Margaret Fell probably could not be Anne Askew's granddaughter, on chronological grounds. Askew died in 1546, while Margaret Askew Fell Fox wasn't born until 1614. That would mean that Margaret's father would have had to be in the neighborhood of 60 (or more) when she was born, which -- though technically possible, is almost unheard of even now -- let alone then.
I have wondered about some sort of relationship between the two, but is Margaret Fell's parentage even known?
