It sounds a bit far-fetched! To understand it better, you could break it up as: "his face had scored a trench. The trench was a foot deep and a couple of yards long." Feet and yards are old units of length. To score is to scratch a groove in something, and to make that big a scratch with someone's face would either kill someone, or be in a very soft material.

here's a leave out of "that is", imean the whole sentence is “his face had scored a trench (that is) a foot deep and a couple of yards long."this is a attributive clause, the leading word "a trench" here is served as Object, which means there promises a ellipsis of pronoun_"that".