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?