Chesapeake Landforms - Sandy River Bluffs

American Holly

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One of the interesting parts of my job these days is to oversee projects associated with the restoration & long-term stabilization of tidal shorelines in the Chesapeake Bay. One of the landforms on which this type of conservation work is frequently done is at the base of sandy river bluffs on small tributaries of the Chesapeake Bay. Several processes are occuring that are eroding tidal wetlands at the base of these bluffs, including sea level rise, freeze/thaw erosion, wave action from power boats, unattenuated wind wave action resulting from a lack of bay grasses (seaweed), and extensive runoff and erosion from the top of the sandy bluffs themselves. Whooo...that's a lot of stuff....and we'll tackle it sometime later!

Typical eroding sandy bluff on the South River in Central Maryland
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But these bluffs, and their constant erosion and movement, have a really interesting history to themselves, as well as a very important ecological role to play in what has become a highly developed part of the earth.

Based of a sandy bluff - the beach sand is eroded off of the slope. These beach habitats are very important for Diamondback Terrapins and Horseshoe Crabs.
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I wanted to tell the story of these unique landforms, but since I am not a geologist, I thought it might be best to contact some geologists. I was able to get in touch with Bob Conkwright and Dr. Jim Reger, both from the Maryland Geological Survey, and ask them a bit about their landforms. Not surprisingly, I got a taste of my own medicine - way more information than I was expecting - which is what folks get when they ask me, "What's wrong with my plant?" (which coincidentally, is the name of a great book).
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According to Bob, most of the sandy river bluffs in the Central Chesapeake Bay tributaries are (geologically) recent features, ranging from 3.5 million to 65 million years of age. In the coastal plain (where the rivers are generally affected by the tides), there is no bedrock within the top 100 or more feet below the surface. So where did all of this sand come from? And keep in mind that the Chesapeake Bay, which cuts rivers and creeks through these sandy cliffs, is largely a product of natural global warming & sea level rise over only the last 10,000 to 12,000 years.
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Bob Conkwright calls these sands "lowland deposits," which means that surface water brought the sand to its approximate current location in horizontal layers, causing extensive shelves. International Geological Congress Field Trip Guide T211 (Elk Neck, MD) shows that some of the sources for the deposits of this age were "upland gravels" and "bay and river fill." We can presume that "upland gravels" were washed down from the piedmont and mountains as eroded pieces of bedrock, while "bay and river fill" are layers of sediment laid down during long term (thousands of years) or short term (perhaps just a few hours) flood events.
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In fact, a trip up just seaward (east) along the Fall Line boundary from Georgia to Delaware, between the piedmont and coastal plain geologic provinces will show us just that - significant efforts at sand and gravel mining....it's really just sand and gravel digging, since the desired materials are at or near the surface, not buried like a true "mine."
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One thing I teach my physical geography class is that glacial rock deposition occurs at the retreat of a glacier - not during its construction. According to The Geological History of the Chesapeake Bay (Hobbs, 2003), this is also true for sandy river bluffs (hence, the "bay and river fill"). As the higher sea levels began to recede (oncoming ice ages and global cooling periods), significant layers of sand, silt, and clay were left behind. At the same time, piedmont rivers began to adapt to this lowering sea level, and with gravity, carved directly into the sand, forming the series of interspersed creeks and cliffs that we see today in the central Chesapeake Bay region.
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Sandy river bluffs are important areas today for an interesting set of reasons. Due to their instability and steep slopes (and resulting environmental & insurance regulations), many of these bluffs can only be partially developed for real estate purposes. Often, the steepest area is the eroding cliff closest to the waterfront. In effect, a band of wildlife habitat occurs in an area (waterfront) that is normally heavily impacted by human activities and development. These areas are generally left alone for wildlife - and their heritage as rich marine sediments - though sandy - provide some great native plants to support wildlife. These plants are very hardy and withstand one of nature's common ironies - near desert conditions just a few feet away from flowing fresh water.
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Let's take a look at some of these plants, in addition to the American Holly at the top of the post.

Virginia Pine - the seeds from its cones are eaten by doves, quail, turkey, and other birds. Some songbird species prefer dried needles for their nests.

Pokeberry is a pesky "weed" whose berries are eaten by birds as soon as they are ripe

Blackhaw Viburnum - berries are treasured by small mammals and birds during the winter months.

The acorns of the Post Oak help fatten up wildlife for the cold winter months. Post Oak is susceptible to the Chestnut Blight Fungus, responsible for eliminating the American Chestnut from the forests of eastern North America.

The leaves of the Mountain Laurel are poisonous to deer and other hooved animals (a very specific evolutionary adaptation!), but the high-protein berries are important food for wildlife in the winter.

Greenbriar is one of many thorny woody vines that inhabit sandy river bluffs. Greenbriar also produces a very high protein berry that is nutritious for songbirds.

The Shad Bush mainly reproduces mainly by cloning, but also provide high quality food for songbirds. The plant gets its name from the time of year that it blooms, which roughly coincides with the time of spring that shad migrate from salt water to fresh water rivers to spawn.

Red-Cedar (a juniper) is a shrub used for living fences, wildlife plantings, and even furniture. The needles and berries of the plant are highly edible to a very wide variety of wildlife species, from deer to songbirds. The berries maintain their nutritional quality throughout the winter months, providing a great emergency source of food for late winter birds.

Over 60 species of songbirds eat Poison Ivy's waxy white berries, which are extremely protein-rich and available through the first half of the winter. In fact, this is why poison ivy mysteriously shows up in your garden - having passed through the digestive tract of a songbird. Unfortunately for humans, we now know that Poison Ivy thrives in air pollution and high CO2 environments, and in fact, produces even more poison than usual.