Solution Subsidence Structures (Dolines) in Malta:
Their Physical Development and Human Use
Ivan Calleja
Introduction
Dolines have often been referred to as the diagnostic1 karst landform and this confirms
the importance of such features in karst terrains. Dolines can be referred to as a natural
enclosed depressions found in karst landscapes. They may occur as isolated individuals, as is
Il-Maqluba (Plate 1), or densely packed together occupying most of the terrain. They come in
various shapes and can range in depth from less than a metre to hundreds of metres. Doline
width varies substantially, from a few metres to more than a kilometer while their sides may
range from being gently sloping to completely vertical.
In the Maltese Islands, these karstic features are more commonly known as solution
subsidence structures. They have been in existence for a very long period of time. A
substantial number lie buried beneath metres of soil and sediment while others are mere
remnants of much larger structures. Furthermore, some dolines are found in positive relief
while a significant percentage exhibit clear surface evidence of their existence.
Dolines are generally formed by four main mechanisms: (i) dissolution; (b) collapse; (c)
suffosion and (d) subsidence. However, in most cases, dolines form as a result of a number of
mechanisms and not one single process. These often work in combination or sequence to
produce the various types of dolines.
Plate 1 – A bird’s eye view of Il-Maqluba doline, Qrendi, Malta as seen from the northern
side of the structure.
Source: Photo taken by Robert Camilleri (2007).
1
This term is generally used to emphasize the fact that dolines serve to identify and characterise karst terrains.
They are a clear indication that karst exists in that particular area.
Methodology and Results
The principal aim of this research study was to present a comprehensive study on the
formation, evolution, distribution and land use of subsidence structures in the Maltese Islands.
A total of 57 solution subsidence structures located on the main islands of Malta, Gozo and
Comino have been inspected and photographed. The main outcome of this exercise was the
production of a doline catalogue. The latter contains a collection of aerial photos, topography
maps, field images and a brief description of all the structures under study. A total of seven
subsidence structures were also included with those already listed on The Geological Map of
the Maltese Islands (1993). These were: (1) Aħrax Point, Mellieħa; (2) Il-Latnija, Mellieħa;
(3) Għar ix-Xagħra, Qrendi; (4) Mistra, St. Pauls Bay; (5) Tal-Pitkal, Dingli; (6) L-Għar ta’
Bla Saqaf, Comino and (7) Il-Mixta, Comino.
In addition, two maps illustrating the distribution of subsidence strucutres across the Maltese
Islands have been produced and presented through the use of Google Earth (Figures 1 and 2).
The most representative dolines were further investigated and discussed as case studies. This
meant that these structures were inspected in more detail through:
(i) the investigation of the physical geography of the doline;
(ii) a land use evaluation of the structure;
(iii) a photographic record of the doline under study; and
(iv) the recording of a number of geometric parameters.
N
Figure 1 – Spatial distribution of dolines in Malta.
N
Figure 2 – Spatial distribution of dolines in Gozo and Comino.
A morphometric analysis, consisting of a number of geometric parameters, has been
conducted by means of a hand-held GPS for a total of 11 dolines representing the case studies
under investigation. Among these were: (1) Il-Maqluba, Qrendi; (2) Ħofret ir-Rizz,
Rabat/Dingli; (3) Ta’ San Martin, Rabat; (4) NE of il-Mansap, Mellieħa (Figure 3); (5) LGħar ta’ Bla Saqaf and (6) Qawra Inland Sea in Gozo. The nine morphometric parameters
chosen gave a clear indication of the perimeter, area, maximum and minimum diameter and
altitude (of various points along the doline rim and bottom) of the dolines under investigation.
Name of Doline
NE of Il-Mansap
Reference
Number
M19
N
GPS
GOOGLE EARTH
PLEN
250 m
244 m
ADOL
4492.1 m²
3980 m²
DMAX
80 m
88.5 m
DMIN
64 m
61. 9m
PMAX
84 m
83 m
PMIN
77 m
78 m
FMIN
77 m
78 m
HPER
7m
5m
HMAX
7m
5m
Figure 3 – Subsidence structure M19 and the geometric
parameters under investigation.
Furthermore, the pattern of distribution of dolines was assessed by means of the nearest
neighbour analysis. Two separate tests were carried out, one for dolines on mainlaind Malta
and another one for subsidence structures in Gozo.
Analysis and Conclusion
Around 90% of all subsidence structures in the Maltese Islands have formed in the
Upper Coralline Limestone. Furthermore, most of these structures have developed either in
the Mtarfa Member (especially on mainland Malta) or Tal-Pitkal Member, mainly as a result
of their high secondary porosity. Most of the other remaining dolines have formed in the
Lower Coralline Limestone while few are thought to have formed in the Blue Clay.
The greatest concentration of dolines in mainland Malta is found along the Rabat-Dingli
plateau with a total of 15 structures (26% of dolines in Malta). On the island of Gozo a cluster
of 6 subsidence structures (out of 17) is found in the Qawra/Dwejra area (35%). There is an
important relationship between the spatial distribution of dolines and faulting on both islands.
In Malta, 29% of dolines are bisected by one or more faults while in Gozo this figure stands at
23.5%.
Solution subsidence structures is the term most widely used to describe dolines in the Maltese
Islands. The research has concluded that, although the term is still valid and relevant for some
dolines, it would be better substituted with other terminology frequently used in the
international literature. As a result, dolines in the Maltese Islands should be referred to as
subsidence dolines or collapse dolines or buried dolines. This classification depends mainly
on the dominant process of formation of these structures or, in the case of buried dolines, their
present state.
The results obtained from the neareset neighbour analysis show that subsidence structures
located on mainland Malta are randomly distributed with a tendency towards clustering
[Rn=0.85]. This clustering is most evident across the Rabat-Dingli plateau. On the other hand,
the dolines found in Gozo are randomly distributed with an Rn value of 0.97.