定语从句中介词加关系副词的用法.比如from where

在北师大版高一新教材第3模块第7单元第1课课文中有这样的一句：He was blown off course and found himself in an unkown land,from where he eventually reached Greenland.
这里的where指的是in an land而不是land,因此只能用关系副词,而不用from which.再如：
He stood under the tree,from where he could see his son swimming.
You can go across the river,from where you can take a boat.