See previous discussions of the mystery surrounding Joshua Tanner's burial: (Surname books and the Reed's Corner Mystery) and (Joshua Tanner and the Elusive Reed's Corner).
In 1917 volunteers associated with the local chapter of the Daughters of the American Revolution (DAR) scoured the towns of Greenwich and Easton, New York, looking for old cemeteries and burials. A distant cousin and local farmer, Oscar W. Tefft (1865-1934), drove around and cataloged all the old burial plots tucked into corners of area farms. Oscar was very interested in family and local history. He helped with the research for the book The Tefft Ancestry (Stocking, 1904).
When he drove up and down the old farm roads and highways in 1917, the burial locations he found had between one and a couple dozen headstones as well a variety of graves marked only by slates. 
Oscar listed 19 farm cemeteries and gave each a name: Koert L. Foster's Farm, Alpheus Barber Farm, Daniel Tefft Farm, William Hartshorn Farm, and so forth.
After Oscar and others finished their explorations, they compiled the lists and sent them to be published in The New York Genealogical and Biographical Record.
Some of the cemeteries, particularly the larger ones, are still in existence, but a century later some may be unknown, or grave markers could be missing or inscriptions worn off, so Oscar Tefft and the others provided a valuable service to the community.
In recent years websites including GenWeb put this data online, but as with any type of indexed information, it's worth going back to the first known compilation.
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Several months ago, Karen Bray Keeley noted on Family Tree that Joshua Tanner was buried in the William Hartshorn Farm Cemetery. She had seen the name "Joshua Turner" and recognized that the other burials in the small cemetery were Joshua and Thankful's grandchildren, so "Turner" was a misreading of Tanner, and now thanks to Karen we know where Joshua is buried.
I saw her note on Friday and although she used one of the secondary sources it was enough to find the original list.
As I read through the lists, many names were familiar: Tanner, Tefft, Barber, Kenyon. Since so many Tanner relatives migrated in great waves from Washington County, Rhode Island, to Washington County, New York, these families lived and died within miles of each other. 
The list cleared up a few mysteries.
John Tanner's first wife, Tabitha Bently Tanner, is buried in the Koert L. Foster Farm Cemetery, one mile north of Greenwich.  She is buried next to her father Elisha Bently, her sister Mercy Bently, and her mother Sarah Bently Rose.  This means that the Elisha Tanner who migrated west with John Tanner and his family was not Tabitha's father as Pioneers and Prominent Men claims. Instead the Elisha mentioned in the book may be a brother or other relative.
Here is the list showing Joshua Tanner, transcribed Turner, five grandchildren, and son-in-law John Wellwood. The death dates of the grandchildren are between 1816 and 1826, which indicates that Tanner relatives were living on the farm until at least that date.
Based on the Greenwich grave records, the information in this Tanner genealogy is largely incorrect.
From Maurice Tanner, Descendants of John Tanner, The Tanner Family Association, 1923.
Many of the family trees show the John and Esther Tanner Wellwood family living in Mexico, Oswego, New York, but they were clearly located in Greenwich, and Esther continued to live in the area until at least 1855. By 1860 she had joined one of her sons in Mexico, but it is incorrect to list her children as being born in Mexico. Additionally, many families list a son John in this family with the same death date as Esther's husband John. Due to the possible confusion created by this list, the existence of a son John needs to be proved separately.
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The list of burials at the William Hartshorn Farm Cemetery includes an inscription on a stone that says simply "T.T." Thankful Tefft Tanner died in 1822, so there is a good chance that she died and was buried next to her husband on the farm where she had lived for decades.
Why would she have been buried in Greenwich when she was shown living in Greenfield, Saratoga, New York in the 1820 census? A quick look at Google suggests an answer: Greenfield was adjacent to Saratoga Springs, which was where people, including invalids, went to "take the waters," or use the mineral waters for their supposed health benefits. It makes more sense that she would have relocated temporarily for health reasons than that she would have randomly moved 23 miles west to a town with no known relatives.
Why didn't Thankful have a normal gravestone? There are at least four possible reasons.
First, she may have had one and it may have been broken or disappeared during the intervening century.
Second, she was a widow. It is likely that she would only have had a stake in any real property (farm land and dwellings) until her death, and living as a widow for so many years could have reduced her circumstances.
Third, her death occurred during the aftermath of the Great Panic of 1819. Times would have been hard and cash would have been scarce for farmers.
Fourth, her oldest son John Tanner had moved to Bolton Landing and had a large family and many financial demands. Her daughter Esther Tanner Wellwood who, as the deaths of her children attests, lived there on the farm, was recently widowed. Thankful's youngest son William may have taken over the farm after John Wellwood died, and he was just starting out in life, so there may have been no family members able to purchase an expensive headstone.
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These records leave a few questions.
First, is there anything I missed? Do you read any of the records differently?
Second, where are these graves? I cannot find any of these farm cemeteries in FindAGrave or Billion graves except for the Alpheus Barber Cemetery. (That is the burial location for another of John Tanner's sisters and her gravestone is still there.) I have put out some requests for information, but have not heard back from anyone yet.
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Thanks to Oscar Tefft we have a better picture of the Greenwich Era in the Tanner family history. In appreciation I left a "flower" and brief note on his FindAGrave entry.
 Although the following article is primarily about Dutch settlers in the region, it explains the different type of markers found in cemeteries from this era. (Brandon Richards, "Fieldstone Burial Markers in the Upper Mid-Atlantic Colonies," January 23, 2010, link.)
 The great family migration that John Tanner participated in while he was a boy certainly prepared him with the practical knowledge and experience he would need much later in life when his family moved in other great migrations: first to Kirtland, then to Missouri, Illinois, and Utah. The practical training he gave his sons benefitted many others in the Church as they helped lead wagon trains and freight back and forth between western settlements.
 The name is alternately listed Bently and Bentley. The government records from the time use "Bently" so I mostly use that, but it isn't too important; either spelling works.
 The others in the Koert Foster cemetery may also be Bently relatives.
Picture of rural Westchester County, New York cemetery from Flickr.