Information gathered from Ancestry.com as I could find no entry in Paxton’s Marshall Family.
Phoebe was the daughter of Martin Pickett Marshall and Elizabeth Colston Marshall. This information came from her head stone and from her death certificate

1910 US Census shows her as head of household living with her sister Susan Massie.
Beyond this I found no more details. I hope others will be able to correct, extend and clarify the life and times of Phoebe Marshall
Newspaper Clippings
Last Will and Testament
Sale of Farm to settle Estate