The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on a weekly schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Tuesday, February 11, 2014

February 11. The concept of stratigraphy

Today I thought we’d talk a bit about the concept of stratigraphy, some ideas that will help with understanding of the geologic events we’re talking about.


Stratigraphy is the study of strata, or layers within the earth. Stratum, the singular of strata, comes from Latin for a bed, and ultimately from a word meaning to spread out—and that's what geological strata do: they spread out over wide areas. The science focuses mostly on sedimentary layers, beds of sandstone, shale, limestone and so on. One key aspect of stratigraphy is the law of superposition – an fancy way of saying that lower layers are older than higher layers. This may seem obvious – if it doesn’t, think about throwing some red sand into a pail on Wednesday, then on Thursday come back and throw in some lime. The sand is older than the lime. It was not obvious to early scientists, and it was Nicholas Steno, a Danish Catholic Bishop, who lived in the 1600s and pioneered and promoted this and other basic aspects of geology. He also conceived the principle of original horizontality, which says that layers of sediment – sand, silt, mud – were laid down in horizontal layers under the action of gravity. There are some obvious exceptions to this, such as deposits on mountain or undersea slopes, but it’s a general principle that matters greatly when we look at rocks that have been deformed by faulting or folding.

If you’re interested in Steno’s story, I can recommend a book by Alan Cutler called The Seashell on the Mountaintop.

While in many ways Steno was the father of stratigraphy, the one who really implemented stratigraphic ideas in a modern way was a British surveyor, William Smith. To this day I still think of him as William “Strata” Smith, as he was called when I first took physical and historical geology classes back in the 1960s. He recognized that different layers or strata of rocks had distinct fossil assemblages, and that he could recognize those characteristic fossils to help him identify the rock packages elsewhere, even if they were distant and disconnected from the original rocks. And even if the kind of rock changed. That meant that the same kinds of fossils, in a sandstone here, but in a limestone there, meant those diverse rocks were of the same age.

Smith made the first geologic map of England and Wales, published in 1815. That was The Map that Changed the World, in the title of the book by Simon Winchester that recounts Smith’s story.
—Richard I. Gibson

The "map that changed the world"

1 comment:

  1. For fathers day my son, who just finished his dissertation (Archaeology) at Cambridge, sent me a color copy of Smith's map he purchased at the Sedgwick Museum where the restored original is on display. There is a youtube video on Smith and the map that was posted about 10 months ago. Thanks for the info that made us aware of its existence.