burthen vs overburden

burthen and overburden both are nouns.

Word	Noun	Adjective	Verb	Adverb
burthen	Yes	No	Yes	No
overburden	Yes	No	Yes	No

As verbs, overburden is a hyponym of burthen; that is, overburden is a word with a more specific, narrower meaning than burthen:

Other hyponyms of burthen include plumb, saddle.

burthen (noun)	overburden (noun)
a variant of `burden'	an excessive burden
	the surface soil that must be moved away to get at coal seams and mineral deposits

burthen (verb)	overburden (verb)
weight down with a load	burden with too much work or responsibility
	load with excessive weight