The 20th Maine Infantry served as a member of Vincent’s Brigade in Barnes’ Division of the Fifth Corps, Army of the Potomac, a Fighting 300 Regiment. The 20th Maine was organized in the state of Maine and mustered into Federal service on August 29, 1862 The unit was most famous for its defense of Little Round Top at the Battle of Gettysburg. The infantry was commanded by
Colonel Joshua L. Chamberlain (1828-1914), a professor at Bowdoin College. He was wounded six times and awarded Medal of Honor for actions at Gettysburg. In his post-war life he was the governor of Maine and President of Bowdoin College. He was given the honor of commanding the Union troops at the surrender ceremony for the infantry of Robert E. Lee's Army at Appomattox, Virginia. Under his command, the 20th brought 386 men to the battlefield and among them 29 were killed, 91 were wounded and 5 went missing. All totaled the 20th had a total wartime enrollment of 1,621 men (including initial muster, replacements, and drafts), losing 147 dead from combat, 146 dead from disease, 381 wounded, and 15 in Confederate prisons.
The 20th Maine Infantry Monument is located off of Wright Avenue, up a walking trail. The climb is steep. The monument is 115 feet from the Wright Avenue. This area and monument are located on the rocks in the Vincent Spur or saddle area between Big Round Top and Little Round Top. The NPS walking trail will bring you everywhere the is to see in this area. If walking a bit higher, a gorgeous view of Devil's Den will be afforded to you. 
Parking is on Wright Avenue. Stay off the grass or anything green or you will be ticketed by the Park Police. I visited this site on Monday, July 1, 2013 on the 150th anniversary of the Battle of Gettysburg @ 1:41 PM, EDT & @ an altitude of 602 feet, ASL. As always, I used my trusty and oft abused Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
Draw the Sword, using descriptive help from the NPS site and the SIRIS site, offers the following description: Polished granite rectangular block monument with apex top and cross emblem on front, set atop boulder. Monument is a polished granite shaft three foot square with apex top on a four foot polished base. Overall height is 5.4 foot. The whole rests on a boulder. Inscriptions are cut into all four sides. Located on rocks in the Vincent Spur or saddle area between Big Round Top and Little Round Top. The monument is installed on the rocks where the infantry’s colors where planted and where Col. Joshua Chamberlain stood when he refused his line into a V to meet the oncoming Alabamains on the late evening of July 2, 1863.
The 20th Maine Infantry Monument was dedicated on June 1, 1886 by the State of Maine, making it one of the earliest monument erected at Gettysburg. The monument is composed of Hallowell granite (more on that). The sculpture is approximately 4 feet 4 inches x 3 feet 3 inches² and the base is approximately 1 foot 2 inches x 4 feet². Although no artisans are on record of having made this sculpture, I'd wager it was fabricated by the Hallowell Granite Company as Hallowell. The 
6th Maine Infantry Monument, another Maine monument is also constructed of Hallowell granite and fabricated by the Hallowell Granite Company. The only problem is the 6th Maine was erected three years after the 20th Maine so I could be wrong. There are lengthy inscriptions on all four sides, all of which read:
(Front):Twentieth Maine
Third Brig. - First Div.
Fifth Corps
(Left):Here the 20th Maine Regiment. Col. J. L. Chamberlain Commanding, forming the extreme left of the national line of battle on the 2nd day of July 1863. Repulsed the attack of the extreme right of Longstreet's Corps and charged in turn, capturing 302 prisoners. The regiment lost 38 killed or mortally wounded, and 93 wounded, out of 358 engaged.
This monument, erected by survivors of the regiment, A.D. 1886, marks very nearly the spot where the colors stood.
(Back): Names of the officers and men of the 20th Maine Volunteers who were killed or died of wounds received in this action.
Capt. Charles W. Billings - Co. C
1st Lieut. Arad H. Linscott - Co. I
2nd Lieut. Warrent L. Kendall - Co. G
Corp. Joseph D. Simpson - Co. A
Priv. John Reed Jr. - Co. A
1st Serg. Isaac W. Estes - Co. C
Priv. Moses Davis - Co. C
Priv Oliver L. Stevens - Co. C
Priv. Charles M. Beadle - Co. C
Corp. Willard Pinkham - Co. D
Priv. Stephen A. Prescott - Co. D
Corp. Paschal M. Tripp - Co. F
Corp. John Foss - Co. F
Corp. William S. Hodgdon - Co. F
Priv. Seth W. Clark - Co. F
Priv. John Wentworth - Co. F.
(Right):Priv. Oscar Wyer - Co. F
Priv. Charles F. Hall - Co. F
Priv. Benjamin W. Grant - Co. F
Priv. Elfin J. Foss - Co. F
Serg. William B. Jordan - Co. G
Corp. Melville C. Law - Co. G
Priv. James A. Knight - Co. G
1st Serg. Charles W. Steele - Co. H
Serg. George W. Buck - Co. H
Serg. Isaac M. Lathrop - Co. H
Priv. Aaron Adams - Co. H
Priv. Goodwin S. Ireland - Co. H
Priv. Iredell Lamson - Co. H
Priv. Alexander E. Lester - Co. I
1st Serg. George S. Noyes - Co. K
Priv. James R. Merril - Co. K
Priv. William F. Merrill - Co. K
Priv. Stephen C. Chase - Co. K
Priv. Willard W. Buxton - Co. K
The 20th Maine 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 MN089-B.
From the Nomination Form:
Located on rocks in Vincent Spur or saddle area bet. Big and Little Round Top; where colors were planted & where Col. Chamberlain stood when he refused his line into a "V" (marks apex) to meet oncoming Alabama on late p.m. July 2, 1863. Position marker is E, marks where Company B was stationed.
Short Physical Description:
Mn, 1 flank marker, 1 position marker. Polished base, 4' square. Polished shaft 3' square w/ apex top. All 5'4 high. Inscriptions cut into 4 sides of shaft. Whole rests on boulder. Flank marker: slant face 1'6"x1'x1'. Position marker, 2'x2'x2'2".
Long Physical Description:
Monument that has one flanking marker and one position marker. Monument is a polished granite shaft three foot square with apex top on a four foot polished base. Overall height is 5.4 foot. The whole rests on a boulder. Inscriptions are cut into all four sides. Flanking marker is slant faced, 1.6x1 foot. The position marker is two foot square. Located on rocks in the Vincent Spur or saddle area between Big Round Top and Little Round Top.
My Sources
1.
NRHP Nomination Form
2.
SIRIS
3.
Stone Sentinels
4.
Virtual Gettysburg
5.
Draw the Sword
6.
Historical Marker Database