what引导的从句叫做名词性关系分句，而这个名词性关系分句又是个嵌入式关系分句。即what所在的分句是另一个分句的宾语从句。

my map would later tell me what was a stream called the Loch

划线部分为what从句，你可以看出what在划线这个分句中作主语，而这个从句整体是my map would later tell me这个分句中的宾语。即what从句嵌入到另一个分句中作成分，这种分句就叫做嵌入式分句，该分句为名词性关系分句，所以就叫嵌入式名词性关系分句。

the place which my map would later tell me was a stream called the Loch.

用the place which替换what后，划线部分就是嵌入式定语从句，which是was a stream的主语，which并不在my map would tell me这个分句作成分，which从句整体在这个分句中作宾语。