59. Roadside Geology of Louisiana by Darwin Spearing
published: 1995
format: 220-page paperback
acquired: August 2020
read: Nov 20-25
time reading: 8:43, 2.4 mpp
rating: 4
locations: 😊
about the author: publisher website says he was exploration geologist in Louisiana and now lives in Colorado.

Louisiana is a pretty young place, without a whole of rocks on the surface, and those mostly being pretty soft and friable, making for terrible outcrops. Everything on the surface, except the salt domes*, is tertiary or younger in age, or less than last 60 million years old. But that doesn't fully capture it. The whole southern part of the state (everything within the lines at the link below), formed in the last 7500 years, all since the last ice age.

What Louisiana lacks in rocks it makes up for in living natural processes, in the interplay and movement and control of massive rivers and the Gulf of Mexico. The Mississippi and Red rivers have never been stable. They have moved dramatically, switching valleys, building large deltas, and then abandoning them to slowly sink. The Mississippi's birds-foot delta is only 600 years old (with documented settlement during half its life, since the 1700's). The Mississippi would have switched itself to the much shorter Atchafalaya river in 1973, a major flood year, if there weren't structures in place to prevent it. (Barely. Those structures physically shook throughout the flood.)

The book itself is an entertaining overview. It is supposed to be designed for a reader to check out the sections they are driving trough. But, as I learned with Spearing's book on Texas, it reads best if read straight through, if you have the time. Recommended basically to those who likely have already decided to read it.

*An extra note on those salt domes: The salt is ~160 million years old, but is light and mobile and gets forced upward though younger rocks. It has formed local diapirs that occasionally reach the surface, dragging older rocks up with them. In places they create local rises, such as what are called the fives islands where the land rises over a hundred feet above the surrounding flat marsh. You can pick them out on Google Maps (Satellite view).

2021
https://www.librarything.com/topic/333774#7664350 ( )

My first thought on seeing this one was, “What geology? Louisiana has mud, not geology.” However, I was mistaken; what we have here is a thorough discussion of deltaic and fluvial geomorphology; how rivers make landforms that will, given time, become – geology. The most intriguing thing is how fast all this happens - especially if you are used to thinking about “geological time”. The Mississippi abandoned the Lafourche delta and moved to the current Birdfoot delta about 1200 AD; and it abandoned the Birdfoot Delta and moved to the Atchafalaya Delta in 1973 (except it didn’t, thanks to the Army Core of Engineers and the Old River Control Structure). I wonder why the environmentalists who are so concerned about humans changing the Earth never comment on that – that the big river port on the Mississippi should be Morgan City, not New Orleans. The book is full of astonishing little details like this; at least they should be astonishing to those who think the planet is an unchanging place. Prior to the retreat of the Laurentian ice sheet – maybe 11000 years ago, scarcely a tick of the geological clock - the Mississippi wasn’t even there – and before the ice most North American rivers flowed to the Arctic or Atlantic. As if that wasn’t enough, many of the changes have happened in historic time, or even in our own lifetimes – Grand and Six Mile lakes have been filled in by the Atchafalaya since 1917; Atchafalaya Bay has become the Atchafalaya delta since 1967; Grand Isle moves east at about 16 feet a year, and the land between the Birdfoot distributary channels has all appeared since 1838 (in fact, we know the exact source for filling one of these – a fisherman named Cubit cut through a levee in 1862 to allow a shortcut to his oyster grounds, and the resulting sediment filled in Cubit’s Gap).


Well written and very well illustrated, especially the maps that show geological changes. Probably should be required reading for everybody thinks the landscape has been here since time immemorial. ( )