re: jarosite (also the pyrite and/or sulfur possibility)
Sure, why not?
from the image site:
Jarosite on pyritic, argillaceous sandstone in the Wealden Group at the eastern end of St Oswald's Bay near Dungy Head. Oxidation of the iron sulphide pyrite can produce sulphuric acid and iron oxides. Reactions lead to the formation of the hydrated iron-bearing sulphate jarosite (with either sodium, potassium or ammonium). This yellow surface encrustation is often mistakenly referred to in old literature as " sulphur ". It is slowly soluble in water and is washed away to some extent by heavy rain but develops again in fairly dry weather.http://www.soton.ac.uk/~imw/dungy.htm