5 Underwater Landscapes and Implicit Geology. Marseilles . . .
57
issued from the dismantling of rudist colonies. These extinct bivalves, organized
in “spud-like” reefs make the fine mud seabed very uneven, and reflect a shallow
underwater landscape on a carbonated platform which evokes the present-day Bahama Islands. The current underwater morphology is essentially dominated by these
compact but fractured limestones, which are particularly vulnerable to karstic dissolution (Fig. 5.4). This karstification of limestones deriving from extinct underwater
landscapes exhibits characteristic morphologies. These normally terrestrial shapes,
arches, caves, karrenfelds, dolines and resurgences, are still easy to recognize beneath the sea as the landscapes were submerged very quickly during the last glacial
period and were thus not eroded.
• Landscape of underwater arches:
At the southern extremity of Ma¨ ıre Island, just off the Farillons Skerries, a series
of spectacular underwater arches forms a grandiose landscape, famous among
divers (Fig. 5.5). There are similar isolated arches on Jarre Island, at a depth of
− 30 m, at the foot of the “Pierre de Brégançon”, but also off the shore of the
“Tombant de la Pierre à la Bague”, between − 35 and − 45 m west of Planier
lighthouse.
• Cave landscapes and karstic galleries:
Underwater caves are another element of the karstic landscape. They are well
known to divers because of their obscure ceilings conducive to the development
of red coral which has been exploited in the region since Antiquity (Fig. 5.6).
Cosquer cave is now famous for its prehistoric paintings. The upper third of the
cave was spared by the Tardiglacial rise in sea level. This cavity is part of a
network of submerged galleries associated with a typical karstic pocket valley,
which is now submerged. This network opens up under the sea (Cosquer Cave
at − 37 m, Triperie Cave at − 17 m, Figuier Cave at − 20 m). Other caves exist
at the foot of the deep wall (− 35 to − 60 m), between Caramassaigne Point (at
the eastern extremity of Riou Island) and the Grand Conglu Skerry. One of these
caves, with the floor currently at a depth of − 58 m, is located at the foot of this
wall which dominates the sand plain extending seaward. Further east, at the Cassis
Port outlet, the vast porch of Trémie Cave, at a depth of − 9 and − 20 m extends
through a secondary room into a landscape of stalagmites and stalactites. A survey
conducted by the prehistorian Eugène Bonifay yielded several flint prehistoric
tools, evidence that humans lived in these now totally submerged landscapes.
• Karrenfeld landscapes:
Offshore, between the Croisette Cap and Ma¨ ıre Island, the Veyron Plateau is a
typical karrenfeld landscape, covered with a spectacular network of galleries,
between − 15 and − 20 m (Fig. 5.7). These galleries, formed by aerial karstic
dissolution have yielded ferruginous sediments and continental limestone floors.
To the west of Planier lighthouse, the “Plateau de la Pierre à la Bague” is a karstic
karrenfeld plateau at a depth of − 1 and − 15 m with pinnacles and typical karstic
incisions (Fig. 5.8). Similar submerged karrenfeld landscapes are present around
the eastern point of Planier Island towards the Souquet shoal. Karrenfeld and
57
issued from the dismantling of rudist colonies. These extinct bivalves, organized
in “spud-like” reefs make the fine mud seabed very uneven, and reflect a shallow
underwater landscape on a carbonated platform which evokes the present-day Bahama Islands. The current underwater morphology is essentially dominated by these
compact but fractured limestones, which are particularly vulnerable to karstic dissolution (Fig. 5.4). This karstification of limestones deriving from extinct underwater
landscapes exhibits characteristic morphologies. These normally terrestrial shapes,
arches, caves, karrenfelds, dolines and resurgences, are still easy to recognize beneath the sea as the landscapes were submerged very quickly during the last glacial
period and were thus not eroded.
• Landscape of underwater arches:
At the southern extremity of Ma¨ ıre Island, just off the Farillons Skerries, a series
of spectacular underwater arches forms a grandiose landscape, famous among
divers (Fig. 5.5). There are similar isolated arches on Jarre Island, at a depth of
− 30 m, at the foot of the “Pierre de Brégançon”, but also off the shore of the
“Tombant de la Pierre à la Bague”, between − 35 and − 45 m west of Planier
lighthouse.
• Cave landscapes and karstic galleries:
Underwater caves are another element of the karstic landscape. They are well
known to divers because of their obscure ceilings conducive to the development
of red coral which has been exploited in the region since Antiquity (Fig. 5.6).
Cosquer cave is now famous for its prehistoric paintings. The upper third of the
cave was spared by the Tardiglacial rise in sea level. This cavity is part of a
network of submerged galleries associated with a typical karstic pocket valley,
which is now submerged. This network opens up under the sea (Cosquer Cave
at − 37 m, Triperie Cave at − 17 m, Figuier Cave at − 20 m). Other caves exist
at the foot of the deep wall (− 35 to − 60 m), between Caramassaigne Point (at
the eastern extremity of Riou Island) and the Grand Conglu Skerry. One of these
caves, with the floor currently at a depth of − 58 m, is located at the foot of this
wall which dominates the sand plain extending seaward. Further east, at the Cassis
Port outlet, the vast porch of Trémie Cave, at a depth of − 9 and − 20 m extends
through a secondary room into a landscape of stalagmites and stalactites. A survey
conducted by the prehistorian Eugène Bonifay yielded several flint prehistoric
tools, evidence that humans lived in these now totally submerged landscapes.
• Karrenfeld landscapes:
Offshore, between the Croisette Cap and Ma¨ ıre Island, the Veyron Plateau is a
typical karrenfeld landscape, covered with a spectacular network of galleries,
between − 15 and − 20 m (Fig. 5.7). These galleries, formed by aerial karstic
dissolution have yielded ferruginous sediments and continental limestone floors.
To the west of Planier lighthouse, the “Plateau de la Pierre à la Bague” is a karstic
karrenfeld plateau at a depth of − 1 and − 15 m with pinnacles and typical karstic
incisions (Fig. 5.8). Similar submerged karrenfeld landscapes are present around
the eastern point of Planier Island towards the Souquet shoal. Karrenfeld and