I've been playing around in ArcGIS (a mapping program--there are many similar free ones) with digital elevation models. They're easy to work with, and lots of cool things can be done with them. They're basically big files that describe elevation every few meters or so across the landscape. They cover the whole country.
With them, you can create topo maps, contour lines, simulated shadows (e.g., sunrise vs. sunset), etc. You can even mark a point on a map and have it display which topographical features will be visible in the distance if you were standing there.
I took DEM files for Kentucky and constrained the data to only display slopes >75°. In other words, I wanted to look at clifflines exclusively. Searching for clifflines using satellite images can be easy (i.e., google maps), but usually the shadows from tall cliffs only indicate one side of an amphitheater and not the other, as satellite imagery is a snapshot. Theoretically, this approach should display all steep drainages. The results are patchy, and clearly inaccurate in some places... but fun to look at anyway. I color coded them to indicate steepness from "cool" to "hot" (pink).
Here is Bedtime for Bonzo, I think:
The Red River is crudely drawn in MSpaint, here:
Muir Valley worked pretty well:
Here's part of the PMRP. Anyone ever been to the hot pink area toward the bottom? It may not have any cliffline (satellite imagery doesn't show any shadows there....) but it is probably steep as hell. Who knows.
The other ?? area at the top left doesn't show up well in my map, but google imagery clearly shows some MASSIVE amphitheaters. Again, they may only be 20' tall of rock... or on private land.... anyone ever been out thay way? Looks like a bushwack:
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