Fort Peterson BlockHouse – Peterson, Iowa
Blockhouses were solidly constructed enclosed wooden fortifications designed to allow small garrisons posted at isolated locations to protect themselves from attacks by superior enemy forces. They were also used as interior keeps within larger earthen field works. Like most other minor field fortifications blockhouse designs ranged through almost all levels of structural complexity from simple single level squares to large elaborate cross and hexagon shaped works capable of receiving an artillery armament.
However simple or complex, almost all blockhouses were constructed according to a few commonly accepted conventions that insured both their structural soundness and usefulness as defensive works. Blockhouses had to be constructed from material capable of sustaining and absorbing multiple bullet impacts on their walls and roofs and put together in a manner designed to maintain their structural integrity under stress of severe lateral shocks. Logs at least 12 inches thick when squared on either two or four sides were considered the minimum material necessary for both walls and roofs to prevent common musket balls from penetrating through to the interior. The walls could be formed either by placing the logs upright and side by side in a manner similar to palisadings or they could be laid horizontally on top of each other and joined with notches in the manner of a common log cabin. The logs composing a vertical wall had to be buried at least three feet into the ground or mortised into a ground-sill and their tops had to be mortised into a cap-sill, or head piece, to keep them from spreading and separating.
The minimum height considered necessary for a blockhouse’s wall was nine feet so that troops sheltered in the place could work their ramrods without excess interference. Twelve feet was considered the minimum acceptable length for each wall since a shorter wall would not provide space for enough loop-holes to allow an adequate defense. Due to the weight of the heavy roof 24 feet was considered the maximum structurally sustainable length for each wall, any walls 16 feet or longer would require girder and shore support framing to carry the weight of the roof.
Roofs of most blockhouses had to be constructed to sustain the same structural shocks as the walls. Most single level blockhouses were given roofs composed of 12 by 12 logs rabbited at the ends to fit down snugly onto the cap-sills or topmost horizontal logs of opposite walls. The resistant roof could be covered by a simple framed top roof to help the building shed water or it could be covered by at least three feet of soil to help absorb the initial impact of bullets striking the roof. Vents, chimneys, or hatches had to be cut through the roof to ventilate the interior spaces and allow smoke to escape.
Blockhouses with an upper level would, of course, usually only have light framework on girders to form the ceiling of the lower level with the resistant roof covering the upper level. The second level could be constructed to project beyond all four walls of the lower level or built at an angle to the lowed level with just the four corners projecting over the central sections of the lower level walls. Those parts of the upper level floor projecting beyond the lower level would be reinforced and loop-holes cut to allow troops to fire down on the heads of an enemy attempt to shelter themselves along the lower level walls.
Loop-holes were cut at three foot intervals along each wall and at least six feet above the ground to prevent an enemy from using them to fire into the blockhouse. The loop-holes were designed to be wider on the interior side of the wall to allow muskets to be pointed in all directions and narrow on the exterior side to maintain as much of the protection afforded by the walls as possible.
The weakest point on the blockhouse was the doorway and special precautions had to be taken to prevent direct and unimpeded access to the door. Some two level blockhouses had the door on the upper level which was reached by a free standing staircase some four to six feet from the exterior wall. Loose planks that could be quickly taken up were laid from the staircase landing to the door way to provide access to the blockhouse. This method of protecting the door had the great disadvantage of trapping the garrison inside the work, especially if the staircase was damaged… an unpleasant situation if the enemy succeeded in setting the blockhouse on fire. A better method was to construct a narrow re-entering gallery that opened on one end of the wall and turned to the right or left before it reached the door way. The exterior gallery wall prevented enemy fire from reaching and weakening the door while the enemy could only make an attempt on the door in single file under fire from several loop-holes.
The walls could be further protected by a shallow ditch dug around the blockhouse with the soil thrown up against the walls. In many cases the ditch was dispensed with and soil was brought to the site and piled on the exterior walls almost to the level of the loop-holes to prevent enemy troops from finding shelter along the walls underneath the loop-holes. But this arrangement had the very distinct disadvantage of allowing an enemy easy access to the loop-holes.
Blockhouses were really quite useful little fortifications that contributed no small measure to the success of Federal field armies dependent on railroads as their main lines of supply and communications. Almost as soon as Federal armies entered the Southern and border states their lines of communication were subjected to attack by guerrillas and other generally displeased Southern sympathizing citizens. Blockhouses allowed Federal authorities to maintain hundreds of small garrisons to watch over isolated, but important, points along hundreds of miles of railroad tracks that could not have been (at least early in the war) easily or quickly rebuilt. Although the number of garrisons required a relatively large number of troops, blockhouses allowed the guards at each point to be reduced to a minimum while allowing the guards to remain in position in relative safety 24 hours a day.
The enclosed nature of blockhouses allowed them to be placed in positions near the object being guarded without regard to commanding terrain around the blockhouse. An enemy firing down on the roof of a blockhouse from a distant hill was bothersome, but hardly dangerous to a garrison holed up in the blockhouse. Blockhouses were versatile and could be constructed to suit the needs of any location. An important bridge surrounded by fairly open country could be well protected by a single large blockhouse; in rugged terrain two or three small blockhouses could cover the approaches to a bridge both from the tracks and the slopes of the ravine crossed by the bridge. In ideals situations blockhouses would be positioned to cover the approaches to the guarded structure, the structure itself, and other blockhouses or stockades with fire.
Though the presence of a blockhouse protected garrison might intimidate small guerrilla bands and keep them from even attempting to attack bridges and trestles, they were no match for larger raiding parties, especially those equipped with artillery. Many engineers seriously believed that blockhouses could be made to resist artillery fire by giving their walls a second layer of 12 inch thick logs. But most types of wood tended to lack the resilience necessary to withstand repeated heavy impacts and none could break the penetrative power of rifled artillery fire; few blockhouse garrisons could withstand the threat of artillery and those that did, didn’t last long once the splinters began to fly inside the blockhouse and the walls started giving way. When the almost inevitable Confederate cavalry raids came roaring down the railroad tracks, the many blockhouse garrisons were recalled from their posts and concentrated in larger field works at central points where they had a better chance of waiting out a raid or repelling an attack.
When you visit the Blockhouse in Peterson you will enjoy having the construction logic to evaluate the structure.
Visit the Blockhouse in Peterson, Iowa 51047