Mishná sur Kéilim 2:14

A thick woolen jacket or a thick wooden block does not bring uncleanness until they are one handbreadth high off the ground. If [garments] are folded one above the other they do not bring uncleanness until the uppermost is one handbreadth high off the ground. Tablets of wood [placed] one above the other do not bring uncleanness unless the uppermost is one handbreadth high off the ground; But if they were of marble, the uncleanness cleaves upwards and downwards.