Describe the planet of Uriel that the children have entered.

From the text:

Meg looked upward and indeed the mountains seemed to be reaching into infinity.

They left the fertile fields and flew across a great plateau of granite-like rock shaped into enormous monoliths. These had a definite, rhythmic form, but they were not statues; they were like nothing Meg had ever seen before, and she wondered if they had been made by wind and weather, by the formation of this earth, or if they were a creation of beings like the one on which she rode.

They left the great granite plain and flew over a garden even more beautiful than anything in a dream. In it were gathered many of the creatures like the one Mrs Whatsit had become, some lying among the flowers, some swimming in a broad, crystal river that flowed through the garden, some flying in what Meg was sure must be a kind of dance, moving in and out above the trees.