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How do sinkholes combine to form larger depressions called poljen?
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Stalactites and stalagmites in the Queen's Chamber, Carlsbad Caverns National Park, southeastern New Mexico.
In the study of karst topography, a polje is an elongated basin characterized by a flat floor and steep walls. These large depressions are formed through the coalescenceāor mergingāof several individual sinkholes.
As sinkholes develop and expand due to the dissolution of underlying limestone bedrock by groundwater, they may eventually grow large enough to intersect with neighboring depressions. Over time, the boundaries between these merging sinkholes disappear, resulting in the creation of a single, expansive basin. The flat floor of a resulting polje is typically covered with soil composed of the insoluble residues left behind after the limestone has been dissolved.12