Several surnames derived from placenames not found in Cornwall have either become numerous here or largely confined to Cornwall over the centuries. These have to all intents and purposes become Cornish surnames. some, such as Chesterfield or Kendal, may have origins many hundreds of miles away, but the largest number, as we might expect, originated in Devonshire.
We can take all the surnames listed in The Surnames of Cornwall where I identified an origin in a Devonian placename which did not also occur in Cornwall and map them in the mid-1600s. We get the following distribution.
Most of the surnames from Devonian places stayed close to the Tamar, but occasional examples cropped up quite early in the west. The presence of both Seccombe and Lamerton on the Lizard in the 1500s might suggest sea-borne migration along the coast, as does the map above, where districts on the south coast seem to have a higher number of such surnames.
While some of these placenames only appeared in Cornwall at a later date some were already present by 1500. Lobb is an example of a surname that was established quite far west by the 1500s.