  The Tibet AS$\gamma$ collaboration has reported a diffuse $\gamma$-ray
emission signal from the Galactic Plane. We consider the possibility that the
diffuse emission from the outer Galactic Plane at the highest energies is
produced by cosmic rays spreading from a single supernova-type source either in
the Local or Perseus arm of the Milky Way. We show that anisotropic diffusion
of multi-PeV cosmic rays along the Galactic magnetic field can produce an
extended source spanning ten(s) of degrees on the sky, with a
flux-per-unit-solid-angle consistent with Tibet AS$\gamma$ measurements.
Observations of this new type of very extended sources, and measurements of
their morphologies, can be used to characterize the anisotropic diffusion of
PeV cosmic rays in the Galactic magnetic field, and to constrain the locations
and properties of past PeVatrons.
