
form. These phenomena show great evidence in the 
two ridges located respectively near Malga Bosco and 
the Spiazzoi, presenting ruiniform characters and which 
seem to be the result of the imprinting of segments of 
relatively large terminal morainic arches (Figure 8, Fig-
ure 11).
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