Large closed depressions with flat floors across which streams flow are called poljes. These features can be compact or irregular, and may be oriented along a tectonic axis. A sharp break exists between the floor and steep walls. The streamsinks in the poljes are called ponors; residual hills of limestone protruding from the floor are called hums.
A tower karst is similar to poljes. This type of karst can occur (often in tropical regions) where residual hills, which may be vertical or overhanging, exist in a swampy alluvial plain. The towers may be up to 1500 feet high.