Viewpoint: Yes, desert varnish (rock varnish) may be an accurate indicator of ancient desert wetness.

Viewpoint: No, rock varnish does not accurately record ancient desert wetness because it cannot be dated effectively and its mineral composition cannot exclusively be attributed to climate change.

The subject of desert varnish, or rock varnish, as we shall see, carries with it a controversy concerning the relative levels of wetness that once existed in what are today desert environments. But this particular topic also illustrates several persistent themes in the earth sciences as well. Among these are the long periods of time usually required to bring about any noticeable change in geologic features (that is, aside from dramatic instances of tectonic activity, such as volcanism). There is also the matter of the gradualism that characterizes the transport of...