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Climatic Geomorphology
Figure 3.5. Glacial polished surface and till. Pordoi Pass, Dolomitas, Italy.
the action of subglacial water currents or a similar preglacial morphology where these
characteristics are emphasized later by the ice.
The action of constant abrasion produces glacial polishing (Fig. 3.5). Observed with a
magnifying glass or under the microscope, a glacially polished rock has a surface furrowed
by fine scratches corresponding to small striations. This polishing is more evident on fine
grained rocks. It also disappears quickly when being exposed to weathering agents.
2.2. Friction cracks and P-forms
These are small forms related to glacial action appearing by fracturing of rock or by
concavities where extraction of rocky substratum material might have taken place or not.
The most classic works in which both differentiation and origin of the friction cracks are
considered are from Gilbert (1906), Harris (1943) and Dreimanis (1953).
Friction cracks are developed better on intermediate grain-sized rocks, in which
striations and polishing are less evident. Normally they appear on hard and fragile rocks
such as granite, basalt, quartzite, and so forth. Four types are distinguished by Embleton
and King (1975a) (Fig. 3.6):
(a) Lunate fracture (Fig. 3.6a), in which the horns point in the direction of the ice
movement. These features are formed by two fractures, one of low angle and another
subvertical; the intersection between them gives rise to a rock slab.
(b) Crescentic gouge (Figs 3.6b and 3.7), the horns point opposite to the ice flow. Its size
ranges between 2 cm and 2 m. They usually appear in two to six systems and generally
one behind another. They also have the two types of fractures mentioned above.
Climatic Geomorphology
Figure 3.5. Glacial polished surface and till. Pordoi Pass, Dolomitas, Italy.
the action of subglacial water currents or a similar preglacial morphology where these
characteristics are emphasized later by the ice.
The action of constant abrasion produces glacial polishing (Fig. 3.5). Observed with a
magnifying glass or under the microscope, a glacially polished rock has a surface furrowed
by fine scratches corresponding to small striations. This polishing is more evident on fine
grained rocks. It also disappears quickly when being exposed to weathering agents.
2.2. Friction cracks and P-forms
These are small forms related to glacial action appearing by fracturing of rock or by
concavities where extraction of rocky substratum material might have taken place or not.
The most classic works in which both differentiation and origin of the friction cracks are
considered are from Gilbert (1906), Harris (1943) and Dreimanis (1953).
Friction cracks are developed better on intermediate grain-sized rocks, in which
striations and polishing are less evident. Normally they appear on hard and fragile rocks
such as granite, basalt, quartzite, and so forth. Four types are distinguished by Embleton
and King (1975a) (Fig. 3.6):
(a) Lunate fracture (Fig. 3.6a), in which the horns point in the direction of the ice
movement. These features are formed by two fractures, one of low angle and another
subvertical; the intersection between them gives rise to a rock slab.
(b) Crescentic gouge (Figs 3.6b and 3.7), the horns point opposite to the ice flow. Its size
ranges between 2 cm and 2 m. They usually appear in two to six systems and generally
one behind another. They also have the two types of fractures mentioned above.