fold up vs undulate

fold up is not an adjective while undulate is an adjective.

Word	Noun	Adjective	Verb	Adverb
fold up	No	No	Yes	No
undulate	No	Yes	Yes	No

fold up (adjective)	undulate (adjective)
	having a wavy margin and rippled surface

fold up (verb)	undulate (verb)
become folded or folded up	increase and decrease in volume or pitch, as if in waves
bend or lay so that one part covers the other	move in a wavy pattern or with a rising and falling motion
	stir up (water) so as to form ripples
	occur in soft rounded shapes