Historical phonetics: "wh" --> {"w", "hw"}

[Lonhyn T. Jasinskyj]

Could anyone point me to some material on the origin of the English
"wh" (as in "wheat", "which", etc.) and how the two alternate
pronunciations ("w" and "hw" came about)?  This subject occupied all of
dinner last night and a good bit of poking around today. If anyone has
any suggestions or thoughts on this I would greatly appreciate it.
 
Many thanks,
Lonhyn