From Wikipedia on the 20th Iowa:
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"The 20th Iowa Infantry was organized at Clinton, Iowa and mustered in for three years of Federal service on August 25, 1862. The 20th Infantry moved to St. Louis, Mo., September 5; thence to Springfield, Mo., arriving September 24. They were attached to the following units over the course of their service:
1st Brigade, Herron's Division, 13th Army Corps, Dept. of Tennessee, to July, 1863.
1st Brigade, 2nd Division, 13th Army Corps, Dept. of Tennessee, to August, 1863, and Dept. of the Gulf to October, 1863.
The 20th took part in the following movements, actions, and engagements:
Moved to St. Genevieve June 3, and to Vicksburg, Miss., June 6–14.
Siege of Vicksburg June 14-July 4.
The regiment was mustered out on July 8, 1865.
The 20th Iowa mustered 1026 men at one time or another during its existence. It suffered 1 officer and 13 enlisted men who were killed in action or who died of their wounds and 2 officers and 157 enlisted men who died of disease, for a total of 173 fatalities. 52 were wounded."
Union Order of Battle fron Civil War Home:
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"HERRON'S DIVISION.
Maj. Gen. FRANCIS J. HERRON.
First Brigade.
Brig. Gen. WILLIAM VANDEVER.
37th Illinois, Col. John C. Black.
26th Indiana, Col. John G. Clark.
20th Iowa, Col. William McE. Dye.
34th Iowa, Col. George W. Clark.
38th Iowa, Col. D. Henry Hughes.
1st Missouri Light Artillery, Battery E, Capt. Nelson Cole.
1st Missouri Light Artillery, Battery F, Capt. Joseph Foust."
Text of Marker:
(shield with olive branches)
IOWA
20th Infantry
1st Brigade Herron's Div.
Sharpshooters Line
June 24 - July 4 1863
About 750 yards in front of marker