Morfologija šavrinskega gričevja in njegovega obrobja

Abstract

Greatest part of the NW corner of Istria is composed of the Savrini
Hills along rivers Rižana and Rokava (Dragogna). This hilly area, roughly
NWof Buzet is built of beds of eocene flysch (sandstones and partly marls)
deposited in the form of a syncline (the syncline of Trieste) but later subject to folding. A fault line rim ti i g WNW—ESE along the lower Rokava sharply separates Savrini Hills area from the mezozoic limestone plateau
of Kras of Buje in the south. On the NE side i,s an area, 3—5 miles wide,
of recumbent folds with alternating beds of eocene flysch and limestones
dipping NE. It is partly faulted on the western side and of plateau-like
character (Kras od Podgorje). It is a transitional belt between Savrini Hills
and eocene and mezozoic limestone mountain area of NW čičarija rising
steeply above it.
Morphogenetic processes leading to existing landforms started by dissecting of the major peneplain surface at the height of about 500 m. It cuts across both central flysh zone as well as surrounding limestone area reaching eastwards to the foot of Čičarija mountains, standing above that surface. Čičarija was the source region of two main streams starting the
dissection of the 500 m surface, when the uplift began in pliocene, while
a third major stream began to develop in the entirely flysch-built zone
between both. The first of these streams developed along the fault line in
the south side on the boundary betvveen flysch and limestones (the pliocene Rokava). The second developed on the north eastern side, roughly on the zone of alternating flysch-limeistone beds (the pliocene Reka). The
third stream began to developed between them roughly in the middle of
the flysh-zone (Pliocene Rižana). Nine surface Ievels in the area were
found down from the peneplain surface of about 500 m. Karstification on the
limestone area went hand in hand with the dissection by normal river
erosion, the latter being the only procces on the continuous area of flysch.
Some main stages in the evolution of landforms and drainage can be
discerned. Initial dissection of the 500 m surface was strongest along the pliocene Rokava and Reka rivers. In the area of recumbent folds flysch belts have soon been largely stripped away, being thinner at the edge of
the synclinal zone and most exposed to denudation. This led to the formation of two lower escarpments with broad, but shallow valleys between, cut 50 and 100 m below the 500 m surface. Then karstification prevailed
and stopped normal surface drainage and dissection, leaving the valleys
dry. Thus the upper part of the drainage basin of pliocene Reka in the
eastern part was put out of function and remained active longer only on
that part of recumbent folds lying more towards the west, because of the
flysch bed s being thicker there. The pliocene Reka began to cut down
along the north eastern edge of continous flysch zone and started to form
the great escarpment on the contact with the paeket of recument folds,
this being built mostly of limestone (Kras of Podgorje). Along the pliocene
Rokava upper left confluents also successively disappeared underground
along the contact with limestone area of the Kras of Buje. These separated
sireamlets continued for some time cutting down below the abondoned
bottom plain of the original river but were finally left as blind valleys,
with bottoms progressively lower towards the west. Thus the drainage
basin of pliocene Rokava has been continuously shrinking in the headwater parts of them. Along Rokava only more western remaining confluents of the river could progressively cut backwards, approaching the headwaters of the third mam stream, developing in the nliddle of the flysch
zone, roughly along the line Trebeše—Marezige—Koper. At the stage of
300 m surface one cvf the left confluents of pliocene Rokava captured the
headwaters of pliocene Rižana and later one of its confluents stili another
part of the upper Rižana drainage basin streamlets.
On the other hand one of the confluents of pliocene Rižana captured
the remaining part of the upper course of pliocene Reka at the stage of
200 m terrace level. Thus the present devide between Rižana and Reka came
into existence. Further morphogenetical evolution was characterised by
cutting down of streams along the river net existing stili today (except for
submerged lower courses). Submersion of main river mouths in pleistocene
time was later followed by the deposition of alluvial plains which is stili
in progress.