Although I have many geologic illustrations which I would like to show, the one which has the most meaningfulness to me is the geologic cross-section of Minnesota from the Minnesota Geological Survey.
As far as cross-sections go, there isn't much here which couldn't be seen in other parts of the world. The reason this means so much to me is because it started the process which "awakened" me to the world of geology. In my third year of undergraduate studies (while declared as a physics major but not really into it) I decided to take the Introductory Geology and Geology of Minnesota classes, both taught by the same professor. The beginning of both classes was the same: basics on rock types, geologic processes, etc. But even still, all those concepts were things that happened "somewhere else". Geology was still something that happened in some mountain somewhere else on earth.
This cross-section was eventually shown to us (probably the first geologic cross-section I had ever seen. The idea of a cross-section was so foreign to me, how could we possibly "see" underground like that?). It was displayed by itself on the screen for a good while as the professor talked. This illustration declared, "Geology is closer than you think, it's right beneath you, all around you, is not all the same and tells a fascinating story". Even Minnesota, a state I always considered to be rather boring (flat farmland or flat forest), was interesting underground. The school I went to was down in the Paleozoic sediments, but eventually I would see all the other parts of the state, with evidence of the Midcontinent Rift, the St. Cloud granites, and so forth. It was diagrams of local geology like this one which got me so interested in geology in the first place.