Numbers 19:16

על פני השדה [AND EVERYTHING WHICH TOUCHES] ON THE OPEN FIELD — Our Rabbis explained that this is intended to include the upper board and the side board of a coffin (which were on the surface of the field — that these, too, render a person unclean) (Chullin 72a). The literal meaning of the words על פני השדה is, however, that even where there is no אהל, covering, there too, the dead body may cause uncleanness, but only by contact with it.

וְכֹ֨ל אֲשֶׁר־יִגַּ֜ע עַל־פְּנֵ֣י הַשָּׂדֶ֗ה בַּֽחֲלַל־חֶ֙רֶב֙ א֣וֹ בְמֵ֔ת אֽוֹ־בְעֶ֥צֶם אָדָ֖ם א֣וֹ בְקָ֑בֶר יִטְמָ֖א שִׁבְעַ֥ת יָמִֽים׃

And whoever touches one that is slain with a sword in the open field, or a dead body, or a bone of a man, or a grave, shall be unclean seven days.