。
To the northeast, the land lay low.
Literal
Northeast direction [in-に] land [subject-が] low was-lying.
北東方向 ('northeast direction') is compact compounds all the way. 横たわる ('to lie down, to stretch out [horizontally]') is a formal/literary verb for landscapes or reclining bodies. 低く is the く-form of 低い ('low') used adverbially. A geographical description with a literary tone.