Stratigraphy

There is this road cut on the Windham/ Gray border that I drive by every afternoon and I can't stop thinking about it.  The rock face is a puzzle.  With three rock types, spanning hundreds of millions of years of time, it has taken me awhile to figure out the geologic history that lead to the formation of the object of my perseveration.  I'm hoping it will offer my students an equivalent feast of thought in our next unit on plate tectonics. 

There are a couple principles of geology that I had in mind as I sorted through the history of the site.  The law of original horizontality told me that the sedimentary rocks that flank the site were not always askew, in that unsettling way that many New England rocks seem to be (can't geology be flat and straight forward like it is in the Midwest?).  The principle of cross-cutting relationships told me that the foot-wide basalt dike that cut through the center of the site was younger than the granite that it bisected.  The granite, in turn, was younger than the sedimentary rock that it split.

While no laws lay out the order in which I teach geology, the puzzle creates its own structure: series of sub-questions (mystery questions) the answer of which everyone must know to arrive at the next step of the mystery.  In its current iteration, my unit is made up of the following questions:

1. What types of rock make up the road-cut rock formation?
2. What order were the rocks lain down in?
3. What processes led to the formation of each type of rock?
4. At what time period in geologic history was each rock lain down in?
5. What was the arrangement of the continents during the period that each rock was formed?

An understanding of each question is necessary to answer the question that follows it.  This stratigraphy of unit questions was the foundation for my lake mystery unit (stay tuned for my students' solutions to these questions) and unless my next unit fails will continue to serve the role in all of my units for the school year.