This gravestone stands in Stretham churchyard, Cambridgeshire, amongst many other Yarrow gravestones. There is something engraved near the foot of the stone but I can't make it out now, and perhaps didn't spot it at the time.
However, this stone appears to have some errors.
The Stretham burials transcript gives William as being 71, and Elizabeth having been buried in 1837.
The Little Thetford burials transcript (Little Thetford being a hamlet of Stretham and it being common for inhabitants to be buried at Stretham), gives a different story: "YARROW Elizabeth otp 50 wife of William farmer, died in London was carried home and buried at Stretham" (Nov 23 1837).
This is two points for 1837, rather than the stone's 1839. The Stretham transcript gives the right age for her, but not for him.
There's no mention of William in the Little Thetford transcript.
Looking at FreeBMD, there's only an Elizabeth Yarrow death (so far) available, but that's registered in the June Quarter of 1838 at St. Pancras!