The theory is that huge rivers and wind carried the sand west where it mixed in with the sand that was already there.

理论是巨大的河流和风将沙子带到西部，在那里它与已经存在的沙子混合在一起。

In desert places, like in the Sahara Desert, you have a " sandstorm" where suddenly a big wind carries all this sand, and you can't see anything, and it gets in your eyes, and not much fun.

在沙漠的地方， 比如在撒哈拉沙漠，你会遇到一场“沙尘暴” ，突然一阵大风把所有的沙子都卷起来， 你什么都看不到， 它进入你的眼睛， 没有什么好玩的。