First, I'll post a caveat. This is not my primary area of study, and therefore I have not spent a lot of time looking at field fortifications. I have, however, spent some time with Dennis Hart Mahan's Treatise on Field Fortifications, and have climbed around a number of earthwork forts. Using those tools, I'll attempt to describe earthwork fortifications as a contrast/comparison to masonry fortifications. Comments and corrections from the readers will be greatly appreciated.
A significant difference between an earthwork fortification and permanent fortification is in the outworks. The outworks of an earthwork fort are often nonexistent, with the exception of the ditch. The excavation of the ditch not only increases the effective height of the scarp, but it also provides the earth for the ramparts of the fort. While there is not usually a sloped glacis, the area in advance of the fortification is usually cleared of all trees and other obstructions - in the same manner that a glacis is cleared in advance of a permanent fortification.
The other outwork that often is constructed in advance of a field fortification is an abatis. An abatis is an entanglement of sharpened branches that greatly slows the advance of an attacker, leaving the attacker under the cannon and riflery of the fort for an extended period of time.
The exterior slope of an earthwork fortification generally follows the natural angle of the earth, and thus varies in the angle of the slope. The natural angle of clay, for example, is much steeper than the natural angle of sand. Unless a revetment is used to steepen the angle, the natural angle of the material dominates. To protect the slopes, and if time permits, the slopes are sodded to add a level of permanence.
The exterior slope in an earthwork replaces the scarp in a permanent fortification. It runs from the ditch to the superior slope, which provides the parapet. This can be equivalent to, or at least analogous to, these works in a permanent fortification. At the parapet, a vertical slope, called the interior slope, descends to the banquette. While this slope can be earthen, for the practical reason that a rifleman/musketeer would prefer to stand against this slope, it is usually revetted with wood - generally making it a vertical wall. This wall is often called the breast-height wall, as in permanent fortifications.
The banquette and banquette slope are the same as in permanent fortifications.
In plan, field fortifications use the same shapes as permanent fortifications. Salients (acute angles) are used facing the attacker, and flanks are used for enfilading fire. Bastions are generally used in the same manner as permanent fortifications, although the flanking fire is from the parapet rather than from casemates as in masonry fortifications. The same angles used in permanent fortifications to eliminate any areas of the ditch that would not be under fire (dead space) are used in earthen fortifications.
The curtains of earthen fortifications - the walls between the bastions - are identical in function to those in permanent fortifications.
A field fortification may have a terreplein that is at the base of the banquette slope, or the banquette slope may terminate at the parade, depending on the height of the ramparts. To minimize excavation, the parade is generally at the level of the country.
I will follow this post with some illustrations of field fortifications. I hope it was helpful.
An abatis in advance of the fort.
The ditch and exterior slope
Two bastions joined by a curtain, from inside one bastion
Wooden revetments at breast-height wall.
The covert way at a water battery
The sally port of a field fortification is generally quite primitive, comprising simply a wooden gate. Flanking defense is provided by bastions adjacent to the curtain with the sally port.
Unlike masonry fortifications, embrasures were generally open-topped and often wood-revetted, like those shown here. In other cases, no embrasures were used - the guns were simply mounted to fire over the parapet.
A significant difference between an earthwork fortification and permanent fortification is in the outworks. The outworks of an earthwork fort are often nonexistent, with the exception of the ditch. The excavation of the ditch not only increases the effective height of the scarp, but it also provides the earth for the ramparts of the fort. While there is not usually a sloped glacis, the area in advance of the fortification is usually cleared of all trees and other obstructions - in the same manner that a glacis is cleared in advance of a permanent fortification.
The other outwork that often is constructed in advance of a field fortification is an abatis. An abatis is an entanglement of sharpened branches that greatly slows the advance of an attacker, leaving the attacker under the cannon and riflery of the fort for an extended period of time.
The exterior slope of an earthwork fortification generally follows the natural angle of the earth, and thus varies in the angle of the slope. The natural angle of clay, for example, is much steeper than the natural angle of sand. Unless a revetment is used to steepen the angle, the natural angle of the material dominates. To protect the slopes, and if time permits, the slopes are sodded to add a level of permanence.
The exterior slope in an earthwork replaces the scarp in a permanent fortification. It runs from the ditch to the superior slope, which provides the parapet. This can be equivalent to, or at least analogous to, these works in a permanent fortification. At the parapet, a vertical slope, called the interior slope, descends to the banquette. While this slope can be earthen, for the practical reason that a rifleman/musketeer would prefer to stand against this slope, it is usually revetted with wood - generally making it a vertical wall. This wall is often called the breast-height wall, as in permanent fortifications.
The banquette and banquette slope are the same as in permanent fortifications.
In plan, field fortifications use the same shapes as permanent fortifications. Salients (acute angles) are used facing the attacker, and flanks are used for enfilading fire. Bastions are generally used in the same manner as permanent fortifications, although the flanking fire is from the parapet rather than from casemates as in masonry fortifications. The same angles used in permanent fortifications to eliminate any areas of the ditch that would not be under fire (dead space) are used in earthen fortifications.
The curtains of earthen fortifications - the walls between the bastions - are identical in function to those in permanent fortifications.
A field fortification may have a terreplein that is at the base of the banquette slope, or the banquette slope may terminate at the parade, depending on the height of the ramparts. To minimize excavation, the parade is generally at the level of the country.
I will follow this post with some illustrations of field fortifications. I hope it was helpful.
An abatis in advance of the fort.
The ditch and exterior slope
Two bastions joined by a curtain, from inside one bastion
Wooden revetments at breast-height wall.
The covert way at a water battery
The sally port of a field fortification is generally quite primitive, comprising simply a wooden gate. Flanking defense is provided by bastions adjacent to the curtain with the sally port.
Unlike masonry fortifications, embrasures were generally open-topped and often wood-revetted, like those shown here. In other cases, no embrasures were used - the guns were simply mounted to fire over the parapet.