It is shown that when two nominally flat rock surfaces are placed together and subjected to a normal load, the resulting closure closely follows a constitutive law predicted by theory. However, the constitutive law exhibits hysteresis. The hysteresis is evidently produced by friction on parts of the surface where the contact is oblique to the load. This behavior, which should be shared by joints and other fractures in the upper part of the crust, suggests that the elastic properties of the near-surface parts of the earth may be far more complex than previously thought.