It gradually accumulates around these, until the heaps become very large, often forming dunes or sand-hills. 佚名.神奇的知识之书.
How are Sand-Dunes Formed? 佚名.神奇的知识之书.
Sand-dunes are composed of drift sand thrown up by the waves of the sea, and blown, when dry, to some distance inland, until it is stopped by large stones, tree roots or other obstacles. 佚名.神奇的知识之书.