Parrish was a fabulously successful illustrator. He earned over $100,000 per year in an era when houses cost only $2,000. His time was so valuable, you'd think he would've found a shortcut for the menial labor of painting tiny crevasses in dumb rocks.
To get the proper "feel" for rocks, Parrish used to bring actual rocks into his studio, paint them a flat color and
light them to accentuate their shadows.
The following detail from an original Parrish painting, photographed from 3 inches away, shows you how tedious it must have been for Parrish to paint all those damn crevasses:
Obviously Parrish decided that these details made an important contribution to his paintings, and that there was no simpler, faster way to achieve the effect he wanted.
Bernie Fuchs was another great illustrator whose time was very much in demand. He was an economical painter who abhorred unnecessary detail. Yet, he too seemed to believe that rocks in the background were worthy of his sustained attention.
There's nothing fake about these rocks; they required thousands of deliberately placed brush strokes. If Fuchs tried to get away with random marks, we would've seen the difference.
This is not an issue of mere realism. Fuchs wove more design into the details than Parrish did, but that was Fuchs's nature.
It's not necessary to paint rocks in great detail to be persuasive. Illustrator Harold von Schmidt simplified desert rocks using black poster paint and a wide, flat brush:
Von Schmidt grew up spending a lot of time staring at rocks in the desert.
On the other hand, when even talented painters disrespect rocks and attempt to fake it, as Frank Frazetta did in these two pictures, the rocks end up looking phony: