Phrase used to describe breadth
(at the expense of depth
Can be applied to a generalist or "jack of all trades" (who know a little bit about a lot of things).
It's also been applied to everything from science curriculum in American schools (too many topics, not enough understanding) to television programming (great variety of shows for a wide spectrum of interests, none of which explore topics very deeply).
Probable origin: 19th century wagon trains travelling across the Western United States, found they could cross the muddy, shallow Platte River in Nebraska and Colorado by driving across it. "A mile wide and an inch deep," they said of the river. Humorist Artemus Ward said the
Platte would be a considerable river if it were turned on edge.