Early Eocene
London Clay
Isle of Sheppy, Kent, England
18mm nut from the famous London Clay Flora
Seed,

Here’s a plant fossil that drifted out to sea.  The London Clay is famous for its marine fauna but a surprising amount of land plant fossils have been found in it too…as in several hundred species.

See:

“Fossil Plants of the London Clay” (The Palaeontological Society, 1983) by Margaret E. Collinson.