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The DWEJRA - GOZO
The spectacular landscape of Dwejra is dotted with a 400 metre-wide circular depressions that are the result of catastrophic geological processes that go back millions of years and reflects the dynamic nature of the land and seabed.
Subsidence features are typically associated with karst processes in limestone regions. Rainwater is naturally slightly acidic and slowly dissolves limestone until a cavern is produced. When the lateral extent of the cavern reaches a threshold size and its roof thins by rockfall, the entire roof of the cavern collapses, leaving a circular-shaped depression called a sinkhole or doline.
However, Dwejra does not fit in this model of limestone dissolution. It seems that rather than having formed by collapse of a thinning roof of a cavern, the Dwejra features were formed by the collapse of a cavernous void beneath western Gozo, hundreds of metres below the present surface. The collapse created a depression over an area of four square kilometres. The cavern collapse also triggered the several closely-spaced large circular sinkholes. The caverns are closer to the surface towards western Gozo and formed when a deep layer of gypsum was rapidly dissolved by the infiltration of freshwater.
The Dwejra area is made up of marine tertiary sedimentary rock deposited around 24 million years ago. In this area one finds Globigerina Limestone, Lower Coraline Limestone and even some Blue Clay.
The Lower Coraline Limestone is mainly exposed in the sheer cliffs on the coastline and also in the crust of the inland sea. The Blue Clay in this area is composed of medium grey and soft pelagic marls, with pale bands rich in planktonic farominfera and lower in clay content.
The Gobigerina Limestone found in this area is mostly of the lower type. This is a white weathering grey marly limestone which supports numerous nodules. The Globigerina Limestone is widely used in the building industry on the islands of Malta.
Part of Dwejra is the inland sea known as “Il-Qawra” by the locals, constitutes the lowest spot of Gozo. The inland sea is an expanse of shallow water set in a deep recess in the rock coastline produced by the caving in of the surface above subterranean caves.
Because of the fact that Qawra area is an enclosed sea, the presence of rock fragments clearly indicates that the Qawra area was totally different in the past and also experienced different geomorphologic processes. The rocks in the area are packed with fossils: the actual remains of plants and fish buried and preserved in sedimentary rock by a natural process.
The pool connects to the sea by means of a narrow tunnel cutting through the cliffs. This only entrance for the pebbly lake is called “Ghar iz-Zerqa” (Azure Cave) because of the azure colour of the surrounding sea.
N.B. Getting out of the tunnel and looking on your right, you can view a natural rock formation of a face sculptured in the cliff.
Logging this cache
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1. What do you think is the kind of stone the cave is formed in? Describe briefly your answer.
2. Keeping in mind that the location at the GZ was a cave or a formation of caves and noting the above illustration, how do you think the tunnel in front of you have formed? What could have contributed to the formation of this tunnel? Describe briefly your answer.
3. How long is the tunnel to the open sea? Give an approximation in metres.
4. How high are the cliffs? Give an approximation in metres.
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