Some example of the use of "shallow" in the horizontal dimension would lend some credibility to the statement that it can be used in this context. Surely "shallow" is exclusively used in reference to the negative vertical dimension.
We don’t put examples of shallow on the page for short, but an example of horizontal shallow could be a shallow dent in the door of my car. Not necessarily a small dent, since it could have a large circumference, but it might be shallow regardless of size. —Stephen 22:07, 9 November 2008 (UTC)