CELEBRATING RJEČINA DAY – HAS THE MYSTERY OF THE PEBBLES IN RJEČINA CANYON FINALLY BEEN RESOLVED?
On last year’s Rječina Day we called attention to some interesting pebbles on the canyon’s edge, located about one hundred metres above the river’s current level. This year we invited geologists to take a look at this interesting phenomenon and here’s what we discovered.
GEOLOGICAL POINT OF INTEREST IN RJEČINA CANYON (on the occasion of Rječina Day)
While exploring Rječina Canyon near Strmica at Trsat a few years ago, we came across some interesting, rounded rocks – pebbles, gravel and blocks of various sizes, shapes and lithological composition. For example, we found carbonate rocks (foraminifera fossils can be seen in some of the limestone pebbles) as well as sandstone rocks, and there were also conglomerates in which small-sized quartz pebbles could be seen in the predominantly carbonate binder (see: www.ju-priroda.hr/zanimljivosti.asp?id=2zanimljivosti/gea/6-dan-rjecine.html). We first assumed that these materials, very diverse in terms of lithology, which we found on the canyon’s steep slopes (as high as 100 metres above the present-day level at which the Rječina River flows through the canyon below Trsat) were deposited by some ancient watercourse, perhaps by a paleo-Rječina.
Geologists Ljerka Marjanac, PhD, at the Institute of Quaternary Palaeontology and Geology of the Croatian Academy of Sciences and Arts (CASA), and prof Tihomir Marjanac, PhD, at the Geology Department of the Faculty of Science Zagreb, accepted our invitation to come study this phenomenon. After reconnoitring the area for only a short while, we discovered possible evidence showing that possibly we were not dealing with fluvial deposits but rather with glacial deposits. So, it seems that the pebbles were not carried to that spot by a watercourse but by ice in an ancient glacier! Clear evidence of this would be glacial striations* on the surface of pebbles and cobbles, together with specially shaped pieces of differing rocks, so called bullet-shaped clasts and faceted clasts, that did not receive their round form through the action of water but through the action of glaciers. Because the canyon of the Rječina River is a tectonically disrupted area and is still tectonically active, it is possible that other processes could have caused the shape of these sediments. So the final interpretation will require further in-depth geological research. Findings of sediments bearing the features of glacial sediments (till*) could shed new light on evaluating and understanding the range and distribution of glaciers in the environs of Rijeka, as well as of the ice sheets on the mountains surrounding the Rijeka Bay at the time of the Late Pleistocene glaciations more than 250,000 years ago and the last Ice Age some 20,000 years ago. At present, very little is known concerning this issue and it would be worthwhile researching it more closely. Hence, plans have been made to carry out an in-depth study of the site.
*Glacial striations are narrow groves, gouges, scratches and cuts in rock or pieces of stone (clasts) created by the abrasion of rock material incorporated into the ice of a moving glacier or by the scouring of the bedrock over which a glacier flows. Incorporated into the glacier are rocks that it has broken off and transports (moraine material), and the scraping of the rocks, embedded in the base of the glacier, creates striations. It is fairly easy to distinguish between striations caused by tectonic or anthropogenic activities.
**Till is an incoherent, chaotic and poorly sorted sediment containing rounded pieces (clasts) of different types of rocks with glacial striations and a muddy/sandy foundation into which the clasts are incorporated. Till is the material which a glacier transports while it moves and expands. When the glacier begins to retreat, this material is deposited on the bedrock over which the glacier flowed. The resulting layer of till is also called ground or bottom moraine.
Key words: Rječina River, canyon, glaciation, glaciers, glacial striations
Marko Randić (“Priroda”), Ljerka Marjanac (Institute of Quaternary Palaeontology and Geology of CASA) and Tihomir Marjanac (Geology Department of the Faculty of Science, Zagreb)