The Index holds 1 or more Eras.

Each Era has:
	Exactly one name (text string)
		(eg: "Paleolithic Period").
	One or more Sites (list)
	Exactly one colour (text string)
	  (hexedicimal colour value).

Each Site has exactly one:
	Unique id ref (text string).
	Non-unique os grid ref (text string)
		(8 figures describing a 10m squared area).
	Non-unique description (text string)
		(Short description. May be used to search for desired key words).
	Non-unique source (text string)
		(The author(s) of the description).