One of the formations is called 'cascada chica' or small waterfall. It was pretty easy to access, unlike the larger waterfall which required a guide to hike to the top. We didn't see any way to access the bottom of either waterfall.
More information as well as maps to Hierve. The new cuota just opened and the toll is 43 pesos ($2.65 U.S.). It's the easiest route and there are signs all along the way. Once you get to Hierve, there are places to camp, restaurants, rest rooms and what looked like a hotel. Old maps will show going through Mitla but the new cuota goes around Mitla and is worth it. The new road has curves but not as many as the map shows. Once exiting the new road and on a paved road to San Lorenzo Albarradas the road gets more and more curves, then turns to dirt road.