The Sinking Coast
The Devonian of Greene County Part Ten
Updated by Robert and Johanna Titus
We usually think of environmental disasters as great, awful catastrophic events. What could be worse than a sudden earthquake or volcanic eruption? But sometimes these disasters proceed quietly, even stealthily with almost nobody taking notice until the worst of the damage has been done. People are alarmed about catastrophic events, but it is easy for them to remain blissfully unaware of those more subtle, downright sneaky problems.
One of those today is the sinking coastline of the Mississippi Delta in southern Louisiana. There the great mass of earth that is the delta has been subsiding, very slowly so that coastal regions have sunk into the Gulf of Mexico. Homes and villages, right on the shore, have had to be abandoned to the advancing waves. Millions of acres of coastal land have thus been lost over the decades and centuries. But, have you ever heard about this? Quite possibly not; it is such an inconspicuous process, who notices?
Why is the Louisiana coastline sinking? Many of the reasons are quite natural. Over the millennia, as the Mississippi has carried sediment to this coastal realm it has piled this material up in increasingly thick masses. Just the weight of all this sediment has pressed down on the crust and caused subsidence. Then too, there is the normal process of compaction. The sediment has simply settled and that causes still more sinking.
It probably won’t surprise you to learn that man has interfered with Nature’s balance. Normally the Mississippi replenishes most of the land that has been lost. Floods rise up over the banks of the river and carry new sediment to fill in and replace volumes of sediment that have been lost to subsidence. The land surface is maintained. Sadly, that important process has been halted; over the past three centuries man has built levies along the banks of the Mississippi in a successful effort to thwart flooding. Now there are many fewer threats from flooding, but the benefits have also been lost. It is impossible for floods to carry new sediment onto sinking landscapes. Ironically man’s efforts to control riverbank floods have helped let coastal flooding get out of control. It gets worse; over the past century so many oil wells have been drilled in the region and so much oil has been pumped out of the ground that this has hastened the rate of subsidence. The oil had helped buoy up the ground, and now where it is gone the land is sinking.
Well, for these and other reasons, much of coastal Louisiana has been condemned to sink beneath the waves. It will take decades or even centuries, but a lot of Louisiana is doomed. Some have decried this as one of the greatest looming environmental disasters of our time.
It couldn’t happen in Greene County, could it? Well no, but it has happened and of course that was back during the Devonian. In the recent installments of this series we have watched Greene County rise out of the sea as a great expanding delta, the Catskill Delta, advanced westward across much of New York State. For a considerable length of time Greene County lay along the front of that delta and, just like the Mississippi Delta of today, it was subject to the natural effects of subsidence. There were no Devonian age levies or oil wells, but Nature herself caused just the sort of sinking that we see today in Louisiana. And, of course, we have the rocks to prove it.
Take Rte. 32 south from Freehold until you arrive at Rte. 23. Turn left and travel east a quarter mile, or so. There on both sides of the highway are some fine outcroppings of what is mostly Devonian age sandstone. It is the north side of the road where you can see the best exposures so find a good place to turn around and park at the outcrop.
Pause and survey the whole outcrop. You will, we hope, be able to see that it is broken up into three separate layers of stratified rock. In other words, there seem to be three packages of rocks here, laid out, one atop the other, in a vertical sequence. As geologists, we always start at the oldest layers of rock and those are the ones at the bottom of the outcrop at its western end. That first “package” of strata is the least well exposed but let’s start there. You will see a sequence of thickly bedded, light colored sandstones. Above them the stratigraphy grades into finer grained, thinner bedded material. This has a greenish gray to brick red color.
This stratigraphy is repeated in the next package and in the third. In other words, we are looking at cyclical events in a cyclical stratigraphy. In the second cycle you can see that many of the thick sandstones are inclined to the west (left). This is typical of river sediments and we have found, in recent columns, that such sandstones are, indeed, river channel deposits. That’s the case here; each of the three cycles begins with river channel sandstone. The overlying finer grained material is a petrified soil profile, literally a fossil soil. So, if you follow all this, each cycle represents the presence of a Devonian age Catskill Delta river channel overlain by a floodplain soil.
So, what is going on here and how does it relate to today’s Louisiana? There were two dynamics going on here back in Catskill Delta days. First those ancient rivers were what we call meandering streams. They formed beautiful, sinuous channels that literally snaked back and forth across their delta floodplains. This process, called river meandering, is a very slow one but it is effective over time and it can still be seen in many modern rivers. But it is slow and that gets us to the second dynamic.
Remember how Louisiana is sinking and that the sinking is slow? Well, our Catskill Delta was sinking slowly also. Slow river meandering was matched with slow crustal subsidence. There was a back and forth motion. First the river would meander one way and then it would return. “Back” was easy, but by the time a river meandered “forth” the crust has already sunk quite a distance. A new river channel/ floodplain “forth” deposit would be laid down on top of the old “back” one. If meandering continued, and it would, then given time a third deposit (cycle) would be deposited on the same sinking delta.
That’s what we see on Rte. 23. Did one river deposit all three cycles? We don’t know but it might have been. Was one river or several rivers meandering across this site? I don’t know but it doesn’t much matter. The important thing is that we can look into the stratigraphy here and recognize chapters in the history of the Catskill Delta. It was sinking and its streams were meandering, and it behaved very much like the Mississippi of today. And that is because Greene County, back during the Devonian, was very much like the Louisiana of today. Only time has changed.
Let us add a note about the names of these geological units. This Devonian sequence has been classified and reclassified over the decades. Different names have been applied to the several units of rock described in this series. The rocks described in this installment are probably the upper portion of the Ashokan Formation. I called the rocks in the last installment the Plattekill Formation but that is likely in error. Those outcrops probably belong to the lower Ashokan Formation. To the average reader these will not be very important issues but to professional geologists they are the subjects of often heated debate.