Kalkara Valley and Rinella Valley form part of the sunken valley system of the Grand Harbour area and make up the greatest part of the remaining natural open space in this area. For this reason these valleys are considered to be important from both the ecological and the geological points of view.

The fault system in the Grand Harbour area led to the formation of fluvial valleys such as Kalkara Valley and Rinella Valley. Due to the post-Miocene (23-5 million years ago) tilting of the Maltese islands to the north northeast, parts of these fluvial valleys along the northeastern coast were sunk. This is further confirmed by the continuation of the fluvial valleys in the seabed, buried beneath deposits of silt and mud, beyond the mouth of the harbour where the drainage once discharged its flow into the sea. These valleys constitute what is left of the emergent parts of the Grand Harbour sunk river-channel system.

The tilting of the Maltese islands to the NNE facilitated the surface water flow through these valleys during the Quaternary pluvial climates. For this reason, the Grand Harbour area is considered to be an important hydrogeological unit because it represents the major drainage system of the Quaternary age.

Mepa scheduled Kalkara and Rinella Valleys as a Level 2 Site of Scientific Importance (Geology) and a Level 4 Area of Ecological Importance as per Government Notice No. 930/02 in the Government Gazette dated October 18, 2002.

A definition of certain terminology used above can be found on http://www.mepa.org.mt/file.aspx?f=3438 .