They moved in with a local family, the Harrisons, and, like them, had little privacy, rare baths, and a blanket of snow on their quilt when they woke up in the morning. Some mornings, Rosamond and Dorothy would arrive at the schoolhouse to find the children weeping from the cold. In spring, the snow was replaced by mud over ice.

这是2022高考英语全国卷一阅读理解B中的一个段落。请问其中的 mud over ice 是一种什么状态？冰上的泥土？冰上面怎么会有泥土呢？泥土不是应该在冰的下面吗？