From 1879 to 1903 the federal government furnished tombstones for Union veterans of the Civil War. index cards with information on these veterans and their burials are available on microfilm in many libraries. The index is also available on line via Ancestry.com and Family Search. Robert Crosby was listed as a Sargent,Company 1, unit US C H Art, buried in Concord Church in Warrensburg, Green County, TN, died April 12, 1877.
The 1970 census for Tn,Greene County,District 19, listed Robert Crosby, age 38, male, black, born in TN. It also listed his wife, Harriet Crosby, age 40, female, black, born in TN.
It is possible Robert Crosby was a black slave to one of the Crosby families in Greene County TN.
After search this cemetery, I did not find the gravestone. However there are numerous unmarked graves, unreadable stones, and sandstones that are extremely worn down.