Sitting about for eons
Score one for the missus. The correct word is sedentary. It means “marked by much sitting about.”
Sedimentary is “having the nature of, or containing sediment.” Sediment is “matter that settles to the bottom of a liquid [or] matter deposited by water or wind.” (All definitions are from Webster’s.)
Let’s give hubby a break, though. One could argue that sediment does a lot of “sitting about” after it’s deposited.
Want proof? Look no farther than the Colorado National Monument. Though the monument is testimony to the effects of erosion, some of its geological layers have sat for millions of years. To better understand this natural wonder, go to http://www.nature.nps.gov/geology/parks/colm.