I believe the monument in Rossville Cemetery is a memorial. He is buried in in Corinth National Cemetery.
Image found on Find a Grave
Lewis Powell
Corinth National Cemetery, Plot B, 3254
Corinth, Alcorn County, Mississippi
(Mar. 1, 1841 - Feb. 16, 1863)
The image below was also found on Find a Grave
Rossville Cemetery
Jefferson Township, Allamakee County, Iowa
(Mar. 1, 1841 - Feb. 16, 1863 Co. I IA 27 Inf)
Inscription
Lewis
Son of
A & E Powell
Died
Feb 16, 1863
Aged
?
Co I 27th
Inf. Iowa Vol.
Buried at Jackson, Tennessee