In 'English Rock Terms' by Arkell & Tomkeieff, one definition of 'head' is:
'Subaerial detritus coating the surface of the rocks, especially in Devon
and Cornwall. Cf head on beer. First recorded 1839 (de la Beche, Report
on Cornwall, &c., 432)'
Like other contributors to this thread, I don't know whether this helps or
not.
Stafford
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