Fort Union Historic Structure Report

Defense. With the advent of the Civil War, Fort Union mobilized for possible action. Because it housed an arsenal, supplies, and material, Fort Union became a prime target for the Confederate Army in the west. Fort Union was, after all, the principal supply depot for federal troops, and control of the fort meant command of its arms and materiel and command of the Santa Fe trail and communication with the States.

Figure 6. The second Fort Union was an earthen fortification laid out in the shape of an eight-pointed star. This photograph was taken from the southwest in 1930. Fort Union National Monument.

Major William Chapman, commander of Fort Union, began construction of the second fort (HS-200) when the threat of the Confederates attacking the fort was imminent. The first fort, directly below the bluffs, was in an indefensible position. Chapman noted to his superiors in Santa Fe that he could construct nothing for its defense, since anything he constructed could be "commanded by higher ground in the rear and on both flanks." [1] The idea was to move the post "out of range of field pieces & small arms . . . construct an entrenched camp with bomb-proof Magazine and store houses sufficient to contain all the stores." [2] Also, Chapman planned to burn the old post before allowing it to fall into the hands of the enemy.

Earthen Fortifications. In terms of tactics, entrenchments and earthworks had well-known advantages. The function of earthworks was one of defense, with the added advantage that good troops within an earthwork could withstand an assault of three to four times as many equally good troops. Most commanding generals made their battle calculations accordingly. [3]

During the Civil War, the size of entrenchments and fortifications varied tremendously based on the time allotted for construction, the strategic location of the fortification, and the available materials. Each entrenchment had a mass or embankment covering it called a parapet. The purpose of the parapet was to "intercept the enemy's missiles, to enable the assailed to use their weapons with effect, and to present an obstacle to the enemy's progress." [4] Each fortification also had a ditch constructed with the twofold purpose of providing material for the construction of the parapet and for increasing the size of the fortification. [5] Often the top of the parapet was capped with a head log from which the men could fire. The parapet was often 10 to 15 feet of solid earth to protect against cannon fire. [6] The fortification at Fort Union was no different.

The Star Fort. [7] The construction of the fort, then, followed fairly standard contemporary army practice. When the fort was officially completed on August 26, 1861, Chapman stated that all it needed was some dressing off, and that it could be defended with 600 men. He had 1,034 troops. [8]

The plan of the fieldwork was done by Captain Cuvier Grover of the 10th Infantry. Captain Grover and Lieutenant William Nicodemus of 11th Infantry oversaw construction of the fortification. [9] Work on the fortification had progressed at a fever pitch. At one point, the army was rotating a force of 200 men—volunteers and regulars—every four hours day and night to complete the entrenchments. [10]

While the star fort was still under construction, other strategic concerns weighed on the minds of the officers of Fort Union. Military Storekeeper William Rawle Shoemaker wrote to his superiors in Washington about the gravity of the situation at the beginning of August, 1861. Shoemaker noted that they had just received word that Fort Fillmore, New Mexico with its garrison of 500 regulars, had been surrounded and taken without a fight by 300 Texans who were said to be headed next to Fort Union. Shoemaker noted that at the time there were 1,000 men at Fort Union, and two or three hundred more were expected to arrive shortly. The army, he said, would soon be putting 14 pieces of artillery in the earthwork. He noted that the earthwork was constructed a mile to the east of the first fort in the open prairie near water. Shoemaker wrote: "It is intended to get all of our (Ordnance) stores within the works and if necessary to destroy all the present buildings, and possible much property.— I will do the best I can for the preservation of the stores with the determination, however, that nothing shall fall into the hands of the enemy." [11] Women and children were removed and sent to Las Vegas or Mora shortly thereafter. [12] The length of their stay in neighboring towns did not appear in the record.

While the fortification was under construction, the various departments erected temporary storehouses in and around the star fort to protect their goods while they awaited new spaces within the bombproof structure. Storehouses were under construction at the fieldwork by September, 1861, but some perishable goods had to be kept outside under tarpaulins while construction was underway. [13] Canby in Santa Fe approved using the demilunes for storehouse construction. [14] Military Storekeeper Shoemaker wrote to the chief of ordnance in Washington that all of his stores would not fit in the earthwork, so that "we shall be obliged to erect temporary storehouses of some kind, as the other departments are doing." [15]

Apparently all of this temporary construction caused enough confusion that the department commander entered into the picture to resolve some conflicts. A letter to the commander of Fort Union from Department headquarters in Santa Fe stated that "The construction of all works of a defensive character are under the charge of the Engineer Department. . . The entrenchments at Fort Union are of that character, and all buildings, and structures of any kind within the work, or within the range of its fire are under the superintend of the officer charged with the execution of the work." [16] By October 20, 1861, work had slowed to a point where all of the men were relieved of extra duty "and work on the storehouses and barracks," and they were put back to their regular duties. [17]

Even Mr. Levi Spiegelberg, the sutler of the volunteers encamped at Fort Union at the time, began constructing buildings at the volunteer camps to serve the troops. Fort Union's commanding officer quickly forbade that situation when he noted that the structures might interfere with the line of fire from the fieldwork (which meant that Mr. Spiegelberg's buildings were probably somewhere between the first fort and the second fort). Also, the commander expected the volunteers to be moved out of Fort Union at a moment's notice, so he recommended that the sutler be ready to move with his troops. [18]

In October of 1861, troop movements out of Fort Union left the fort with so few regulars that the commander thought that all remaining stores should be moved to the fieldwork so that he would not have to split his command in case it was attacked. This meant that some of the stores remained at the first fort up until that time. [19] By January of 1862, nearly all of the quartermaster property, ordnance stores, and provisions had been moved into the fieldwork. [20]

Contemporary Descriptions. An article in Denver's Rocky Mountain News in February, 1862, described the star fort as:

. . . one of the strongest forts in the Western frontier. Its size is seven hundred and fifty feet square, parapets seven feet high. From the level of the ground on the inside with a ditch on the outside eight feet deep and fifteen feet wide. Quarters for two companies built on the insides with a large magazine, and quarters are built outside of the fort in an acute angular form from the sides of the fort, on each of the four sides with officers quarters intervening. The ordnance will be put in position early in May. Also other necessary buildings will be erected as soon as weather permits. The force at this fort is six companies numbering about three hundred men. [21]

Undoubtedly the publication of this description provided additional material for confederate military intelligence; the confederate troops, however, had been spying on the fort since August, 1861.

Contemporary with that description is one written (then later published) by one of the Colorado volunteers. In March, 1862, Ovando Hollister described the area:

Within a mile of the west side of the vale, on a gentle swell, is the fortification. A simple field-work of moderate size, with bastioned corners surrounded by dirt parapet and ditch, with a slight abatis at exposed points. The armament is poor, consisting mostly of howitzers, but the supply of ammunition is deemed sufficient for any emergency. It has bomb-proof quarters in and surrounding it forming part of the works, sufficiently large to accommodate 500 men besides the necessary room for stores. [22]

Hollister noted that he and his command were quartered in a log house below the fortification.

Figure 7. This photograph shows the approximate scale of the earthen fortification as well as some aspects of its construction. This is the south angle of either HS-206 or 207, the enlisted barracks in the redans. The branches overlying the parapet retarded erosion. Log framing outlined the entrance. Fort Union National Monument.

Figure 8. This photograph of the star fort shows the deterioration of the earthwork due to erosion. The brick chimneys pierce the roof of the redan, HS-205, at fairly regular intervals. The Third Fort Union is in the background. National Archives, Still Pictures Branch.

Figure 9. The rivulets of erosion are again obvious in this photograph of the earthworks taken in 1865. The lack of sod on the structures accelerated deterioration. National Archives, Still Pictures Branch.

Completing the Star Fort. On August 27, 1861, the department commander sent word to Fort Union expressing thanks to Captain Grover and Lieutenant Nicodemus for their zeal, energy, perseverance and alacrity in completing the fieldwork. [23] Although Chapman felt that all the star fort needed at the time was some dressing off, other correspondence indicated that Fort Union was a tad short of completion one year later in the summer of 1862. An inspection that summer found that:

The new Post, which is being built, according to plans of Capt. C. Grover, was commenced in 1861, and is not yet completed, as orders were received from Dept. HQrs. to discontinue the work. The four angles (HS-204, 205, 206, 207), designed for store houses and company quarters are completed, each wing is 200 feet in length and 26 feet in depth, which is subdivided into a storehouse 100 feet in length and 6 rooms designed for the use of one company. The condition of these houses are good, being just completed, but being partly underground, when heavy rains occur, the roofs being of earth, leak badly, and the water collects and runs in at the doors.

According to the plan, there should be eight sets of officers quarters, two of which are occupied (HS-201, HS-202). The rest now being completed. Each set forms an angle and is composed of eight rooms. One side of the angle is composed of three rooms, two of which are 16 feet by 18 feet, and one 12 feet by 16 feet, the other side is composed of five rooms, two of which are 14 feet by 16 feet, one 12 feet by 16 feet, one 16 feet by 16 feet, and one 8 feet by 16 feet. They are built partly underground and during heavy rains the rooms are subject to inundation.

The parapet (HS-200), forming the breastwork, is fast washing away and filling up the ditch around the works. This cannot be prevented, unless the slopes are sodded. There are two sets of Company Quarters and one set of Officers Quarters of four rooms, inside the works, which are put up temporarily which to render substantial buildings, would have to be rebuilt. The only board floors in the garrison are in the two sets of Officers Quarters outside the field works, all the rest are dirt floors. I would respectfully state that the buildings forming the Officers Quarters, Company Quarters, and Storehouses cover the curtain of the field works, to such an extent, as to weaken the defence of the place and as stated before, all being underground and without ventilation are unhealthy to men and subject all the stores placed in them damaged. [24]

The star fort had a flagstaff by the summer of 1862 (HS-225), and the quartermaster sent a request to Santa Fe for a garrison flag and halyards so that the flagstaff could be put to use on the Fourth of July. [25]

Additional Problems. The strategic vulnerability of the star fort was disclosed in June, 1862, when the post commander arranged for a test involving a six-pound and a 12-pound howitzer and the fortification. The six-pounder was placed at the foot of the hills, and the twelve-pounder at the crest of the hills above the first fort. Both were loaded with ordinary charges and shot off. Both guns had the fort within range. When a six-pounder was set off from the western bastion of the earthwork, its range only reached halfway to the hills. Also, the commander commented that the fortification "has a 'dip' towards these hills which causes its whole interior to be revealed." [26]

2nd Lt. A.W. Robb told Capt. Peter Plympton (then commander) that not only was the fort vulnerable, but also it was wholly unsuitable for occupation. Excessive dampness in the walls and flooring was causing disease among the men. Canby came to check it out personally, and decided that because of its location Fort Union would make a decent site for a depot; but he also conceded that just about all forts could be made vulnerable by contemporary gunfire. As a result of the visit, Canby gave the order to proceed with additional structures above ground. [27] That same month, Plympton received another letter from headquarters in Santa Fe asking him to examine "the field work for the purpose of seeing whether the interior of the work can be defiladed and if it can, to suggest the best method of doing so." [28]

Captain Plympton looked into the possibility and concluded that the star fort could not be defiladed to protect it from fire from the mesa above because of the size of the work and its type of construction. Because the sets of quarters, store houses, shops, and offices formed the demilunes and were outside the flanks, Plympton concluded that any system of traverses would afford minimal protection from shelling. Plympton wrote: "In a word the site should have been selected at a point beyond the reach of the shot, and shells of the enemy or if it had be necessary to put it where it is, which was not the case, its interior should have had a much greater command." [29] Plympton also complained that the storehouses and quarters were badly ventilated, that the magazine (HS-208?) was too damp to store ammunition, and that the soil contained a large percentage of clay so it retained moisture for long periods of time. Despite Plympton's dim view of the fortification, a company was sent up from Fort Union to cut trees for abatis for the fort. [30]

Other controversies arose that summer about the suitability of the star fort. The commander of the Department of New Mexico wrote to his superiors in Washington defending the fort. First, he noted that the site of the fort was chosen to be at a good location point for a general depot—a spot that was accessible and easily defensible at the same time. Although other points on the Mora and Gallinas had been considered, both were rejected, and by that time Fort Union had already been in use as a general depot or sub depot for a few years, so it had storehouses and barracks available for occupancy while the earthwork was under construction. He noted that even if a more strategically opportune location had been available to construct the fieldwork, he did not want to separate his command and supplies and take a chance on losing both his men and materiel.

He noted that when Grover and Chapman chose the site for the fieldwork, it was thought to be beyond the range of field guns—which turned out to be false. But also the fieldwork had to be located there to control the water supply needed for a thousand men and a thousand animals. The commander went on to say that the original idea was to build storehouses and barracks to the right and rear of the fieldwork "and to complete the arrangements for defense of the Depot by the construction of a similar fieldwork at the opposite angle, but this was changed upon the recommendation of Col. Chapman and Captain Grover by making provision for the storehouses, &c. in outworks (demilunes) of the original work, bringing the whole into smaller space & rendering them defensible by a smaller number of men." He went on to say that the original redoubt was completed shortly after construction began, but the out works and storehouses were not completed and work was suspended on them in early June, 1862.

The commander concluded that: 1) the fieldwork was within range of field guns; 2) the places he approved earlier for barracks and storehouses did not really meet the need; 3) the construction of a redoubt on the hill above the earthwork would prevent "the establishment of batteries within the range of any except rifled guns"; 4) that the construction of a redoubt would cover depot buildings from fire; 5) that the redoubt was necessary to control the water supply. So the commander ordered a survey of the area to pick a suitable spot for the construction of a redoubt. [31] The plan was submitted in November, 1862, but it was never built.

At least part of the arming of the fortification took place in October, 1862. With the arrival of the Colorado Volunteers, the Department Commander issued an order arming the volunteers with two 32-pounder field howitzers, two 12-pounder field guns, and two 3-inch parrott guns to defend Fort Union. [32]

Also in the late fall of 1862, General Carleton visited Fort Union and noted that the buildings were in bad shape, but some of the structures were in a tolerable state of preservation. His greater concern was that there was not enough room in the extant structures to meet the needs of the quartermaster. So Carleton ordered that a roof be put on the completed adobe walls of the first storehouse being built at Third Fort (HS-40, 41, or 42), and he had the soldiers pile up the adobes that were already finished to save them from disintegration in the winter weather. [33]

By mid-December, 1862, the abatis were in place. They had been under construction since November, 1862. [34] An article in the Mesilla Times that was carried in other papers described the fort as follows:

New Fort Union, situated one mile due east of the old fort, is considering its position and the material at hand, one of the best pieces of engineering ever done in America. It is an octagon . . . the walls are double rows of large pine logs en palisade, 12 feet between the rows, and filled with sod. The ditch is 20 feet wide at the top, 16 feet at the bottom, and 12 feet deep. The abatis is firmly studded with dwarf cedar trees, the branches trimmed short, case hardened with fire and sharpened to a point. These are firmly driven in, and present a bristling array upon which it would be impossible to force cavalry. The cannon enfilade the ditch at all points, and there is no cover for the approach of an attacking party within cannon shot. The magazine, quarters and all the garrison buildings are half basement, bomb-proof buildings, some of these are entirely under ground. Four large bomb-proof ware-houses have been built, fronting the salient angles of the fort, and in the shape of a wedge. There are in this post two years supplies of all kinds for two regiments. Ten 12 pounders are mounted, and several guns of larger calibre were being mounted. [35]

Additional Changes to the Star Fort. When queried by Fort Union's commanding officer about the appropriate way to construct a magazine inside the earthwork, M.S.K. Shoemaker responded that the troops should make an excavation about 60 feet long by 25 feet wide, and eight feet deep, "the walls to be formed by setting timbers upright and lined with rough boards, to the height of fourteen feet to receive a plate, on which the roofing timbers will spring to a longitudinal view, supported through the center by uprights, the roofing timbers to incline one foot in twelve, and a half, with boards to receive the earth which has been excavated. The room to be floored with 1-1/2 inch plank on sills or joists. The rooms to have a ventilation, and a door at each end with steps to descend, these doors to be protected by splinter proofs composed of inclined timbers. The whole to be covered with earth at least three feet in thickness, in order to be Bombproof." [36]

Ten days after Shoemaker described plans for the magazine within the fieldwork (late fall of 1862), Captain Plympton received a letter from department headquarters in Santa Fe enclosing an approved copy of a report entitled "Changes deemed necessary in the defense of Fort Union N.M and the means of conducting its defense in case of attack." Lieutenant Anderson, "acting as an Engineer officer" drew up the plans. [37] Plympton was directed to have one whole company—officers included—work on the fieldwork "until the work is done commencing at once . . . The work must be done with a will, and as soon as possible." The order also encouraged Plympton to proceed quickly to implement all of the changes, and noted that the brigadier general was counting on Plympton's personal supervision of the work. [38]

In December, 1862, the commander of Fort Union received a directive from headquarters in Santa Fe to construct additional bombproof spaces in the fieldwork. On the chance that Colonel St. Vrain headed down to the fort with over 100 men from Taos, headquarters wanted to have a large, underground room "made for contingencies." The order stated that the room's interior should measure 100' x 25' with a ceiling 12' high.

The purpose was to shelter part of St. Vrain's troops in case of bad weather. [39] Other directives concerning construction also appeared about the same time. Special Orders 209 stated all building materials that had been collected to construct the Quartermaster Depot but might be required to strengthen the defensive fieldwork would be used for that purpose at the discretion of the fort commander. The orders also approved the employment of thirty "first rate" laborers to build a magazine within the fieldwork at Fort Union, so Shoemaker's new building was being realized. [40]

Also in early December, 1862, Carleton wrote to Ceran St. Vrain in Taos that "The defenses at Fort Union need a great deal of work to make them strong, and we are bending all our energies toward completing them." [41]

More Construction. In February, 1863, 1st Lieutenant Cyrus DeForrest of headquarters, Department of New Mexico wrote to Captain William Craig, the depot quartermaster at Fort Union, directing him to tear down "the old house on the hill known as Col. Sumner's house which was formerly the hospital at Fort Union [HS-126]." DeForrest ordered Craig to salvage lumber, doors, and windows and to construct a new set of officers' quarters of "say four rooms and a kitchen, with a yard &c, complete and comfortable—over near the Redoubt." The directive went on to say that the building would be the new commanding officer's quarters (probably HS-224) and that the commanding officer could select whichever side of the fieldwork he chose for its construction. The building was to be a temporary structure built of logs plastered on the interior "with blinds for the windows and a gallery [porch] running along its front, say ten feet broad." The building was supposed to have a roof of lumber and chimneys of stone. Craig was also ordered to complete the project quickly and report its completion to headquarters as soon as possible. [42]

In April, 1863, Captain John Court McFerran of the Quartermaster's Office in Santa Fe gave orders to Captain N. S. Davis of the same office to go out to Fort Union and complete the new quartermaster depot and post at Fort Union (the third fort) and to finish the work as soon as possible using "the strictest economy, consistent with a rapid completion of the work." McFerran also ordered him to salvage windows, doors and all other building materials in the quarters of the demilunes of the field work. The demilunes were slated for demolition specifically because the building materials in them needed to be salvaged for use in the new post. McFerran also requested an estimate of roofing zinc or tin for covering the new officers' and soldiers' quarters and the storehouses for the third fort. [43]

Four days after the above order was issued, Carleton issued Special Orders 23 stating that Fort Union's commanding officer was to move into the new set of quarters (probably HS-224) near the field work. Also, all enlisted men and laundresses were to be quartered in the demilunes of the fieldwork (HS-204, 205, 206 and 207) until they ran out of room, at which time they were supposed to pitch tents near the redoubt. [44]

In 1865, an inspection of Fort Union described the situation at the Fort:

Fort Union is near the western limits of the great plains which extend uninterruptedly from Fort Leavenworth to the Rocky Mountains. Here there is a defensive earthwork with temporary quarters in the demi-lunes for some eight companies. There are but five companies of infantry at present at Fort Union.

The depot for quartermaster stores and the depot of subsistence stores are building by order of the War Department at Fort Union: and new permanent quarters for four companies are also in process of erection at that post. When the latter are completed, the temporary quarters in the demi-lunes will be abandoned, and the materials of which they are constructed will be used for other purposes. [45]

Later Occupancy. Despite the problems that appeared in the star fort in its first two years and the fact that the imminent threat of invasion by confederates diminished after the end of March, 1862, the Army continued to occupy and use the second fort into the autumn of 1866. The temporary nature of the earthworks again became apparent in comments that M.S.K. Shoemaker wrote to his superiors in Washington. Shoemaker was dissatisfied with the magazines within the earthworks. In particular, he was not happy with his ammunition storage space, and he wanted to get his stores properly taken care of before the next rainy season. [46] By 1864, Shoemaker had moved his entire ordnance operation back to the first fort area, but most of the other Fort Union operations remained in the vicinity of the second fort. Even when the commanding officer's quarters (possibly HS-224) burned on November 25, 1864, the troops remained entrenched in the fortification.

The constantly damp conditions in the star fort increased the incidence of disease among the troops. A report from General Carleton, Commander of the District of New Mexico, noted in September, 1865 that the earthwork at Fort Union had "temporary quarters in the demi-lunes for some eight companies." The report noted that the depot for the quartermaster's stores and subsistence stores, as well as the quarters were under construction (the third fort). At the time the new buildings were completed, the commander intended to abandon the demilunes and salvage any usable building materials. Carleton also complained that at that time all of the ordnance stores for New Mexico were stored in "a confused group of log and adobe buildings [the first fort] which have been erected from time to time since 1851 as temporary shelter until a proper arsenal could be constructed." [47] This again emphasized the need for a new arsenal.

On October 16, 1866, a huge rainstorm flooded out the barracks of the star fort with 8 to 12 inches of water. A report written by the post surgeon the following day said that all the men were damp. The dampness coupled with the intermittent storms convinced the surgeon that the incidence of disease including fever, rheumatism, and heart complications were "due in great part to the casemated barracks occupied by the troops at this post." He told the post commander that the only way to ameliorate the situation was to repair the old barracks, move into tents, or move early into the new quarters at the third fort. [48] Authority to occupy the new quarters "any time you desire" was granted the same day, pending their completion, of course. [49]

After that last flood, the troops moved out into tents because nothing at the third Fort Union had been completed by that time except one new officers' quarters. At the same time, the Army issued an order to move the district headquarters of the Department of the Missouri from Santa Fe to Fort Union. [50]

Demolition. By March, 1867, the endorsement came through on the order to demolish the remaining buildings "known as 'Old Post of Fort Union'" except those necessary for housing authorized laundresses (HS-204, 206, and 207) and stabling horses and mules (HS-205). The order also said that any woodwork that could be salvaged from the demolition would be turned in to the depot quartermaster. [51]

Although the order specified the old post, another contemporary report noted what was left of buildings "around the 'Old Post' or Earthwork, and find them to consist of three rows of partially underground frame structures in a very dilapidated state, fast falling to decay and ruin." The report recounted that a number of people still living in the structures were causing problems. The inspection stated: "There are always a lot of Mexicans and unknown Americans harbored around these buildings, Gambling, Drinking, and Prostitution seems to be the principal use to which many of the rooms are appropriated, and soldiers of the Garrison are enticed and harbored there to carouse all night." [52] At that time, the post commander requested permission to demolish all of the buildings of the old post "with the exception of sufficient quarters for the authorized Laundresses, and two angles of buildings, one of which is now used, and other can be used for Cavalry Stables (HS-205)." [53]

In April, 1867, the final order came through: prior to demolishing the buildings at the old post, all window frames, sashes, doors, and other serviceable materials were supposed to be taken out of the buildings and turned over to the depot quartermaster. [54] Apparently not all of the buildings were demolished, because the infantry was storing fresh vegetables in the "(Bomb-proof Hospital) Old Fortifications" six months after that order came through. [55]

The Tunnel (HS-222). Access to a convenient and steady water supply was a principle reason for deciding the location of the second fort. In 1861 the soldiers built a tunnel that began in the ditch of the fort and connected it to a cistern beside the creek. This tunnel collapsed soon after its completion, but wells excavated inside the fortification made it unnecessary. [56] In January, 1862, a well was under construction in the star fort. Period correspondence stated: "A well has been commenced inside the fortification & every arrangement has been made to receive the enemy properly should they come here." [57]

The presence of the tunnel was discussed in a letter from Harry LaTourette Cavanaugh to James Arrott in 1950. Cavanaugh spent 1882 and 1883 at Fort Union. He reported to Arrott: "We four boys, playing near the old earthwork, discovered a deep hole in the ground. It was explored and it was found to connect the earthwork with the creek (about 1/4 mile away). I was told that the tunnel was later filled up as it was dangerous." [58]

The tunnel was small and was lined with boards. [59] Park employees traced the route of the tunnel in 1961, and found enough remnants to ascertain its approximate dimensions—three feet, widening to five feet, and with a circular, cistern-like depression of ten feet in diameter near its outlet at the creek. [60]

Summary. The site for the earthen fortification at Fort Union was chosen for its proximity to the water supply needed to maintain the troops and the animals for the fort. The fort was vulnerable to howitzer fire from the mesa above for two reasons: its distance from the mesa, and the way its interior was exposed by its slant toward the mesa. Although the army acknowledged its vulnerability, based on a field test on the fortification in June, 1862, the earthwork never saw a battle. When the confederates were repelled at the Battle of Glorieta Pass in March, 1862, the Civil War in the west virtually ended. After that, the vulnerability of the second fort was most likely of lesser concern at the time of the field test than it had been at the beginning of the war.

The earthwork was of simple construction: digging the ditches outlining the interior of the fort provided earth for the parapets. Interior room features included shoring timbers, board ceilings with dirt above, brick chimneys, and either dirt or board floors. Although the exterior never received a sod covering, the branches and abatis protected parts of the exterior. Living conditions were far from satisfactory due to dampness, lack of light, and lack of ventilation. The slope of the land and the partial construction underground made the fort susceptible to flooding. By the time construction began on the third Fort Union in 1863, this army facility, too, had outlived its usefulness. Like much of the first fort, the army salvaged building materials from the fortification for re-use in the third fort.