56
Climatic Geomorphology
Figure 2.25. In the foreground in shadow, ogives in the Mer de Glace Glacier, Mont Blanc Massif,
French Alps. Photo: J. L6pez-Martfnez.
underneath shows a plastic behaviour. They constitute excellent routes for the penetration
of melt water. When the system of crevasses intersects, the surface of the glacier becomes
a fragile mass of dentated pinnacles known as sdracs (Fig. 2.26).
Crevasses are grouped in systems (Fig. 2.27) (Sharp, 1960), differentiated into the
marginal or in chevron crevasses formed by extensive stress generated by a higher speed
of the glacier flow in the centre rather than at the margins as a result of the friction with
the rocky walls. They intersect the edge of the glacier at angles of --~ 45 ~ Transverse
crevasses (Fig. 2.28), located in zones of higher speed, are perpendicular to the flow.
Extension crevasses occur where the valley glacier gets wider, or in bed threshold zones.
These crevasses form smaller angles of --~ 45 ~ with the edges. At the end of the glacier
terminus, crevasses of radial extension are developed. Once formed, the systems of
crevasses are modified by glacier flow. Marginal crevasses can undergo rotation and the
transverse and extension crevasses are narrowed until they finally close, being then
recognized by a blue ice vein.
Climatic Geomorphology
Figure 2.25. In the foreground in shadow, ogives in the Mer de Glace Glacier, Mont Blanc Massif,
French Alps. Photo: J. L6pez-Martfnez.
underneath shows a plastic behaviour. They constitute excellent routes for the penetration
of melt water. When the system of crevasses intersects, the surface of the glacier becomes
a fragile mass of dentated pinnacles known as sdracs (Fig. 2.26).
Crevasses are grouped in systems (Fig. 2.27) (Sharp, 1960), differentiated into the
marginal or in chevron crevasses formed by extensive stress generated by a higher speed
of the glacier flow in the centre rather than at the margins as a result of the friction with
the rocky walls. They intersect the edge of the glacier at angles of --~ 45 ~ Transverse
crevasses (Fig. 2.28), located in zones of higher speed, are perpendicular to the flow.
Extension crevasses occur where the valley glacier gets wider, or in bed threshold zones.
These crevasses form smaller angles of --~ 45 ~ with the edges. At the end of the glacier
terminus, crevasses of radial extension are developed. Once formed, the systems of
crevasses are modified by glacier flow. Marginal crevasses can undergo rotation and the
transverse and extension crevasses are narrowed until they finally close, being then
recognized by a blue ice vein.