A glacis (//; French: [ɡlasi]) in military engineering is an artificial slope of earth used in late European fortresses so constructed as to keep any potential assailant under the fire of the defenders until the last possible moment. On natural, level ground, troops attacking any high work have a degree of shelter from its fire when close up to it; the glacis consists of a slope with a low grade inclined towards the top of the wall. This gave defenders a direct line of sight into the assaulting force, allowing them to efficiently sweep the field with fire from the parapet. Additionally, but secondarily, the bank of earth would shield the walls from being hit directly by cannon fire.
Though defenders on a high ground already have a direct line of sight, a glacis allows defenders to sweep the field more efficiently by minimizing changes to the angle of their guns while firing. Furthermore, the glacis prevents attacking cannons from having a clear shot at the walls of a fortress, as one usually cannot even see the walls until the glacis is crossed and the ditch, bounded on either side by the smooth, masoned scarp and counterscarp, is reached.
More generally, the term glacis can denote any slope, natural or artificial, which fulfils the above requirements. The etymology of this French word suggests a slope made dangerous with ice, hence the relationship with glacier.
The term glacis plate describes the sloped front-most section of the hull of a tank or other armored fighting vehicle. In a head-on-head armored engagement, the glacis plate is the largest and most obvious target available to an enemy gunner. Anti-tank mines which employ a tilt-rod mechanism are also designed to detonate directly underneath the glacis plate. As a result, the glacis plate is generally the thickest, most robust armored section of a tank, followed by the turret face and mantlet.
An erosional or depositional landform, with little slope. Erosional glacis occur mostly in arid regions, and result from intense weathering and surface transport via laminar, episodic water flow.
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