So...what do Leonardo da Vinci paintings and cartoons have to do with our journey to Death Valley last week? It has to do with the intrinsic and tangible value of the field trip in geoscience education. You can show draw cartoons on a chalkboard, you can show digital images on a screen, but nothing can ever substitute for the value of students standing on a mountainside, laying their hands on the rocks and making their own observations and judgements about the story told by the rocks.
For my students, and for the first pioneers of geology, telling the story of the rocks was a huge step. The mere recognition that rocks have a story was one of the great advances in the science, because in the European worldview of the middle ages, rocks were simply created as is, and changes since their origin have been minor. They barely deserved attention unless they contained valuable ores. Nicolas Steno (1638-1686), a Danish physician who spent much of his life in Florence, changed that.
St. Nick (yes, he was beatified in 1988!) first annunciated three of the four founding principles of stratigraphy, and each of them can be connected to the cartoons I used above. Take a look again, only now with labels:
Did these sedimentary layers form originally as sloping layers? If you think layers form horizontally, in the bottom of lakes, on the bottom of the sea, or on a floodplain, then you understand the Principle of Original Horizontality: most sediments originate in horizontal layers. If the layers are tilted, some outside force put them that way.
Did layer E or F originally end in a cliff? If you think not, you are on the verge of understanding the Principle of Lateral Continuity: sediments are continuous unless they abut against the edge of the original sedimentary basin, or they thin out. We understand that one can expect to find more of layer E or F in the general vicinity, because although erosion has removed the intervening rock, the original layers continued across a larger area than just this outcrop.
When did the faulting occur? If you think it came after the sediments were deposited, than you would be right, and you begin to understand the Principle of Crosscutting Relationships: faults and intrusive rocks are younger than the rocks they cut across. Steno may have written something about this, but the principle is associated more often with James Hutton in the latest 1700's and Charles Lyell in the early 1800's.
With this basic background (oh my, this post reads like a classroom lecture, doesn't it?), you can begin to understand the enthusiasm with which I presented these topics on our Death Valley trip. I was standing in front of the cliff in the picture below:
After presenting the basic principles we walked around the corner and encountered another outcrop, as seen below: