[The gravestone cannot be read in the photo, too distant. The memorial page, posted by a reputable contributor who is familiar with Warwick genealogy, gives the following inscriptions:
Stephen Johnson died Jan. 20, 1857, AE. 78.
Polly White, his wife, died May 18, 1858, AE. 75.
Daniel M. Johnson died Mar. 2, 1822, AE. 5 ys.
Mary Johnson died Nov. 4, 1889, AE. 74.
Source:Allred, Marilyn. Town of Warwick, Franklin County, Massachusetts, Genealogical Records 1739-1900, Births, Marriages,, p. 154, gives the death date as 18 May 1850 age 75, which not only doesn't calculate to the right birthdate, but is contradicted by Polly's presence in the 1855 census (left side, line 4). No actual death record has been located in Warwick records (e.g., here.) or elsewhere.]