formation/rock

Some formations within the park look as though the gentlest wind could send huge pieces of rock crashing to the ground. These are called balanced rocks. The most famous of all is simply called Balanced Rock. This sandstone rock is the size of three school buses. It sits on top of a tower of darker rock called mudstone. The formation is about 39 meters tall. This bottom rock narrows sharply where it connects to the sandstone rock on top.
Source: VOA Learning English - America's National Parks - The Strange and Beautiful World of Arches National Park

Can I say rock or sandstone rock in this context?

Formation is better in this context, especially since the passage uses those words just prior to the sentence in question.

The rock would make sense, belongedto, but formation is common and suitable in references to rock formations. If the word is not familiar to you, perhaps this definition of formation from the American Heritage Dictionary will be helpful:

5. Geology A primary unit in the mapping of rock strata, consisting of a set of contemporaneous rocks having the same characteristics and origin.

Cross-posted.

The formation is about 39 meters tall.
The rock is about 39 meters tall.

No, I don't think you can replace "formation" with "rock". "The rock" refers to the balanced rock. "The formation" means that rock plus the tower of rock.

The formation (the balanced rock plus the tower of rock) totals 39 meters tall. The balanced rock is only about 17 meters tall.

I have rewritten part of the passage to avoid some repetition and eliminate choppiness:

"Some formations within the park look as though the gentlest wind could send them crashing to the ground. The most famous of these is Balanced Rock, which rises to a height of 39 meters. It is comprised of a sandstone boulder the size of three school buses, perched atop a roughly pyramidal tower of darker material called mudstone."