Geo,
How interesting, thank you. The following seems relevant to your observation that the sand hill plum and beach plum appear to have some features in common:
"Improvement of native plums began only when the population of the country spread into climates such as that of the
prairie states and the South, where old-world plums could not survive." (Emphasis added)
I would be very interested to try the Kansan plum jam if you locate it. The Kansas Sand Hills contain some lovely memories for me, as I accompanied my father, my uncle and my brother there on a quail hunting trip one fall when the long grass was red and the skies were slate blue before a rain. It's beautiful country, beautified by plums, no doubt.
Fruit preserves are truly the best of heritage foods, don't you think? I'm currently working on a jar of Georgian cornelian cherry preserves and a jar of mulberry jam from the slopes of Mount Ararat as well as some Connecticut apple butter.
My daughter and I made up a batch of wild grape jam from grapes she foraged near New London. This batch had a winey complexity - a hint of the Concord grape depth, without the cloying quality.
Man : I can't understand how a poet like you can eat that stuff.
T. S. Eliot: Ah, but you're not a poet.