But the little valley down which it came was much steeper and rockier than the one he had just left and much overgrown with bushes.

1. 句中的 down 是介词，down which it came 是定语从句（介词+which 引导的定语从句），修饰名词 valley。

2. the one = the valley。

3. and much overgrown with bushes 不是修饰 the one 的形容词短语，而是与前面的 much steeper and rockier 并列的表语。much overgrown 在此相当于thickly overgrown。

▲句子的基本结构是：But the little valley (down which it came) was much steeper and rockier (than the one he had just left) and much overgrown with bushes.

▲句意：但是，其（指这个 it 所代表示事物，应该是指一条小河）源头上的那个小山谷比他刚刚离开的那个山谷要更加陡峭，岩石也更多，而且还长满了枝繁叶茂的灌木。