There are a few issues here that deserve mentioning.
"planes" just means large flat areas.
"hard slopes" just means steep, edges of hills/mountains.
In other words his face was made up of large flat areas and "craggy" slopes and valleys with little or no expression.
The other oddity in this first part is the "his face".
A "perfect" way of expressing it would be:
His face was all planes and hard slopes...
But, sometimes in English we switch around words and phrases to make them more literary.
Like with Yoda on Star Wars, he speaks as though he is in the 1500's by putting his verbs at the end of the sentence.
The author here, is making the sentence "more than perfect" - making literary - and drawing your attention to the phrase.