Both these leagues have been going for some time now, I think the Saturday league was formed about 25 years earlier though.

Does anyone know how it was formed and if any teams went into the South of Scotland League from there?

not sure how it was formed but do know it used to have 3 or 4 divisions, a lot of the teams came from workplaces i.e uniroyal(gates),penmans,carnation,glaxochem,western buses,ici and the health board etc,also various pubs had teams.several sat amateur teams have gone on to south league,some very successfully!
lincluden swifts,max high fps,blackwood dynamos(now crichton),dumfries high fps(now heston rovers),abbey vale,mids,nithsdale and gretna to name a few.maybe the sat amateur league should join the south and have 2 divisions otherwise cant see it lasting much longer as its already
dying of death,which is a real shame because some clubs are better run than some south clubs!
not sure if any of the above started as sun teams or if any sun tems have eventually ended up in the sos league.

Sunday League was formed in 1975 20 odd years after the Saturday League.

I think St Joes FPs are possibly the only team to progress to the SOSL but that was via their merger with Dumfries HSFPs, though Cricjhton/Blackwood might have played in the Sundays in the 70s too when they were called Auldgirth.

Summerhill Thistle

Maxwelltown Braes

Thornhill Farmers Arms

Stewartry Strollers

White Hart Athletic

Lincluden

There were two more that I can't remember, Cricton Royal under the management of Brian Newton failed to gain entry first time aroun, but did play in the cups.