The River Basin of Kokra during the Pleistocene Period

Abstract

In the river basin of Kokra (Slovenia, Yugoslavia, the Kamnik Alps) traces of
the wiirm glaciation and of the post-glacial stages of the retreat of the glaciers had
already been known. The author has discovered traces of at least one another older
glaciation, which has been proved by the erosion remnants of the cemented moraine.
He has also confirmed and completed the results of the preceding studies, in particular
of those concerning the extension of the wiirm glaciation and the stratigraphical
sequence of the glacial, fluvioglacial and periglacial deposition.
The valleys, in which the older moraine has been deposited, have already been
cut out approximately to the existing level. This is indicated by the moraine along
the Kokra river on the site of the Roblek and Jezersko glaciers. In the case of the
latter site the moraine has also been discovered on the bottom near the end of the
Ravenska Kocna valley. Also the firmly cemented breccia, between Potoce and the
Zagar farm, which dates from the same period, if it is not even older, nearly reaches
the bottom of the Kokra valley. The cemented moraines of the Suhi dol and Podstorzic glaciers, that have nearly entirely filled both the valleys, have probably been removed because only a few remnants still remain on the slopes.
The glaciers, which have left traces in the cemented moraine have been nearly
of the same size and thickness as the wiirm glacier. This has been proved for the
Roblek glacier at Spodnja Kokra and at the Pestotnik farm and, partially, also for
the Podstorzic glacier, where the older moraine and the wiirm moraine are approximately on the same level.
The extent of the valley glacier of the older glaciation can be most firmly ascertained for the Roblek glacier, where the last traces of the cemented moraine have been found above the Preseren farm at Spodnja Kokra; the wiirm moraine extends some
100 metres to the west of the cemented moraine. In a terrace promotory, a little more
to the west of the cemented moraine, conglomerate is preserved at the same elevation
which the author considers to be related to the older glaciation. An equivalent layer
of conglomerate is also preserved at some more spots in the Kokra valley to the
west of this promotory.
The last traces of the Jezersko valley glacier from the older glaciation are found
along the Kokra a little more to the south of Podlog, whereas those of the Suhi dol
glacier are found near the Leskovec farm. The Podstorzic glacier of the older glaciation extended some five kilometres to the northeast in the direction of the Reka
valley. No traces of an older glaciation have been discovered, however, on the sites
of other warm glaciers.
The older glaciation has been followed by intense erosion which has removed
most of the moraine and has, along the Kokra river, at least reached the bottom of
the valley into which the older moraine has been deposited. The wiirm moraine is
namely in many places located on the valley bottom close to the older cemented
moraine.
It is not known to what depth the erosion has cut into the older moraine on the
bottom of the small Jezersko basin; it has been discovered in the ground only at one
spot which has been overlaid with younger quarternary sediments. It is neither
known to what depth the quarternary sediments of the basin have been deposited.
The conclusion can be made, however, that the valleys had been much deeper before
the present basin was filled with the thick quarternary sediments. Several facts seem
to point at such conclusion: the bottom of the basin is comparatively wide and flat,
whereas the surrounding slopes are steep and the valley of Jezernica (fig. 42) between
Zgornje and Spodnje Jezersko is deep and contains a well preserved older moraine
on its bottom at Log. It appears that the valleys in the basin area of Zgornje Jezersko
have been V-shaped in the period prior to the older glaciation, such as they still are
in the upper parts of the small brooks which have not been affected by the glaciers.
The headward erosion in the post-wiirm period has not yet reached the Jezersko
basin.
The Jezersko glacier extended, in the wiirm period, downstream as far as Zgornje
Fužine, which was somewhat further than it had been assumed by Lucerna. In the
Jezersko basin along the present course of the Jezernica river at Gmajna, the glaciers
from both the alpine valleys of Ravenska Kočna and Makekova Kočna have been
joined together. At Podlog the Jezersko glacier has spread a little upward along the
Reka brook and has left traces of both the older and the wiirm moraine, which is
exposed in a gravel pit (fig. 2). For the Podstoržič glacier is also proved to have
been larger than it was established by M e 1 i k and, above all, much thicker. Lucern a considered that the Suhi dol glacier had been slightly larger and that the
Roblek glacier had been of the slope-glacier type. The author's findings, however,
indicate that the Roblek glacier has been in fact a valley glacier and much more
extensive. There have been very good conditions for its existence in the high
alpine karst plateau, called Dolga njiva, of some 2.2 square kilometres in size at an
elevation of 1800 to 2150 metres.
During the pleistocene period several glaciers have descended from the Kočna
mountain (altitude 2484 m) and even from its lower northwestern ridge of Veliki vrh
(altitude 1696 m), down along the steep and mostly impervious slopes and ravines
(Ledeni jarek, Trnovčev graben, Beli potok, the ravine under Stara Povšna). By the
help of the solifluction and fluvio-nival action the glaciers have carried down large
quantities of the carbonate clastic material of all sizes, including large boulders, all
the way down to the Kokra river. Most of this material is preserved at Spodnje
Fužine, at the confluence of the Trnovčev graben brook and the Kokra river (fig. 15),
but large quantities are supposed to have also been deposited between the Beli potok
ravine and the Povšnar farm. The glaciers descending down from the Kamnik Alps to the Kokra valley have
been disconnected. An exception has been the glacier in the Beli potok ravine which
has extended along the Kokra river as far as the Povšnar farm, where it has been
presumably joined by another glacier descending from Jezerska Kočna along the
ravine under the Stara Povšna farm. The Suhi dol and the Roblek glaciers have been
close but have never been joined, since no traces of the glacial action have been
found in the Kokra valley between the Leskovec farm and Podlebelca.
Moraines of the buhl stage of the retreat of the glaciers have been found on
the sites of all former glaciers. They are characterized by the fresh and not very
coarse material which includes boulders, a few straited rocks but mainly rocks with
only slightly rounded edges, that area reminiscents of an ablation moraine. Larger glaciers of the buhl stage have converged in the Jezersko basin where the Ravno and the Makek glaciers remained separate and have reached only its rim. The Makek
glacier has even spread into the Jezernica valley below Zgornje Jezersko. A lake has come into existence behind the frontal moraine of that glacier at Gmajna, where smaller
fluvial-limnic particles had ben deposited (fig. 12). Because the headward erosion
has not yet entirely cut through the moraine a lake has been existing periodically
at that locality still in the historical time (which may explain the name of the
place). The terrain there had still been a marshy one until quite recently when the
drainage of the Gmajna area has been carried out. The buhl moraine of the Ravno
glacier has left at the front of that glacier marked traces of more frontal ridges
(fig. 7, 8, 11) and of the small dry valley, where one of the brooks had been flowing
along the western side of that glacier. In the Suhi dol valley a marked frontal moraine
from the biihl period has been deposited only some 500 metres away from the Kokra
valley. Traces of the biihl moraine of the Roblek glacier have been found near the
Pestotnik farm and in Grda dolina (the Grda valley) to the east as well as further
below on the slopes above the Kokra river. At Podstoržič the moraine of the supposedly buhl stage is found at the end of the ravine in a great cirque called Jekarica
(fig. 19).
Some of the glacial material, derived from the carbonate rocks of Zaplata (1821 m)
and Srednji vrh (1852 m), is considered to have also been included into the rough
clastic material from the upper reaches of the Vobenca and Koritarica ravines, which,
however, consists mainly of the scree from the slopes. Some of the moraine material
is also assumed to have been present in the wide patch of the clastic material on the
eastern slopes of Kozji vrh (1628 m).
Traces of the former glaciers in the upper part of the Kokra valley have also
been found in the Karavanke mountains. The predominant silicious clastic material
of the rocks, however, makes it difficult to distinguish the slope scree from the glacial
material, in particular, because the striated rocks in it are not characteristic and very
scarce; one single piece has been, in fact, found in the clastic material consisting of
the coral-reef limestone. The deposits at the confluence of Zabukovški potok and Reka
at Dol are, however, undoubtedly of glacial origin, although they are covered with
a younger alluvial fan. The only possible explanation for the large lump of the
coral reef limestone, typical of the glacial boulders, on the werfen ground, which are
derived from Stegovnik (1691 m) and Fevča (1605 m) about two and a half kilometres
away, is that, that they have been brought by a glacier descending down along the
Zabukovški potok. The author concludes, by the analogy with the Stegovnik glacier,
that a large part of the material on the eastern slopes of Veliki Javornik and Mali
Javornik (1723 viz. 1684 m) and in the Tržič ravine is also of a glacial origin, just as
that material found on the southeastern slopes of Veliki vrh (1637 m) and of Virnikov
Grintovec (1654 m).
The author considers that the exarative action of the glaciers in the upper part
of the Kokra valley has not been significant, at least not directly. The indirect
influence of the exaration is best recognized at the end of the Ravenska Kočna valley,
where the glacier has hit softer impermeable rocks on the slopes of the Karavanke
mountains and has indirectly widened the side-valleys in a semi-circular from. The
same situation is observed also at the end of the Makekova Kočna valley and traces
of such an action can be observed also on the right side of the Kokra valley opposite
to Suhi dol.
The marked Hariš terrace on the left side of the Kokra river at Spodnja Kokra,
at an elevation of 570 to 600 metres or some 55 to 85 metres above the river (fig. 43),
is indirectly related to the glacier. The lower part of the terrace lies at an altitude
that has been reached by the Roblek glacier in the periods of both the older and the
wiirm glaciation. The terrace and the slopes above it consist of a very brittle triassic
dolomite that has enabled the Kokra river and its tributaries from the Krvavec
mountain, when flowing on the glacier, to widen with ease the valley and to cut a
broader terrace into the solid rock; its final shape has been formed, however, only
after the linear erosion has dissected the deposited ground moraine. The inclination
of the Haris terrace, from the solid rock slopes to its edge, is a result of the solifluction
of the slope scree on the terrace and of the alluvial fan, which has been spread over
it by the Hribnica brook that drains the Kopa mountain (1024 m), to the south of the
Haris farm.
The periglacial material in the Kokra valley is most typical of the slopes sediments which are mostly of the solifluction origin, but there is also to be found the
fluvio-glacial material deposited in the form of the alluvial fans. The largest and
most significant alluvial fan in the Kokra valley — beside that of the Kokra —
is that of the Bistrica brook at Preddvor that has been formed simultaneously with
the gravel terrace I into which it merges. In the Bistrica fan the older cemented
fan material is also exposed at some places and is apparently equivalent to the
conglomerate terrace II of the Kokra alluvial fan. The same terrace is exposed also
in the lower part of the Bistrica alluvial fan near the Hrib castle. Large amounts
of the slope material are found on the impermeable paleozoic rocks in the Karavanke
mountains. It is impossible to date its origin precisely because of the predominant
silicious composition. It is simpler to date the carbonate material where the author
discerns the old, cemented material (breccia) from the younger one that is not cemented (scree). The scree from the steep impermeable slopes above the Kokra has chiefly
been carried down to the river and transported away, so only some breccia has been
left on unexposed sites among the ravines of brooks and torrents (Stara Povsna) or
on less inclined carbonate slopes.
The periglacial material on the slopes has been generated with particular intensity
in the Kokra valley itself, to the west of the terrace promotory at Spodnja Kokra,
that has been in close vicinity to the front of the Roblek glacier. The eastern part
of that valley, between the Mengar and Logar farms, has been filled with the
carbonate scree, mixed with large rocks, that had been brought down to the valley
from the right side, i. e. from under the Potoska gora; at this point numerous carbonate rocks left on both sides of the valley bottom and on the slopes above it. In
the western part of the valley on more gentle slopes of Potoska gora large amounts
of the firmly cemented breccia are preserved between Potoce and the Zagar farm.
The river Kokra has deposited a wide alluvial fan, when it left the mountains
and entered the lowland of the Ljubljana basin, in the area to the north of Kranj.
The fan extends in the south as far as the Sava river and to the Plana gmajna, to
the river Reka and its large alluvial fan in the east, and to the oligocene hills in
the west, where the contact with the deposits of the Sava river have not been clearly
examined yet. The alluvial fan of Kokra has already been studied by several authors,
most often in connection with the whole Ljubljana basin; only the more recent studies
may be referred to: Rakovec , 1927; Ilesic , 1935; Oblak , 1952; Meze ,
1965; Sifrer , 1969; Zlebnik , 1971. The alluvial fan consists of the partly eroded
older conglomerate terraces, which stick out of the wide gravel plain of Kranjsko
polje, that is classified as the terrace I, the lower layer of which consists of conglomerate. The layer of the gravel is increasingly thinner toward the south and west.
The core of the gravel deposition lies somewhere in the belt extending between Hotemaze, Srednja vas, Sencur and Voklo. The present valley of Kokra is therefore
located in the western part of the alluvial fan and has been formed only after the
wide gravel plain had been cut into the fan. In the author's opinion the old, pre-wtirm
valley of Kokra followed a line directly to the south, approximately in the direction
Srednja vas — Sencur — Voklo — Smlednik. The old river bed of Rupovscica has
been paralel to the existing course of the Sava river from Primskovo to the southeast.
The alluvial fan of Kokra consists of three basic types of fluvial deposits: the
compact conglomerate, the loosely cemented conglomerate and the gravel. The compact conglomerate and the gravel can be further differentiated with regard to the
age, the different age of the material, however, is reflected in the terraces of different
age. The number of the terraces does not correspond with the accumulations, especially not after the deposition of the main wiirm gravel accumulation; there are more
terraces than there have been stages of deposition. The author has identified the age
of individual terraces on the basis of their absolute and relative height but has also
considered: the thickness of the layer of the weathered material on the surface of
the terraces; the degree to which the harder pebbles of primarily volcanic origin are
weathered, using an original method developed by Sifre r (1969, 1972); the degree
to which the conglomerate is cemented; the eventual karstification and the dissection
of the conglomerate terraces; the grades in the size of the fluvial gravel.
The older conglomerate terraces are intensely weathered. Conglomerate was
nowhere found to be exposed in the oldest terrace. There only the weathered material
including some intact pebbles, mostly flints, lies directly on the solid rock. The conglomerate, however, has been found exposed under the cover of the weathered material in the terraces II and III. This material is 2 to 5 metres thick on the original
conglomerate terrace.
There are ten pleistocene terraces cut into the Kokra alluvial fan. The main
wiirm terrace was numbered as the terrace I, whereas the older terraces were numbered from II to »the oldest« and the younger terraces from 4 c to 1. The lowest
lying flat land along the river is of holocene age, to the holocene also partly belongs
terrace

(1) since it is in some places covered with the same flood deposits of fine
sand as the lowest one. The older terraces (the oldest to I) and the terraces 3 and
1 are the result of independent depositions, whereas the terraces 4 c, 4 b, 4 a and
2 have been cut by the linear and lateral erosion into the material of the older
terraces.
The valley of Kokra is below Visoko cut into the compact conglomerate and only
at the confluence with the RupovScica brook at Primskovo some more gravel is to
be found, but not in the river bed of Kokra, which is cut entirely into the conglomerate. In the conglomerate the valley is narrow, deep and marked by steep and
even precipitous slopes, typical of a canyon. The left side of the valley between
Visoko and Preddvor consists of the gravel and probably of the slightly cemented
conglomerate. On the right side to the south of Breg the compact conglomerate is
found. To the north of it first the slightly cemented conglomerate occurs and then the
gravel which extends in a narrow belt all the way to Preddvor and even beyond
that locality.
The gravel deposits decrease in thickness toward the south and east of the alluvial fan of Kokra. At Kranj, the terrace I already consists of the conglomerate which
is, however, covered by a thin layer of the gravel to the east of the town.
The relative heights of the gravel terrace I and of the older conglomerate terraces
increase further down the valley, vhereas the younger terraces are all about on the
same level above the river in its entire course along the alluvial fan and even
away from the Kokra. The inclination of the river bed has been during the older
stages of the formation of the alluvial fan therefore smaller than in the younger
stages, in particular, since the main wiirm terrace came into existence.
The alluvial fan has been intensely dissected by the erosion in the compact
as well as into the less cemented conglomerate before the wiirm accumulation. The
abundant deposition of the Kokra and partly also of the Rupovscica has filled, during
the wiirm period, both the old valleys and also wide stretches of the alluvial fan.
The subsequent intense linear erosion has started when the Kokra has been flowing
along the western part of the alluvial fan. The post-wurm erosion has nearly reached
the present bottom of the Kokra valley, but it has been three time interrupted by the
lateral erosion, which has cut under the terrace I three successive terraces (4 c, 4 b and
4 a) in the gravel and even into the underlying conglomerate. The repeated deposition
consisting of the coarse gravel, which is found in the terrace 3, has been some 10
metres thick and a later erosion has cut into its side the terrace 2.
In the period when the last independent stage of the deposition, which has resulted in the formation of the terrace 1, has been taking place, a mighty collapse of the
conglomerate slopes in the Kokra canyon below Primskovo has occurred and dammed
the river Kokra and has thus caused the formation of a lake in the area of the confluence of Kokra and Rupovscica. This is proved by the sandy loam layers in the
terrace 1 along the Kokra (fig. 39) and the Rupovscica (fig. 38) at the confluence.
The leyers of sandy loam descend into the terrace I from the impermeable tertiary rim and from the thick, weathered material of the older conglomerate terraces.
Close to the rim these layers are rather deep and incline toward the middle of the
alluvial fan. Such sandy loam and finer inserted gravel layers are mostly found in
the southern and southeastern part of the Kokra alluvial fan, which consists of the
material derived from the old conglomerate terrace of Plana gmajna and from the
tertiary Tunjice hills in the north. On the site of the Ljubljana airport two layers
of sandy loam and clayish gravel have been discovered by the borings. The first is
12 to 16,6 metres thick and the second more than 35,5 metres. The layer of sandy loam
is also found at several places on the surface of the gravel terrace I (between Cesnjevek and Olsevek; to the southeast of Preddvor) and it is responsible for the wet
ground and consequently for the less valuable land and crops that are cultivated
there.
In Kranjsko polje (the Kranj field) the pleistocene fluvial sediments are very
thick. The Kokra river along its whole course from Hotemaze to the confluence with
the Sava river nowhere reaches the underlying solid rock. 2 1 e b n i k (1971) established, on the basis of numerous borings, that the total thickness of the pleistocene sediments is almost everywhere in the middle of Kranjsko polje at Sencur, Voklo and
Voglje over 100 metres.
The gravel terrace I is related, according to the author, to the wiirm glaciation.
Traces of this terrace are preserved in the valley of the Kokra river above Preddvor
only at one spot, on the left side opposite to Potoce. In other places in the narrow
valley with steep slopes all the traces of this terrace have been removed by abundant
waters of the Kokra river. Two more recent depositions, their result are the terraces
3 and 1, are most probably related to the post-glacial stages of the retreat. The compact moraine is considered to have been related to the deposition of the conglomerate
terrace II. However, no moraines that might be related to two older conglomerate
terraces have been found. The author emphasizes the fact that traces of the old
moraine in the narrow valleys of the Kokra basin are scarce, therefore it is impossible
to classify them exactly. For that reason the compact moraine has been described
as the »older moraine«, but the author permits a possibility, that it might be of
different age on different places. The slightly cemented moraine of the Jezersko
glacier (which is exposed along the steep road between Spodnje and Zgornje Jezersko
and, partly, at Podlog) might be related to the deposition of the slightly cemented
conglomerate. At Podlog is beside the recent uncemented moraine also the older
cemented moraine that is only slightly cemented and without any weathered material
in the soil. Such weathered material has neither been found in the contact of the
compact and the recent moraine. Any relation between the slightly cemented moraine
of the Jezersko glacier and the slightly cemented conglomerate is therefore problematic and subject to many conditions to be examined in further researches.
In the author's opinion the remnants of the eroded conglomerate in that part
of the valley of Kokra, which has been affected by the older glaciation of the Suhi
dol and the Roblek glaciers, must be in connection with the initial stage of the older
glaciation, when the glaciers have not reached the Kokra yet, but the abundant
waters derived from below them have already carried away large amounts of the
gravel and have deposited it down in the valley. When the glacier has later reached
the deposited gravel has not carried it away but has gone over and has left on it
the moraine when again retreated. The subsequent erosion in the period between the
older and the wurm glaciations has cut deep into the moraine and into the conglomerate but has not completely removed them either. Both the cemented and the wurm
moraines are found on the slopes above the Kokra, but the latter is also found on
the bottom of the valley under the remnants of the eroded conglomerate.
The author gives a similar explanation for the remnant of the gravel on the
promotory, where the Suhi dol and the Kokra converge and where the boulders of
the wiirm moraine have been found on the top of the gravel. The gravel has been
presumably deposited behind the Roblek glacier, which has reached the Kokra earlier
than the Suhi dol glacier, and has been later covered by the wiirm moraine of the
Suhi dol glacier.
There are proofs beyond doubt that the gravel deposition and the formation
of the periglacial scree material on the slopes occurred during the cold sections of
the pleistocene period. The géomorphologie investigations made by the author could
not establish to what an extent tectonic movements were involved in the formation
of the pleistocene deposition terraces of the alluvial fan of the Kokra river; 2leb -
nika geologist (1971) considered them to be of great, indeed of decisive importance.
The author considers that these movements have undoubtedly played a certain role,
although not necessarily so important as assumed by 2 1 ebnik. The problem could
be solved by further research cosidering the relation between the climatic conditions
in the genesis of the pleistocene deposition terraces, which — in the author's opinion
— have been of primary importance, and the eventual and partial influence of the
tectonic movements, which might have been, however, also destructive.
On the northeastern edge of the alluvial fan of Kokra, between Cesnjevek and
the rim of solid rocks at the foot of the mountains, are large deposits of the sandy
loams and of the clay, used in an industrial brick-works. These deposits are located
among the large alluvial fan of the Reka brook, the gravel terrace I, the old conglomerate terrace III and the impervious rocks of the mountains consisting mainly of
clayish and marly shales and of sandstones. The clay and the sandy loam are not
homogenous but mixed with thin layers of the fine gravel and of sand from the rim.
In the author's opinion the fine clastic material is a fluvial marshy sediment, which
has been deposited because the Usica brook and its two tributaries, the Lebrant and
the Cesnjevica, have been dammed by the accumulation of the Kokra gravel during
the formation of the terrace I and by the deposition of the alluvial fan of the Reka
brook.