A sinkhole (in international karstological terminology, the Slovene and Slavic term dolina is widely used) is the fundamental, most widespread, and most recognizable closed depression on the karst surface. It is bowl-shaped, funnel-shaped, or cauldron-shaped. Its diameter typically ranges from a few meters to several hundred meters, and its depth from a few meters to several tens of meters.

1. Morphological Classification: Shapes of Sinkholes

Based on the ratio of diameter to depth, the slope gradient, and the shape of the floor, karstology distinguishes several characteristic geomorphological types:

  • Bowl-shaped (or saucer-shaped) sinkhole (skledasta vrtača):

    • Characteristics: A shallow depression with very gentle, sloping sides and a broad, rounded bottom.

    • Ratio: The diameter is substantially larger than the depth (often in a ratio of 5:1 or even 10:1).

    • Comparison: Compared to a funnel-shaped sinkhole, a bowl-shaped sinkhole has significantly gentler slopes, is entirely covered with soil and continuous vegetation, and lacks a prominent ponor (swallow hole) at the bottom. It is common on flat plateaus with a relatively thick mantle of regolith.

  • Funnel-shaped sinkhole (lijakasta vrtača):

    • Characteristics: Slopes converge uniformly at a relatively steep angle (often between 20° and 40°) toward a narrow, distinctly constricted floor or ponor, giving it the appearance of a true funnel.

    • Ratio: The diameter-to-depth ratio is lower (typically between 2:1 and 3:1).

    • Comparison: In contrast to a bowl-shaped sinkhole, a funnel-shaped sinkhole is narrower and deeper, its slopes are steeper, and the soil layer is thinner or locally interrupted by outcrops of bedrock. Surface runoff drains much faster directly toward the localized swallow point at the bottom.

  • Cauldron-shaped (or pot-shaped) sinkhole (kotlasta vrtača):

    • Characteristics: A very deep depression featuring steep, partially rocky slopes and a relatively flat or slightly concave floor.

    • Ratio: Depth can be comparable to the diameter itself (a ratio close to 1:1).

    • Comparison: It represents an intermediate stage between a regular solution sinkhole and a collapse sinkhole; the walls are considerably steeper than in a funnel-shaped depression, but the floor has not necessarily originated from the complete structural collapse of an underground cave chamber.

  • Cylindrical (shaft-like) sinkhole / Pit (valjasta vrtača / brezno):

    • Characteristics: A depression with vertical or near-vertical rocky walls. It is characteristic of collapse events or areas dominated by focused vertical dissolution along distinct fault zones.

  • Compound (coalescent / nested) sinkhole (sestavljena vrtača):

    • Characteristics: Formed by the coalescence of two or more adjacent sinkholes as their separating rims are lowered and degraded by lateral expansion and denudation. Such depressions feature an irregular, wavy outline with multiple distinct deepening points on the floor (a transitional form toward a uvala).

2. Genesis and Genetic Types of Sinkholes

According to the dominant formation processes, four fundamental genetic types are recognized in karstology:

  • Solution (corrosion) sinkhole: Formed by the slow chemical dissolution of limestone or dolomite proceeding downward from the surface. Precipitation water, enriched with carbon dioxide ($CO_2$), drains toward intersecting fissures or bedding planes and dissolves the rock faster there, causing the surface to gradually subside into a shallow bowl or funnel.

  • Collapse sinkhole (udorna vrtača / udornica / koliševka): Formed by the catastrophic structural failure and collapse of the roof of an underground cavity or cave chamber. Collapse sinkholes are characterized by steep, vertical, or even overhanging rock walls and fresh breakdown rubble on the floor (e.g., Velika dolina and Mala dolina in the Škocjan Caves, the Red and Blue Lakes near Imotski).

  • Suffosion sinkhole: Characteristic of covered (mantled) karst, where the carbonate bedrock is overlain by a thicker mantle of unconsolidated sediments (sand, silt, loess, loam). It forms through the mechanical subsurface washing down of fine-grained particles (suffosion): percolating water carries sediment down into open karst fissures or voids in the underlying rock. As material is removed, a subterranean arch or void gradually forms in the cover layer, which collapses once weakened or overloaded (e.g., following heavy rainfall). The slopes of suffosion sinkholes consist of soft, loose material and lack vertical rock walls.

  • Dropout / Sagging (subsidence) sinkhole: Forms in areas with an unconsolidated mantle where ground displacement is caused not by the mechanical removal of cover sediment, but by the gradual dissolution and lowering of the underlying bedrock itself beneath the sediments, resulting in continuous, slow settling of the entire overlying surface.

3. Morphological and Microclimatic Characteristics

  • Soil accumulation: The floor of a sinkhole is frequently filled with a layer of fertile clay or reddish/brown soil (jerina or terra rossa), washed down from the slopes by rainfall. The floor can be permeable (drained by a ponor or fine fissures) or partially sealed by washed-in impermeable loam.

  • Thermal and vegetative inversion (temperature inversion): Because sinkholes are closed hollows without surface drainage for air, cold, dense air pools at the bottom during calm, clear nights. Consequently, the sinkhole floor can be considerably colder than its upper rim. This produces a vegetative inversion: cold-adapted, shade-tolerant plant species (e.g., spruce, ferns) thrive at the bottom, while thermophilic species (e.g., smoke bush, pubescent oak) grow on the warmer upper rim.

4. Significance for Human Settlement and Cultural Landscape

On the arid, rocky Classical Karst, sinkholes were historically the primary centers of agricultural life:

  • Agricultural oases (dolci): To access the scarce fertile soil at the bottom of sinkholes, farmers cleared stones from the slopes by hand and used them to construct protective drystone walls (suhozid), preventing soil erosion and shielding crops from the fierce Bora (burja) wind.

  • Cultural heritage: Numerous terraced sinkholes across the Classical Karst and the Dinaric Karst represent an outstanding example of sustainable, nature-adapted human land use in karst environments.

Karst area of Slovenia