The end of Chapter 20 of Exodus mentions that altars are to be earthen. What is wrong with cut stone altars? There is an idea of the meeting of heaven and earth for the earthen altar, but there is also a connection with the prohibition of idols of silver and gold. There are problematic types and mandated types of intermediaries. There is the Golden Calf on the one hand, and the Mishkan and the earthen altar on the other. The tension lies in the question of what Bnei Yisrael will choose.