I was reading page 5 of my 上級へのとびら textbook, and in line 12, the following sentence caught my eye:
I cannot figure out why the に is used instead of が (南北が長い). Is に the 連用形 of なる here?
Turns a noun 南北 into an adverbial expression, i.e. answers the question "how?" Let's say ..."north-south-directionally".
And the expression then modifies the adjective "long".
In English it would translate to "long in north-south direction".
We usually say "日本は南北に長い" having a meaning of "日本は南北方向に長い". In addition to this, it is very common to use "東西に長い" in place of "東西方向に長い".