The 139th Pennsylvania Infantry served as a member of Wheaton’s Brigade in Newton’s Division of the Sixth Corps, Army of the Potomac, a Fighting 300 Regiment. The unit was commanded by Colonel Frederick H. Collier (1826-1906). He was wounded on July 3rd, and Lieutenant Colonel William H. Moody then took command. The 139th was transferred in October 1862 to the VI Corps. In the Battle of Fredericksburg, it suffered minor casualties from artillery fire, but didn't get a chance to fight. Five months later, however, it did participate in the 2nd Battle of Fredericksburg. At the Battle of Gettysburg in July, it helped defend the left flank of the Union army. At Gettysburg, the unit has 511 troops engaged with 1 killed and 19 wounded.
The 139th Pennsylvania Infantry is located on the right or northeast side of the road if traveling northwest, in a clump of trees, along the driveway to the John Weickert farm. The road leading to the farm is Crawford Road and it terminates at the farm, northeast of this position, 521 feet away. This site is the field north of the "Valley of Death". This position is also 50 feet shy of the four-way intersection of Crawford and Wheatfield Roads to the north. 
Parking is available at small, cutout shoulders along the road, some wide, some narrow. Be sure to stay off the grass or you will be ticketed by park police. I visited this monument on Saturday, March 10, 2012 at 6:08 P.M., exactly 10 minutes after sunset and (just before the clocks were set ahead). I am not sure how I was able to take pictures, but it happened; the sun was setting so I got a really nice orange glow on the bronze. I was at an elevation of 527 feet, ASL. I used a General Electric 10.1 megapixel model # A1050 digital camera for the photos.
This monument, although the primary monument, was not the first raised to the 139th. There is a position marker located along Sickles Avenue @ Excelsior Field. The waymark for that seconday marker can be found HERE. The Draw the Sword site helped out by the NPS narrative and the SIRIS site offers the following description: Monument: granite with bronze adornment; Base: granite. Granite monument with granite shaft and relief of eagle and crossed flags on front, and cap with Greek cross. Bronze reliefs of the Pennsylvania shield with eagle and horses appears below the granite relief. The monument sits on a rough-hewn base. Monument is a 3.11×2.1 foot granite shaft set on a rough hewn base, 6.5×5.4 foot. Overall height is 14.3 foot. The shaft has inscriptions on polished panels, a relief of crossed flags and eagle and a bronze state seal on the west side. It has a relief of a Greek cross on all sides of the cap. Flanking markers are flat topped, one foot square.
The location of the monument marks the position held by regiment on the evening of July 2, 1863, until the close of the battle.
The monument was dedicated September 11, 1889 by the Commonwealth of Pennsylvania and was fabricated by the Rhode Island firm of Smith Granite Company, who had their hands in at least sixty-three monuments scattered about the battlefield. The monument is made of granite with bronze adornments and has the following dimensions: Monument: approximately 14 feet 3 inches x 3 feet 11 inches x 2 feet 10 inches; Base: approximately Width 6 feet 5 inches x Depth 5 feet 4 inches. There are inscriptions on all four sides which read:
(Front):139th Pennsylvania
Infantry.
3rd. Brigade,
3rd. Division,
6th. Corps.
(Left):
Left Manchester, Md at 8 p.m. July 1st and arrives at Rock Creek on the Baltimore Pike at 2 p.m. of the 2d. Towards evening the brigade moved rapidly to the front to support the Union left, this regiment deploying on the right of Little Round Top, and advanced with the 1st. Brigade Penna Reserves driving the enemy into the Wheat Field.
Retired to and held this position until the evening of the 3rd. when the regiment moved with the Penna. Reserves and advanced about 900 yards to the position indicated by a Greek Cross tablet, and assisted in forcing the enemy back. Subsequently returned to this position. Present at Gettysburg 511. Killed and mortally wounded 4, wounded 16.
(Back):Antietam. - Totopotomoy.
Fredericksburg. - Cold Harbor.
Marye's Hights. - Petersburg.
Salem Heights. - Fort Stevens.
Gettysburg. - Opequon.
Rappahanoock Station. - Fisher's Hill.
Mine Run. - Cedar Creek.
Wilderness. - Petersburg (Fort Fisher).
Spotsylvania. - Petersburg (assault).
North Anna. - Sailor's Creek.
Appomattox.
(Right):
Recruited in the counties of Allegheny, Armstrong, Mercer and Beaver.
Mustered in Sept. 1, 1862.
Mustered out June 21, 1865.
Total enrollment 1070.
Killed and died of wounds, 10 officers, 141 men.
Wounded, 36 officers, 424 men.
Died of disease & c. 5 officers, 29 men.
Captured or missing, 1 officer, 54 men.
Total 750.
The 139th Pennsylvania Infantry Monument is a contributing feature to the Gettysburg National Military Park Historic District which is nationally significant under NR Criteria A, B, C & D. Areas of Significance: Military, Politics/Government, Landscape Architecture, Conservation, Archeology-Historic. Period of Significance: 1863-1938. The original National Register Nomination was approved by the Keeper March 19, 1975. An update to this nomination was approved by the Keeper on January 23, 2004. The monument is identified as structure number MN118-B.
From the Nomination Form:
Marks position held by 139th Pennsylvania Infantry from p.m. July 2, 1863 until close of battle. 1 of 110 mns in Park to Pennsyvania commands. Located S end of F.Althoff/John Weikert Farm Lane, N of Wheatfield Rd. in Plum Run Valley.
Short Physical Description:
Mn & 2 flank markers. Rough hewn base, 6'5"x5'4". Shaft 3'11"x2'10", all 14'3" H w/inscriptions on polished panels, relief of crossed flags & eagle, & bronze state seal on W face. Relief of Greek Cross on all faces of cap. Flank markers: flat topped, 1'x1'x1'6".
Long Physical Description:
Monument that has two flanking markers. Monument is a 3.11x2.1 foot granite shaft set on a rough hewn base, 6.5x5.4 foot. Overall height is 14.3 foot. The shaft has inscriptions on polished panels, a relief of crossed flags and eagle and a bronze state seal on the west side. It has a relief of a Greek cross on all sides of the cap. Flanking markers are flat topped, one foot square. Located at the south end of the F. Althoff/John Weikert farm lane north of the Wheatfield Road.
My Sources
1.
NRHP Narrative
2.
SIRIS
3.
Stone Sentinels
4.
Virtual Gettysburg
5.
Draw the Sword
6.
Historical Marker Database
7.
Wikipedia
8.
Antietam on the Web