Has there ever been any written evidence found on the practices of ancient druidism ?

I searched the net and found some stuff on druidism but it doesnt make sense to me .One reason why is man`s intellect . People back then seemed more than smart enough to work out chemical formulae and math was(if used ) of ancient societies was very developed.For people that well developed and not keep some kind record of thier work just doesnt ring true . To not have no way other than word of mouth and to p on from it mentor to student doesnt make sense . druidic law or whatever other pratices and knowledge they had would be lost easily in events such as plagues, genocide,starvation or wars .Isnt it reasonable to ume that there must be or had been some kind records left of these beliefs