RCW 58.28.280

If a stone is used as a monument it must have a cross cut in the top at the point of intersection of center lines of streets, or a hole may be drilled in the stone to mark such point. If an iron monument is used it must be at least two inches in diameter by two and one-half feet in length, and may be either solid iron or pipe. The dimensions of the monuments must be marked on the plat, and reference thereto made in the field notes, and establish permanently the lines of all the streets. The surveyor must make and subscribe on the plat a certificate that such survey was made in accordance with the provisions of this chapter, stating the date of survey, and verify the same by his or her oath.