Why Is There a Flat Field in the Middle of All This Limestone? It Is a Drain

The form is an enclosed depression with a remarkably flat floor, steep enclosing sides, and dimensions that can reach tens of kilometres across, which makes it far larger than the sinkholes and hollows usually associated with limestone. The floor is covered in soil and sediment rather than bare rock, which is why it is farmed while the surrounding hills are not. There is generally no surface outlet, so any stream crossing the floor disappears into a hole at the edge. The name is a Slavic word meaning field, which is exactly how the floor is used.

Several processes work together at one level:

• •The water table sits close to the floor surface
• •Dissolution is fastest at and just below that level
• •So the rock is eaten away laterally rather than downwards
• •Seasonal flooding spreads sediment evenly across the floor
• •Streams entering from the sides deposit their load there
• •Insoluble clay left behind by dissolved limestone accumulates

The seasonal flooding is the defining behaviour and it follows directly from having no surface outlet. Water leaves through holes and cave systems at the low end, and those passages can only carry a certain flow. When winter rain or snowmelt delivers more water than the holes can swallow, the surplus has nowhere to go and the floor fills, sometimes to depths of several metres, turning the whole basin into a temporary lake. It drains through the same holes over the following weeks. Some basins flood almost every year and others only in exceptional ones, and the largest examples in the Dinaric region are famous for it.

The feature sits at the top of a size sequence and placing it there makes the landscape legible. The smallest hollows are funnel or bowl shaped depressions a few metres across, formed where rock dissolves around a crack or where a cave roof collapses. Several of those merging produce an irregular larger hollow with a scalloped outline. Larger again are elongated valleys aligned with the structure of the rock. And the largest are these flat-floored basins, which are not simply bigger versions of the small hollows but are shaped by the water table and by sediment in ways the small ones are not.

Human use of these basins is shaped entirely by the flooding and produces a distinctive pattern. Villages sit on the slopes at the edge, above the flood level, and never on the floor. The floor is cultivated in summer and left in winter, which suits crops that can be planted and harvested between floods. The fertile sediment makes it the best agricultural land for a long way around, in a landscape that is otherwise thin dry soil over bare rock. Attempts to drain them permanently by enlarging the outlets have been made repeatedly, and some succeeded at the cost of the fertility that came with the flooding.

A wide flat-floored basin ringed by steep limestone slopes forms where dissolution works sideways at the water table rather than downwards, and sediment spread by flooding levels the floor. With no surface outlet, water leaves through holes that can only take so much, so winter surplus turns the basin into a temporary lake. Villages sit on the rim and the floor is farmed in summer.