My favorite geological illustration is ...

Bird’s Eye View of the Black Hills, to illustrate the geologic structure.

What a beautiful and memorable illustration!  I discovered it in the late 1970s shortly after moving to the Black Hills, a geological and biological wonderland for a naturalist like myself.  Simplified it may be, but then this illustration is from the report of the Newton - Jenny expedition of 1875.  The basic geomorphology of the Black Hills is clearly shown (see annotated version below):  the eroded Central Core where the Precambrian core is exposed; the high rolling Limestone Plateau to the west; the Red Valley or Racetrack, a strike valley underlain by the easily-eroded Spearfish Formation that almost completely surrounds the higher elevations; and the striking Hogback Rim, a series of hogbacks and strike valleys making up the outer perimeter of the uplift.

Look close (click to enlarge) ... do you see the birds whose perspective was the basis for this illustration?  If that isn’t cool enough, the birds serve as labels for the major rock types:  Archean Schists, Carboniferous Limestone, Dakota Sandstone and Newer Cretaceous (see tiny legend at base of illustration).  Wow!

As the expedition and report were funded by we-the-people, the illustration is free to download at the American Memory website (11.7 MB).  The Black Hills Institute in Hill City, SD, has a 28” x 22” poster for sale.

Geomorphic regions of the Black Hills.