Mudcracks & Ripple Marks

Back in 1788, Scottish polymath and geologist James Hutton, in his seminal work titled Theory of the Earth, came up with a concept in geology that came to be known as uniformitarianism.

The basic assumption in uniformitarianism is that we can interpret Earth's past history by assuming that the processes we see operating today were essentially the same processes operating in the Earth's past. This concept was later summarized by the memorable phrase "The present is the key to the past." While not always strictly true, in some ways the ancient Earth was different from the modern Earth, it is often a good guiding principle.

As an example, I took a picture in summer 2022 of a place called Duck Pond in the Mohonk Preserve on the Shawangunk Ridge near New Paltz. At the time, there was an issue with the dam impounding the pond and water levels were very low exposing the bed of the pond in places. The summer sun dessicated the mud forming mudcracks.

Here's a picture from the same month and year as the previous shot but taken in Sojourner Truth State Park in Kingston. This is a slab of 420 million year old rock (Late Silurian Rondout Formation) showing what appear to be preserved mudcracks. The concept of uniformitarianism tells us that it's reasonable to assume that this structure in the rocks are, in fact, mudcracks and formed in a similar way to those we find in the modern world.

And, importantly, these mudcracks tells us something about the paleoenvironment that existed here in Ulster County in the geologic past. It's not the whole picture, because mudcracks can form in different environments (a drying up lake bed as above or perhaps a muddy tidal flat on a shoreline) but other features in the rock, such as fossils, might help us figure that out.

Similarly, one can look at ripples in sand, like these in shallow ocean water I took at Ocracoke Island in North Carolina a few years ago.

Then we can look at 420 million year old (also Late Silurian Rondout Formation but hear High Falls, NY) rock and find preserved ripple marks.

Here's another picture of ripple marks from the Mohonk Preserve in the Shawangunks in even older rock (Mid-Ordovician Martinsburg Formation). I didn't take this picture, it's from a 2009 New York State Geological Association Field Guide (although I was on this trip).

Once again, the interpretation is that these ripple marks preserved from hundreds of millions of years ago in rocks formed by the same physical processes that form ripple marks in sand today - back and forth currents of water or wind.

These types of features are called sedimentary structures - essentially fossils of features formed in sediments and then preserved in rock as the sediments lithify. They are invaluable features for attempting to unravel the ancient geological history of an area like the Hudson Valley.

Yule

Merry Christmas for those who celebrate. While I grew up with Christmas, I don't find it particularly meaningful as an adult (although I still observe it for my wife and stepson). The winter solstice is, in many ways, a far more interesting seasonal event for me. Occurring last Thursday, December 21, at exactly 10:27 pm, it marks the point of maximum tilt of the Northern Hemisphere away from the Sun in our yearly orbit.

Many people, incorrectly, assume that the change in seasons has to do with our distance from the Sun. This is incorrect as the Earth is closest to the Sun (perihelion) on January 2, 2024 (aphelion, the furthest point, is six months later in early July). The seasons are due to the tilt of the Earth's axis as it orbits the Sun. When the Northern Hemisphere is tilted away from the Sun, the sunlight at our mid-latitudes can't reach halfway around the Earth leading to a 9 hour day here in Ulster County, NY. In addition, the Sun is at a lower angle in the sky (about 25º altitude) meaning the incoming solar energy (insolation) is smeared out more than in the summer when the Sun is higher in the sky (around 70º of altitude with 15 hours of daylight in late June). It's the tilt, not the distance that causes the seasons.

There's a reason why almost all ancient cultures (at least in the mid-latitudes) observed the solstices - they are real events with a real impact on our lives. They especially impacted ancient people since solstices and equinoxes, along with the cycle of phases of the Moon, defined all early calendars (and even some religious calendars today).

I always try to mark the winter solstice with a fire in my backyard fire pit after the Sun goes down. Depending on the outside temperature, some friends or family may or may not join me. For me, like my northern European ancestors, the winter solstice is a time to celebrate as it marks the turning point where the days start to get fractionally longer again even though the worst parts of winter are yet to come. In ancient times, it used to be, and still should be in my opinion, the official New Year's Day. January 1 is an artificial construct due to political considerations in the Roman Empire.

The decreasing hours of daylight in the winter really gets to me. I have to always make an effort to get outside and get some sunlight during these shorter days - something difficult to do when I sometimes find myself inside working for most of those daylight hours and the persistently cloudy Hudson Valley winter days.

So, no geology in today's blog post (although talking about the seasons is basic Earth science) but some of my outdoor winter excursions may well be to geologic locales for future posts!

Minor Flooding in Ulster County

We got a little bit of rain a couple of days ago on Sunday night into Monday, December 17-18 from a coastal storm. In my area, it seemed to have dumped almost 4 inches of rain in less than 24 hours which led to some flooding in the area.

Here are a couple of graphs from the United States Geological Survey (USGS) stream gaging station on the Rondout Creek at Rosendale near where I teach at SUNY Ulster. You can see the pretty dramatic rise in of the stream from 10.25 feet at 8:00 pm Sunday to 19.5 feet at 3:30 pm Monday – 9.25 feet in 19.5 hours or almost half a foot per hour. That’s why the National Weather Service issued flash flood alerts Sunday.

The next graph shows the discharge of the Rondout in cubic feet per second (ft³/s or cfs). This seemed to peak about a half hour earlier than the height but again we see a fast increase from 700 cfs around 8:00 pm to 15,800 cfs at 3:00 pm the next day. That’s an increase of 22.5 times the volume of water going past the gaging station each second in less than a day.

How does this compare to other flood events on the Rondout? I actually have physical geology students at Ulster look at decades worth of data from this local gaging station (available at https://waterdata.usgs.gov/monitoring-location/01367500) to work out a recurrence interval vs. discharge graph as shown below.

From this graph (blue lines intersecting the red best-fit line to the data), we can tell that a discharge of 18,500 cfs might occur, on average, every 6-7 years (the bottom scale may look odd because it's a semi-log graph). Not a major flooding event, but certainly significant – especially for people who build their homes on river floodplains or along normally-sleepy mountain streams which is common in our area.

One place in Ulster County which always floods (almost yearly) is Route 299 and Springtown Road just west of New Paltz on the other side of the Wallkill River. As seen on Google Maps.

The fields there are a natural floodplain of the Wallkill. That’s a great place to grow corn and other crops but a lousy place to build. The road was, as expected, flooded on Monday, December 18. Lots of pictures shared on Facebook. Someone got a drone show showing minor coverage of Route 299 but Springtown Road completely underwater. By the way, obey the barriers and don’t drive through stuff like this, a woman in Catskill lost her life by driving into floodwaters Monday up there.

What’s neat is that I also saw a picture on Facebook from the Woodbury Historical Society from the 1920s showing the exact same place flooded 100 years ago. That’s looking across the Wallkill from the New Paltz side with the bridge to the left. Notice the faint Skytop Tower on the ridge.

These fields have been flooding as long as there’s been a Wallkill River here (it’s not called a floodplain for nothing!). Flooding is, unfortunately for the people who have to deal with it, a natural part of a river’s life.

Tail of the Cock Traced on a Rock

On the side of a busy highway near the Ulster County city of Kingston is an series of outcrops quite well known to local geologists. They are along Route 199 just west of the Kingston-Rhinecliff Bridge. 

One part of these outcrops is composed of the Esopus Shale Formation and, in this location, it's dipping steeply to the east with a nice glacially-polished surface making for treacherous climbing onto it (especially since you'd slide down into a massive thornbush if you lost your footing).

Looking at this nice polished surface, you'll see some swirling patterns etched into the rock (there's a little iron staining from what looks like the weathering of small inclusions of pyrite - FeS2).

They reminded some of "rooster tails" leading to their common name.

A close-up shows the pattern a little better.

So what are these? Let's start with a little background first.

While paleontology is the study of ancient life (as preserved in fossils), ichnology is a branch of paleontology concerned with trace fossils - fossils that represent some trace of the ancient organism's activity. An example might be a dinosaur trackway (or the eurypterid trackway I wrote about a couple of weeks ago).

Sometimes all we have preserved from an organism is a trace fossil and we're not even sure exactly what species of animal made the trace. These trace fossils may be given taxonomic names and known as ichnospecies.

This swirly fossil in the Esopus Shale (it's found in some other units as well) is such an ichnofossil and was was given the ichnospecies name Zoophycos caudagalli where caudagalli means “tail of the cock” or "rooster tail".

The name Zoophycos was originally used to describe what was believed to be a new genus of algae (1855). It was later recognized that it wasn't algae at all but rather the trace of a marine worm - exactly what species of worm and exactly what this worm was doing is up for interpretation (although we can make some educated guesses).

Underlying the Esopus Shale, and just east of this particular outcrop on Route 199, are a series of limestone formations, chock full of marine invertebrate fossils, which formed in the early Devonian Period ( a hair over 400 million years ago). These limestones formed in the ancient Helderberg Sea - a shallow subtropical sea which once covered Ulster County (and beyond). Why subtropical? It's because this area was still in the Southern Hemisphere and closer to the equator than we are today.

This once clear sea began to get muddy, however, as mountain building started to occur to the east. This was the start of the formation of the Himalayan-scale Acadian Mountains. That's a story for a different day but suffice to say that the start of this mountain building brought a lot of sediments into the Helderberg Sea making it more muddy and leading to the deposition of the Esopus Shale. The previously-abundant marine invertebrates we see in the limestones disappeared as they were mostly filter feeders, straining organic material from the water, and those types of animals don't do well in muddy bottom waters.

Marine worms, however, do just fine in the bottom muds, as they churn through and extract organics from the sediments. While worms, and soft-squishy things in general, rarely preserve as fossils, they can leave traces of their activity. For Zoophycos, it's generally believed that the swirls were feeding traces of the worm (called spreiten) that lived in a vertical shaft, or burrow, beneath the traces.

Zoophycos fossil photo from the Kentucky Geological Survey

Zoophycos fossils have been found throughout the geologic record from 541 million years ago to modern times and even recent studies (Zhang, et al., 2015) acknowledge that we're still not sure of the exact group of marine worm (sipunculida, echiurida, or polychaeta) that may have made these fossils. Whichever one it was, it certainly hit upon a very successful strategy for existing in the muddy seafloor sediments for hundreds of millions of years.

Maybe my students don't feel the same, but I am always humbled when standing on such outcrops, ignoring the modern traffic whizzing by, and looking at fossils like this while imagining the seafloor that once existed here deep in the mists of time.