H geomorfologiji Bele krajine

Abstract

Bela krajina is a peripheral Slovene region lying on the contact
of high dinaric karst plateaus and of the low karstic peneplains which
extend along the rivers Kolpa, Dobra, Mrežnica and Korana and surround
the area of unpermeable deposits of the basin of Karlovac. According
to older geomorphological studies (Seidl and Krebs) the relief ob Bela
krajina is considered to be the result of young and very young tectonic
movements which caused the uplift of the whole mountain rim and,
consequently, the dissection of the Pannonian peneplains. Because of
the uneven uplift the differences of their relative heights are now as
much as 700 m. The author has based his study on the investigation of recent geomorphological processes, and on an analysis of the smaller features of karst topography. He did so, because he supposed that the correlation of distant and discontinued surface levels is not enough reliable.
He paid a special attention to dry valleys which are a very common
feature of the Higland rim as well as of the low-lying area of the Črnomelj level. This flat area is separated from similar l'ow plateaus further to the east by a group of hills, which make it appear as the bottom of
a basin. There are rests of the surface water courses in the form of springs and
small brooklets in the upper parts of nearly ali dry valleys. The original
fluvial action in the formation of these valleys is out of question. Two
of the dry valleys lost their water courses only during the historical
tirne. In one valley, a part of the town of Metlika is situated, in the other
(NW of Črnomelj) a village called Gorenja (upper) Paka lies in its
upper part and another, Dolenja (lower) Paka, at the end. The name
»Paka« is given in Slovenia to the villages near a stream. The author thinks therefore that a brook must have flown past the two villages in the time when they have been founded.
Signs of fault-lines can be established in the direction of most dry
valleys. In the highland rim most dry valleys lie in the same direction
as the straight tectonic edge of the Črnomelj level. These dry valleys
are also important for transportation, because they represent the easiest
routes leading from Bela krajina to the rest of Slovenia. Recent
research has not confirmed the existence of a unique fault-line running
from Upper Krka across Crmošnjice valley and along the eastern edge
of the lignite basin of Kanižarica towards SE. A zone of faulting,
however, has been ascertained along the river Kolpa, continuing
towards the hills of Rožni dol in the NW and into the rift—valley
of Ribnica. A faulted zone which runs from Metlika to Jugorje has not
been explored enough.
It is a fact, somewhat surprising from the point of view of theoretical geomorphology, that streams in Bela krajina tend to avoid loose loamy and gravelly sediments, and have cut their gorges in the nearby
limestones inspite of being far more resistant to erosion. Another
apparent anomaly is the fact that both tributaries of Kolpa, Lahinja
and Krupa, have half as small inclination of their longitudinal profiles than Kolpa that carries much more water. The only one justified reason is, according to author's opinion, difference in the size of the
river load. Karstic streams like Lahinja and Krupa carry only loam,
while Kolpa carries large pebbles (because the upper course lies in an
area of non-limestone permo-carboniferous rocks). These differences
appear also in the composition of the pleistocene deposits which, in Bela
krajina, are certainly preserved in one, and possib!y in two series of
deposits. Erosion levels (Map I), which have been studied so far, do not
confirm a sinking of the Črnomelj peneplain after the level of 200—
220 m has been formed. Therefore the relatively lower terrain along
Lahinja, compared with that along Kolpa, is explained by the author
as a result of uneven inclination of the longitudinal profile of both
rivers. Low lying sources of Lahinja present a possibility that, in the
future, they may draw Kolpa towards the middle of the Črnomelj
peneplain by means of a karstic, underground river capture. The great
bend of Kolpa, between the southernmost point of Bela krajina and
Karlovac, might have been very likely caused by such a subterraneous
river piracy. One of its karstic tributaries, ,which also must have had
the former headwaters lying lower than Kolpa, the main river, was
probably responsible for the capture.
New geological explorations of the 400 m thick layers of the neogene sediments have given a strong support to Seidl's presumption that the loose neogene sediments, now restricted to a structural basin some
3 km long, were originally spread over much wider area. New profiles
(Map II) show that lignite lies parallel to the faulted cretaceous limestone substructure. Numerous fault-line crossing that basin date consequently from the time after the deposition of the pannonian layers
and before the erosion level of 200—220 m had been formed. This
surface extends uninterrupted across both the limestone rim and the
neogene sediments of the structural basin. An original cover of unpermeable neogene sediments, 400 m thick and covering a wide area, must be taken into consideration. The sediments have later been sunken in
the area of the structural basin and thus remained preserved there. At
the time when the neogene sediments stili covered much wider area
of Bela krajina, an active corrosion took plače in the bordering limestone area. Erosion by streams, coming from the unpermeable neogene area, was also in action. An analysis of the terraces and rests of the
heigh plaines is depicted on map I. The two terraces at the heigh of
150—160 m and 160—175 m along Kolpa are cut into loam and flint
sand, that one along Lahinja is cut into loam only. The first aggradational terrace is better preserved only near the source of Lahinja. Kolpa was responsible for the damming of Lahinja and the following deposition along Lahinja. These deposits are under arable and meadows, while
the terrace of 160—165 m is covered by sparse and degraded wood
(»steljniki«) which is used also for pasture. The most significant peneplain surface is that of 200—220 m, which is well cultivated and
inhabited more densely than the average for Bela krajina. This surface is
also considerably more karstified than those below it. According to
author's view a larger span of time lies between the formation of the
200—220 m surface and those at a lower level.
Smaller karstic forms reveal a dependence on the strike and dip
of ročk strata. The surface streams flow in the direetion of the strike
and even more along the dip of the strata. Where the streams flow
in another direetion, many oblong incised meanders occur where the
bends follow either the direetion of the dip or that of the strike of
strata. In the dolines, the less inclined slope is that of the dip.
Further on, the author presents the results of the speleohydrographical explorations (especially on the piesometric level) in the western part of the Črnomelj peneplain. He also includes some observations on soil
erosion and points out the fact that there are no sharp boundaries
between the bright-red karstic loams in the lowlands and the brown
soil in the highland. There is in fact only a slow transition, dependant
on climatic conditions, plant associations, depth of soil and its humidity.
In the closing paragraphs the author considers two local topographical terms: draga and vrtača. The former is used for what the
term »doline, dolina« is used in the international literature. The latter
is a special form of the doline, which is larger and where the water
appears at the bottom.