Subsidence / Solution structures (Dolines)

There are various circular subsidence structures and are found in various localities. These are when the land surface collapses into a circular crater like depression hollow. This falls as the underlying rock is removed by solution.

Dolines are mainly associated with the solution of Limestone caused by percolation of water that leads to the collapse of caves that can be either terrestrial or submarine caves. In the Maltese islands we have 2 types of subsidence structures:
 

• those formed underwater due to seafloor collapse in the Miocene. Examples of these are found in Dwejra Gozo.
• those formed on land as result of cave roof collapse in the quaternary period. Example: Il-Maqluba.

These structures formed due to the rainfall increase in the Pleistocene pluvial period that helped their formation. Rainwater percolated through fissures and cracks and met point of weakness in bedding plains. As the process of dissolution continued, underground channels and caves formed. The cavern ceiling collapsed as it was not large enough to hold the overlying limestone rock.

Most of these structures formed in the Upper Coralline Limestone accept for 3 which are:
 

• Il-Maqluba in the Lower Coralline Limestone
• Ic-Cirku at Marsascala in the Globigerina Limestone
• Ghajn Zejtuna in the Blue Clay.