Kvartarni razvoj doline Rašice in Dobrega polja

Abstract

Our investigations in the valley of Rašica and Dobro polje have confirmed the previous findings that the valley of Dobro polje had already been formed in its present main outlines towards the end of the Pliocene and that during the Quarternary it was filled with the sediments onlv that were
brought with an increased alluviation by the Rašica brook from the upper
part of the valley (1).
An interesting contribution to our present knowledge is the discoverv
of siliceous sands and gravels that intermix on the upper slopes of the
Dobro polje valley with a brownish-red loara and the occurrence of siliceous
gravel in a Karstic depression some 50 m above the Rapljeva village (462 m).
A very small admixture of mud and loam in this gravel allows us to conjecture that after its deposition this alluvium did not undergo any essential
changes and that it never contained any larger quantities of carbonate rocks.
This gravel resembles closely the Pliocene sediments in the basin of Novo
mesto and it is believed that it originates from a period when the dissolution
of carbonates was exceptionally intensive so that almost exclusively the
more resistant grains of flint were only brought to this plače.
Essentiallv different is the accumulated material that in the valley of
the Rašica brook and of Dobro polje covers the lowermost floor of the vallev
as well as the first terraces, the highest of which reaches ca 25—35 m above
the lowermost surface of the valley. Al i this alluvion differs from the Pliocene
material above ali in the dimensions of the gravel which in it is not on the
whole considerably thicker, so much so that pieces of gravel more than 20 cm
thick are by no means rare. Furthermore, the alluvion contains besides the
siliceous gravel also manv pieces of siliceous sandstones. of iron ore. and of
slates. Finally, carbonate rocks were also deposited in large quantities into
this alluvion: as a matter of fact, in the area here discussed thev are stronglv
prevalent.
By way of a detailed analysis of this material we have been enabled to
distinguish approximatelv three main phases of accumulation that differ
from each other both in the extent of the deposition and in the degree how
the material is mouldered.
The oldest deposition of this group is preserved on a broad terrace that
is cut into the bedrock and occurs in the vallev of the Rašica brook and of
its tributaries as well as in the Dobro polje valley some 25—30 m above the
lower terrace that is equally cut into the bedrock and is covered by the
gravel of the younger (II) phase of accumulation. Some 5—8 m below the
latter (II) terrace follows the youngest level of accumulation (III) that covers
above ali the strongly eroded parts of the II n d
phase of accumulation.
The surfaces covered by the gravel of the first phase of accumulation
(I) are alreadv stronglv karstified. A similar situation can be seen within
the sphere covered by the gravel of the second phase of accumulation (TI)
with the onlv difference that here — especially in those places where
gravel is deposited in a thicker laver — one can find surfaces that are
even more level. Completelv level are stili the surfaces that are covered by
the IIIrd phase of accumulation. Nevertheless one can find also in tliese
surfaces alreadv individual sinks and potholes. Some of tliem are so young
that their emergence can be remembered by the present inhabitants.
The whole material of the first phase of accumulation (I) is alreadv
strongly mouldered. Thick pieces of iron ore, of siliceous sandstones, and
thinner pieces of flint gravel have only been preserved among the brownishred loam that resulted mainly as a consequence of the mouldering of carbonate rocks. A corresponding gravel in the primary petrographic composition with prevalent carbonate rocks has been found in a cave only some 200m north of the Rašica village where it occurs under a thick stratum of
sinter. scree, and of a red mouldered material.
The gravel of the II1"' phase of accumulation has its mouldered material
onlv 3—4 m thick. Under it occurs the gravel in its original composition. with
carbonate rocks prevalent. The mouldered material of this gravel resembles
closelv the mouldered material of the first phase of accumulation. with the
only difference that the loamy remains of carbonate rocks are of a more yellowish colour and that among the mouldered material we ean find not
only the more resistant pieces of ročk but also -— though rare — better
resistant pieces of gravel composed of dolomite.
The gravel of the third phase of accumulation (III) bears a stratum of
mouldered material that is only 0,3—t m thick. I n the lower part of the
gravel the carbonate rocks prevail, among tliem predominantlv the dolomite
gravel among which we can also find few pieces of gravel composed of
sandstone, iron ore, flint, and red slates.
We have been able to establish the extent of lateral incisions during
the individual phases of accumulation above ali on the basis of a very
detailed analysis of effects of the third phase of accumulation (III) which
is supposed to have been started by the same causes that had also led to
the older alluviations. These investigations have shown that the effects of
latteral incisions were at the time of this alluviation quite insignificant: the
river Rašica was at that time not able to undercut, and even less so to carrv
away, the most exposed fragments of the older alluviation (II).
Such a humble erosion during this phase (III) leads us to suppose that
the two terraces cut into the bedrock which are covered by the gravel of the
first (I) and of the second (II) phases of accumulation must be older than
the gravel alluvion that covers them.
In ali probability these two terraces go back to the Pliocene, a fact
which is perhaps proved by the siliceous gravel found directly above the
upper terrace as well as by the characteristic distribution and extent of the
bedrock terraces that cannot be explained with the fluviatile processes
during the one or the other alluviation. The supposition forces itself upon
us that in the formation of such extended terraces the processes of corrosion
must have had an important role. This is particularly true for the broad
terrace that is covered near Ponikve and in the vallev of Dobro polje bv
the gravel of the II nd phase of accumulation: here we must also reckon with
verv frequent inundations and with a permanent soaking of the ground
during the Pliocene. If we are on a right way with these conclusions, then the first alluviation should have been the most extensive so that the strata of gravel more
than 30 m thick could have been deposited. After an extensive mouldering
and removal of large quantities of this gravel (I) only, this could have been
followed by a new alluviation (II) which. however, should have been less
extensive so that it should have stopped some 20 m below the upper limit of
the oldest first phase of accumulation. An even smaller extent should have
had the third (III) phase of accumulation which had covered with its gravel
the most lowered and eroded parts of the fan of the II n ( i
phase of accumulation.
A more precise determination of the age of these individual phases of
accumulation has been Ieft in this studv completely open. The suggestion
has only been expressed that these alluviations had taken plače during the
older parts of the Pleistocene and that the third phase of accumulation seems
to belong to the last Glacial Epoch. Perhaps it wil l not be wrong if we plače
the traces of the second phase of accumulation into the Riss Age, a supposition which seems also to be supported by a similar mouldering of material as it can be found in the comparative fluvioglacial terraces in Gorenjska
(Upper Carniola) along the Sava river and its tributaries. Much more problematic, however, is the age of the largest, the first phase of accumulation. We can only make suggestion that the material in this accumulation belongs
to one of the Pre — Riss glaciation periods (MindelP).