Glacis

A glacis is a specialized artificial slope used in military engineering.

It serves as a defensive barrier for fortresses or armored vehicles. These slopes can be made of earth for temporary use. They can also be made of stone for permanent structures. In a broader sense, any slope that fulfills these defensive requirements is a glacis. The name comes from a French word. This word suggests a slope made dangerous with ice, similar to a glacier.

In medieval fortifications, these slopes were also called taluses. They functioned through several specific physical mechanisms. First, they strengthened walls against undermining by enemies. Second, they helped hamper escalades, which are attempts to scale walls. Third, they changed how projectiles moved. If defenders dropped missiles from the battlements, the objects would ricochet off the slope. This caused the missiles to bounce into the attacking forces instead of hitting the ground.

History shows that glacis have been used for thousands of years. Ancient Egyptians used them at Semna in Nubia. There, they prevented enemy siege engines from weakening the defensive walls. In Britain, hillforts began using them around 350 BC. Maiden Castle in Dorset is an example of a site with very high slopes. As technology changed, the design of the glacis also evolved to meet new threats.

During the late medieval period, castles were modified to defend against cannons. Engineers placed earthen slopes faced with stones in front of curtain walls and bastions. Bastions are towers that form part of a fortification. These slopes served two main purposes. They could absorb the impact of cannon shots. They could also deflect the shots away from the main structure. During this time, towers were often lowered to the same height as curtain walls. These towers were then converted into gun platforms.

Early modern European fortresses used glacis to keep attackers under fire. On level ground, a glacis consists of a slope with a low grade. This slope is inclined toward the top of the wall. This design provides a direct line of sight for the defenders. It allows them to sweep the field with fire from the parapet. This is more efficient because it minimizes changes to the angle of the guns. Additionally, the earth bank provides a secondary benefit. It shields the walls from being hit directly by cannon fire.

The term glacis is also used in modern armored warfare. A glacis plate is the sloped front-most section of a tank hull. This plate often consists of upper and lower halves. In a head-on armored engagement, the glacis plate is the largest target. It is also usually the thickest and most robust part of the vehicle. It is followed in strength by the turret face and the gun mantlet. This design is a response to the threat of anti-tank mines. Some mines use a tilt-rod fuze to detonate directly under the glacis plate.

Sloped armor provides two distinct advantages for an armored fighting vehicle. The first advantage is deflection. Many projectiles will bounce off the slope rather than penetrating the metal. The second advantage involves the path of the projectile. If a shot does penetrate, it must travel a longer diagonal route. This path is longer than a path through armor that is perpendicular to the trajectory. Because the projectile travels through more material along this diagonal, the armor becomes more effective.