William Morris Davis, Anton Melik, level top ridges and slope processes in Slovenia

Abstract

Professor Anton Melik (1890–1966) as the leading Slovenian geographer (1927–1965) was a crucial person who, with lectures in geomorphology and publications, introduced the concept of erosion cycle of W.M. Davis into Slovenian geomorphology. His morphogenetics of the mountainous part of Slovenia is also based on the Austrian geologists and Slovenian geomorphologist I. Rakovec. For the hilly areas on the margin of Slovenia he expected the best results from using the concept of W.M. Davis and Serbian geomorphologist Jovan Cvijić who studied terraces in northern Šumadija on southern border of the Pannonian Tertiary Sea.

In the 1950s A. Melik sent his assistants and collaborators to map terraces of the Slovenian hills in the drainage area of Danube’s tributaries Mura, Drava and Sava and in the hinterland of the Adriatic Sea – Bay of Triest (Slovenian part of Istria and Brda of Gorizia – Gorica). In these hills we found no proper Pliocene terraces and niveaus. Instead of them, we proclaimed the horizontally leveled tops of ridges for the remnants of primary surface. According to the mapped level ridges, published mostly in the Geografski zbornik, they have even in the 60 km long hills of Slovenske gorice the same altitude and no inclination along the main rivers.

The present article denies the opinion on level top ridges as an indicator of the primary surface. According to the slope processes theory they are the result of the permanent surface lowering by chemical erosion. On the tops of ridges the chemical erosion of the ground water is the only process, as on the water divide the rainwater flows below the surface and there is no surface run-off. But on both sides, this water comes out in springs on the upper slopes. The lower parts of slopes are formed by soil erosion and occasionally faster movements of the weathered mantle (creeping, sliding).

The above-mentioned hills in the hinterland of the Bay of Triest are built of Eocene flysch and in E and NE Slovenia of Neogene marls, sands and clays mostly. As they are more or less carbonatic, the author of the article calculated chemical erosion from mean total hardness and run-off. It amounts between 15 and 30 m3 of CaCO3 and MgCO3 /km2 /yearly, what is equivalent to 15–30 m of surface lowering in one million years. The lowering of surface lasted presumably up to 4 million years, from Lower Pliocene onward.

In the Velenje Basin the hills are built of Pliocene sediments. The depth of the valleys from 40–90 m is also common in the mentioned border hilly areas. Neotectonical subsiding of Velenje Basin is proved by 1000 m thick Pliocene and Quaternary sediments, and is also typical for wide main river valleys and basins in hilly areas mentioned above.

The opinion on the uninterrupted chemical and mechanical erosion of surface is confirmed by geological profile of the Miocene/Pliocene anticline of Kog (Slovenske gorice). From it nearly 900 m of sediments were removed during the Upper Miocene, Pliocene and Quaternary

Spacing of valleys on the cross-section through hilly regions are compared in a special table, where data on the yearly precipitation, potential evapotranspiration, run-off and depth of the valleys for three hilly areas in Slovenia and one in Šumadija (near Belgrade, Yugoslavia) are enlisted, too. A close correlation between the spacing of valleys (this means width of the level surface in between) and run-off is evidently explained.

The role of neotectonics is also evident in the basins of Dravsko polje and Ptujsko polje. The Drava River formed in the same Tertiary sediments at Limbuš (W of Maribor) 1 km wide valley bottom and 30 km to the SE in the Dravsko polje a 15 km wide plain of gravel with different thickness. The character of neotectonic basins is partially evident in the Mura »valley« in Slovenia, in the Celje and Krško-Brežice (Krka) basins and in the basin of Preval in Brda. Lowering of the crests in Slovenian Istria to the west is the result of the oblique tectonic sinking at the flanks of the Friulian basin.

The relief in Slovenia pertains to the Alpides where the active contacts of Eurasian, Adriatic and Pannonian (micro) plates with pressure provoke the local sinking and contemporary rising of the surface. The concept of erosion cycles as predicted by W. M. Davis can be therefore reasonably adopted in Slovenia only in regions where Neogene and Quaternary tectonic stability is assured.