A polje (an internationally adopted karstological term of Slavic origin: polje) is the largest, most complex, and most diagnostic closed surface depression in karst landscapes. It serves as a central focal junction between surface and subterranean drainage, representing the hallmark feature of Classical Karst.
1. Definition and Conceptual Approaches
A. General Geomorphological Definition
A polje is an extensive, closed or semi-closed depression in karst terrain featuring a remarkably flat floor demarcated from steep surrounding slopes by an abrupt basal slope break (knickpunkt). Its evolution is driven by the prolonged interaction of tectonic subsidence, lateral fluvial erosion (planation), and chemical solution (corrosion) of carbonate rocks at the boundary between surface and subsurface water flows.
B. Gams's Criteria (Ivan Gams, 1973, 1978)
Slovene karstologist Ivan Gams established rigorous defining criteria that distinguish a true polje from uvalas, blind valleys, and sinkholes:
- Karst water circulation: The depression must feature karst drainage—either a karst inflow, a karst outflow, or seasonal inundation driven by karst groundwater.
- Planated alluvial or karst floor: The floor is flattened and commonly veneered with semi-permeable or impermeable Quaternary deposits (alluvium, clay, gravel, or glacial moraine).
- Distinct marginal slope break: A sharp morphological junction separates the steep surrounding walls from the level floor, lacking a continuous concave basal transition.
- Floor dimensions: The width of the level floor typically measures at least 0.5 to 1 km (with minor exceptions for alpine micro-poljes).
C. Conceptual Frameworks
- Hydrological Concept (Jovan Cvijić): Focuses on the polje as a hydrological hub where surface streams emerge from karst vauclusian springs, meander across an impermeable floor, and sink into ponors, swallow holes, or estavelles at the opposing margin.
- Structural-Tectonic Concept (Herbert Lehmann, Marjorie Sweeting): Considers the polje primarily as a tectonically preconditioned graben or downfaulted depression whose margins and floor were subsequently widened and planarized by solutional processes.
- Climatic-Corrosive Concept: Emphasizes lateral solution (marginal corrosion), where chemically aggressive water issuing from non-karst catchments or alluvial plains undercuts carbonate slopes, systematically expanding the flat floor.
2. Comprehensive Typology of Karst Poljes
I. Types by Geological Structure and Lithological Contacts
- Border Polje: Positioned precisely at the contact between an impermeable (fluvial, flysch, or palaeozoic) catchment and permeable karst terrain. Streams cross the impermeable plain and sink as soon as they encounter pure carbonate bedrock.
Examples in Slovenia: Cerknica Polje, Radensko Polje. - Structural / Tectonic Polje: Morphology and orientation are directly dictated by regional fault zones and graben structures (in Slovenia, predominantly aligned in the Dinaric NW–SE direction).
Examples in Slovenia: Kočevje Polje, Ribnica Polje, Planina Polje. - Base-Level Polje: The depression floor has been lowered by corrosion down to the regional karst water table. Any rise in the regional water table generates immediate hydrological flooding through bottom springs and estavelles.
Examples in Slovenia: Lož Polje, the basins of the Intermittent Lakes of Pivka. - Blind Polje: Entirely enclosed within carbonate rocks without any surface inflow from external non-karst catchments; recharged exclusively by subterranean cave springs and drained via swallow holes.
Example in Slovenia: Planina Polje (recharged from Planina Cave via the Unica River).
II. Piedmont and Alpine Karst Poljes (Glaciokarst)
- Piedmont Polje (Foothill Polje): A distinctive polje situated at the foot of mountain massifs or within high-alpine glaciokarst plateaus. Its level floor is maintained by thick glacial (morainic) and fluvioglacial deposits, which form an impermeable veneer that prevents immediate vertical drainage into deep karst fissures.
III. Types by Hydrological Regime
- Throughflow / Overflow Polje: Operates as an intermediate transit station within a stepped karst river cascade. It receives discharge from an upstream karst system, channels it across the floor, and drains it through ponors toward a downstream base level.
Examples in Slovenia: Planina Polje, Radensko Polje. - Piezometric (Pressure) Polje: Lacks massive perennial surface inputs, filling hydrostatically "from the bottom up" via estavelles and boiling resurgences when the karst groundwater table surges.
Examples in Slovenia: Cerknica Polje (resurgences like Cemun and Kotel), Lučki Dol. - Dry (Fossil) Polje: Due to incision of subterranean conduits or tectonic shifts, it has permanently lost its perennial surface water; the floor remains dry and is commonly pocked with dolines.
Examples in Slovenia: Babno Polje (a notorious frost hollow), Dobrepolje, Zadlog above Črni Vrh. - Stepped Polje System (Polje Cascade): A succession of connected poljes arranged at descending elevations that pass water subterraneanly along a single catchment basin.
Classic Example: The Catchment of the Seven Names of the Ljubljanica River: Babno Polje (750 m) → Lož Polje (580 m) → Cerknica Polje (550 m) → Planina Polje (450 m) → Vrhnika Springs / Ljubljana Marsh (300 m).
3. Comparative Summary of Karst Polje Types
| Category | Polje Type | Primary Genetic / Hydrological Mechanism | Example in Slovenia |
|---|---|---|---|
| Lithological Contact | Border Polje | Boundary between impermeable catchment and carbonate rocks | Cerknica Polje, Radensko Polje |
| Tectonics | Structural Polje | Subsidence along major fault lines and tectonic grabens | Kočevje Polje, Ribnica Polje |
| Groundwater | Base-Level Polje | Floor planated to regional karst water table | Lož Polje, Pivka Lakes |
| Karst Setting | Blind Polje | Enclosed within carbonates; recharged purely from karst springs | Planina Polje |
| Alpine Relief | Piedmont Polje | Flat floor sealed by glacial and morainic deposits at mountain bases | / |
| Hydrology | Throughflow Polje | Transit basin between upper and lower karst hydrological stages | Planina Polje |
| Hydrology | Piezometric Polje | Flooding driven from below via estavelles and boiling springs | Cerknica Polje |
| Morphology | Dry (Fossil) Polje | Abandoned surface drainage, deeply dropped groundwater table | Babno Polje, Dobrepolje, Zadlog |