The taxonomists are at it again. Most of the guide books still classify the rock cresses in the genus Arabis, but recently most New World species have been moved to either Boechera or Arabidopsis.
lyre-leaved rockcress, aka sandcress (Arabidopsis lyrata, formerly Arabis lyrata)
Lyre-leaved rockcress is a mostly northern species, found across Canada and the northeastern US, with small, isolated populations found further to the south. It’s endangered in Massachusetts, and threatened in Ohio and Vermont
Needing very little soil, it doesn’t tolerate competition from other plants, but will grow happily by itself right out of rocks.
smooth rockcress (Boechera laevigata, formerly Arabis laevigata)
Smooth rockcress seems to enjoy a little more soil than lyre-leaved; it can be found in rocky woods and ledges, but seldom growing right out of the rock. It ranges from Quebec south to Georgia, and west into some of the Great Plains states, and is threatened in Maine and Massachusetts.
The rockcresses are in the Brassicaceae (mustard family).