Picture the line that is left when something is pushed through a surface: a plough through soil, a wheel through mud, water down a . The line is a small valley. The middle is pressed down and the material that was there is up along both sides. It is long, and it is deep enough to lay a finger in. Once you can see that line, the face works the same way. Worry or hard thinking pulls the skin of a forehead into exactly the same shape: a long fold pressed down, with the skin standing slightly higher on each side. Field or forehead, the word is asking you to see one deep line pressed into a surface.
The Wiktionary entry in the packet traces furrow back through Old English furh to an old root meaning to dig. That fits the picture the word still carries: a furrow is something that has been dug or pushed open, never something drawn on top of a surface.
資料包裡的 Wiktionary 條目把 furrow 上溯到古英語 furh，再往上追到一個意思是「挖」的古老字根。這正好對上這個字今天的畫面：furrow 一定是被挖開、推開來的，不是畫在表面上的線。
Wiktionary (kaikki extract in evidence packet), source_ref kaikki:3f6f69a190036ae4:955993085 — Middle English furgh, Old English furh, Proto-Germanic *furhs, PIE *perḱ- (to dig).
Two branches
地面與物體表面上的溝
這一組就是核心畫面本身，沒有轉彎。名詞是留下來的那道溝，動詞是把溝弄出來的動作，兩者互為正反面。
noun · B2
A long, deep line cut into the surface of something, especially the line a plough turns in soil so that seeds can be planted; also a groove worn into wood, metal, rock or a road.
犁溝／深槽
臉上的深紋
可以想成同一條溝搬到臉上。擔心或用力思考時，額頭和眉心的皮被擠壓，凹出一道長痕，兩側微微隆起，形狀跟田裡的犁溝一樣。這是形狀上的比喻，用來幫你記住畫面，不是在說哪個意思比較早出現。
noun · B2
A deep line or fold in the skin of a person's face, especially one that runs across the forehead or sits between the eyebrows.
額頭深紋