Field Log: Bayside Barriers

Last week I had a chance to get some ground time on the beaches in Delaware, so I’m going to put the shoreline change discussion on hold for a moment and do a deepish dive into coastal geology. First, a little primer on the history of the southern Mid-Atlantic coastal plain:

Delware Bay, the Delmarva Peninsula, and Chesapeake Bay occupy an embayment, a technical way of describing a large indentation in the coast. Throughout the Cenozoic era (65 million years ago to present), this structure, referred to as the Salisbury Embayment, gradually filled with river and marine sediments, becoming incised by waterways during the late Pliocene and Pleistocene epochs (~3 million to 12,000 years ago) as sea levels fell in response to glaciations. The history of this deposition can be found in the sand and gravel forming much of the modern coast of Delaware, perhaps most strikingly at a place called Slaughter Beach, along the Delaware Bayshore.

Map of the lower Delaware Bayshore region. Click to enlarge.

Slaughter Beach forms part of a contiguous 14-mile long barrier island that runs from the mouth of the Mispillion River east of Milford to Roosevelt Inlet, in Lewes. Because it is protected within the Bay, the features of the barrier are less pronounced than ocean fronting islands, with a much narrower beach and a dune that is less than 2 meters tall in most places. Nonetheless, it is subject to the same processes affecting ocean barriers, including overwash-driven landward migration in response to sea level rise. In fact, overwash fans and evidence of landward migration are clear in modern satellite imagery:

Like most east coast barriers, this island formed offshore several thousand years ago when sea level was significantly lower than today, and has been migrating inland ever since. Throughout this landward migration, the island was fed by the sand and gravel deposited within the Salisbury Embayment. As a consequence, the sedimentology of Slaughter Beach tell us something about the long-term geologic history of the region.

Slaughter Beach, looking north from the main access point at the public pavilion

One of the unique features of Slaughter Beach is its abundance of gravel and cobbles, some pieces of which approach the size of a baseball. The vast majority of these rocks are quartz, in all types of colors, but a closer inspection reveals pieces that seem out of place. In particular, a keen eye can spot pieces of chert and fossiliferous limestone—some of which contain the traces of creatures from the Paleozoic!  (+250 million years old)

Pebbles and cobbles along the high tide line at Slaughter Beach

Fossil-bearing limestone cobble found by Masters student Chris Tenebruso

But how did these Paleozoic fossils wind up in Delaware Bay? 

The fossil-bearing cobbles were actually sourced in the Ridge and Valley region of the Appalachian Mountains and carried downstream to the Bay. In fact, Geologists studying this region think much of the sand and gravel composing Slaughter Beach was eroded from the Appalachians during the Pleistocene (~2.5 million years ago to 12,000 years ago) and laid down in rivers cutting through the ancestral Delaware Bay. This material is thought to be the result of glacial outwash, or sediment in rivers that was originally eroded by glaciers.

Other components of the gravel/cobble include shale and argillite, which could have originated from within the Mesozoic basins in the Atlantic Piedmont. In New Jersey and Pennsylvania, for instance, the Newark Basin contains a predominantly black/gray argillite and mudstone unit called the Lockatong Formation. Perhaps the argillite pebble pictured below belongs to this formation?

Left: 2x quartz cobbles. Center: Black argillite pebble and quarter for scale. Right: Ironstone conglomerate w/ cemented quartz pebbles.

Not all of the sand and gravel at Slaughter Beach came from Pleistocene glacial outwash, however. In the picture above is a large fragment of ruddy-colored sand and pebbles, a type of ironstone conglomerate. Ironstone is a classic coastal plain sedimentary rock, formed when sand and other particles become cemented together by the precipitation of iron from solution. The process is typically driven by biochemical reactions in swampy environs, leading to the term ‘bog iron’ to describe deposits formerly mined and smelted in places such as Allaire Village in New Jersey.

In Delaware, coastal ironstone conglomerate is typically found in association with the Pliocene (~5 to 2.5 million years ago) Beaverdam Formation. This unit underlies the southern end of Slaughter Beach a few feet below the modern surface. It was laid down by ancient rivers when global temperatures were similar to today or somewhat colder. (Bonus info: there is some evidence that thick sequences of sediment within this and similar formations were deposited in response to enhanced erosion driven by episodes of cooling during the Miocene/Pliocene. See: Pazzagalia, Robinson, and Traverse [1997])

The ironstone most likely found its way into the modern barrier through excavation of the subsurface by inlet activity. However, the area where the pictured sample was located is a spot where the modern barrier is underlain by several dozen feet of peat and mud, and so it must have been carried north by littoral currents. Interestingly, the Beaverdam Formation is thought to be related to New Jersey's Beacon Hill Gravel and Cohonsey Sand, which feature very similar ironstone conglomerates—you can find some washed up on Sandy Hook!

The unique geological history of Slaughter Beach, told in the vast array of pebbles and cobbles stranded on the high tide line, may be significantly modified in the future, as the barrier here is being nourished with sand mined from offshore areas to halt its retreat. Already, places like Broadkill Beach, Prime Hook, and Fowler Beach feature much less pebbles and cobble than they once did, and so to get a glimpse of nature’s story (and maybe a few nice pieces of Delaware quartz and some fossils) I highly recommend stopping at Slaughter Beach sooner rather than later.

Looking south down the barrier at Slaughter Beach towards Fowler Beach and Prime Hook.

For an in-depth read on the coastal geology/geomorphology around Slaughter Beach, check out Delaware's Changing Shoreline.

Also, did I mention Slaughter Beach is the horseshoe crab capital of the world?

The Dynamic Coast

I think a good place to start this blog is to consider coastline dynamics, to get a sense of the scale of changes that occur commonly in sedimentary systems. In the modern era, developed coastlines, such as in New Jersey, are predominantly engineered systems. We stabilize beaches through sand nourishment (literally, piping in sand from offshore sources), as well as construction of groin fields—rock jetties that capture sand moving in shore-parallel currents. Without such interventions, however, the coastline is a highly dynamic environment, especially where barrier islands are present.  

In New Jersey, rapid shoreline changes were abundantly evident in the past, and in some places can still be seen today. One of the most dynamic portions of the modern coast is Sandy Hook, a spit at the north end of the Sea Bright barrier in Monmouth County. Here, sand is transported northward by littoral currents and deposited at the tip of the spit year-after-year, gradually extending Sandy Hook into Lower New York Bay.

Sandy Hook seen in aerial images in Google Earth

Sandy Hook seen from the Twin Lights of Navesink, looking towards New York City

Using a progression of satellite imagery, we can visualize changes in Sandy Hook with time, capturing the transport and deposition of sand around the northern point of the spit. Below, I produced a timelapse of Landsat photos taken from 1984 to the present, using Google Earth Engine. For those unfamiliar, Google Earth Engine (not to be confused with the Google Earth software that lets you view a map of the world) is a database of many satellite images, which allows a user to perform detailed geospatial analyses using scripts—more on that in a later post.

If we want to go further back in time and see changes in Sandy Hook that have occurred over the last hundred years, we have to seek alternatives to the satellite image record, which ends in the 1970s. Luckily, in New Jersey, we have aerial photos of the coast which extend all the way back to 1920.  A great catalog of images can be accessed courtesy of the United State Army Corps of Engineers, which digitized sets of photos from 1920, 1933, 1944, and 1962: http://rsm.usace.army.mil/shore/

Using Google Earth Pro, I georeferenced a 1920 aerial photo of the tip on Sandy Hook and overlaid it on the modern aerial imagery. In the image below, I’ve highlighted the 1920 shoreline in yellow, and the modern shoreline in pink. The spit has grown northward about a kilometer!

1920 photo of Sandy Hook overlaid on modern imagery

More surprising, however, is that at least half of this growth has actually been in the last 20 years, as revealed when overlaying the modern shoreline on an aerial image from 1995.  If the 1995 shoreline is traced (orange), the area deposited along the north part of the spit since that time can be calculated: 358,000 square meters.

Change in area at north end of Sandy Hook from 1995 to 2017

Let’s assume for a moment that the average depth of beach sand at Sandy Hook is a relatively conservative 3 meters (this is in the right ballpark given some data that was recently acquired). If this is true, then the volume change at the north end of the spit since 1995 is +1,074,000 cubic meters. To put that in perspective, the volume of the Empire State Building is 1,047,723 cubic meters.

Such large changes occur elsewhere on the New Jersey coast, especially where other spits are present. A great example of this is the Holgate spit, at the southern end of Long Beach Island, which I constructed another timelapse of, starting in 1984.

Note: Vegetation colored red. I made this video using false-color imagery.

As with Sandy Hook, we can examine changes further back in time, and here, we find something extraordinary. The modern-day version of the spit is at least the second incarnation of this feature.  Early in the 20^th century, Beach Haven Inlet (now closed) severed the spit from Long Beach Island, which resulted in runaway erosion of the original pre-20^th century spit, culminating in its complete destruction. Nearly the entire southern end of Long Beach Island, a more than 2 square kilometer area comprising modern Forsythe National Wildlife Refuge, has regrown since the 1920s. 

Southern Long Beach Island / Little Egg Inlet. The Holgate spit is in the center of the image and extends for ~5 km, fronting the ocean.

Aerial photos showing the destruction of the original spit and growth of the modern spit. Growth from right to left.

In future posts, changes in coastal systems around the U.S. east coast will be considered. I’ve talked a little about barrier islands, and I’ll go into much more detail about these in the next installment, examining even greater changes than I’ve shown here…

Welcome to the Geo Jaunt

The mission of Geo Jaunt is to discuss geology, geomorphology, and geography (that's a lot of geo!), sharing my adventures and studies across the US Mid-Atlantic and beyond. I hope to feature some interesting tidbits that everyone can enjoy, exploring natural and human processes across history.

A little about me: My name is Dan Ciarletta, and I'm a Ph.D candidate in environmental management at Montclair State University in New Jersey. I currently study coastal systems as a field geologist and as a modeler, specializing in the evolution of barrier islands. When I'm not digging up sand and mud, or staring into a computer screen, you can find me on the nearest hiking trails.

Stay tuned...