These notes pick up with a brief history of the late Tertiary development of the Chesapeake Bay region. Check your geologic time scale for approximate dates.
The main point to be made here is that there were alternating periods of rising and falling sea level throughout the Tertiary. These apparently occurred on much longer time scales than during the Pleistocene, and were not attributable to glacial/interglacial cycles, which really didn't get started until the Pleistocene.
Some of the local changes in sea level may have been attributable to regional uplift (question: how would uplift affect local sea level?) Others may have been attributable to climate variations, though not to glacial episodes. But what we can see from our curve showing long-term trends in sea level is that, from about 15 million years ago until the beginning of the Pleistocene, each successive high stand of sea level was a bit lower than the one before it. This means that each new high stand did not extend as far inland or as high over the land surface as the one before it. What is the practical implication of this?
Terraces and inset deposits
Consider for a moment that the overall stratigraphy of the Coastal Plain is that of a sedimentary wedge, thickening seaward, with the oldest layers on the bottom and the youngest layers on top. The oldest sediments exposed on land are fluvial sands and gravels of the Chesapeake Potomac group, and these occur in the westernmost part of the coastal plain near the Fall Line, where they just barely overlap the crystalline metamorphic rock of the Piedmont. As you go to the east/southeast, progressively younger layers are exposed at the surface and the older layers are found deeper and deeper below the surface.
These sediments, however, were not deposited in simple wedge-shaped sheets. The fluvial sediments were deposited on the upland surface by rivers that migrated back and forth and dumped their sediment successively in one place and then another. As these rivers reached the shoreline, which was much farther inland than it is today, the remaining sediment reached the ocean in deltas or in coastal embayments, and there it accumulated as coastal or shallow marine sediments. Calvert Cliffs on the west shore of the bay exposes a rich sequence of fossil-rich layers that is one of the great historic fossil-collecting localities of this part of the world. You can read a little bit about it in one of the links above.
What would happen during a low stand of sea level? The shoreline would move seaward, and the rivers would cut downward into their valleys because lower sea level causes a steeper gradient and more available potential energy for transporting water and sediment. As they cut down, and as they also migrated laterally, there would be an incised valley that formed a trench within the upland coastal plain sediments. Riverine gravels and sands would be deposited in the channel running down this valley.
As sea level rises again, these valleys begin to fill in. In their downstream portions, the fluvial deposits may be buried by younger marine open-water deposits. A bit further upstream, one might find coastal or estuarine or wetland deposits. A bit further upstream still, out of the direct influence of the tides, one might still find the rivers aggrading their beds and building up their floodplains to bury the incised valleys, because with higher sea level the gradient is lower and there is less potential energy to transport the sediment further downvalley.
But keep in mind that each high stand is a little lower than the one before it. Therefore the estuarine fill or the layer of marine sediment, or the floodplain sediments, if we are a little further upstream, are not going to be high enough to reach the same level as the surounding upland. The fill from the current high stand is inset at a level below the upland surface.
With the next cycle of sea-level fall and rise, the next sedimentary fill in turn will be inset even lower.
The result is that our "wedge" of unconsolidated coastal plain sediments is dissected by a series of valleys running toward the sea, and each of these valleys contains a staircase of different levels deposited during successive high stands of sea level. These remnant levels are called terraces. If they were made by rivers, they are fluvial or alluvial terraces; if made by estuarine sedimentation or by marine sediment, then they are estuarine or marine terraces.
If you look at the handouts provided in class, you will see a couple of examples, one from the area around Cecil and Harford county and one from a cross-section of the James River estuary. Although at opposite ends of the bay region, they show similar features. The different terraces get progressively younger as you approach lower and lower elevations. Only the lowest couple of levels are of Pleistocene age. If you look at the cryptic table with the sea-level curve on the right side, you can probably find the names of the formations associated with some of these terraces and the approximate time periods during which those sediments were deposited.
Note that the rolling upland topography of the western shore of the Bay is considerably higher than the topography of the eastern shore. The eastern shore developed more recently, during periods when sea level was not as high. Large parts of today's lower eastern shore developed during the most recent high stand of sea level, which was probably no more than 10 to 15 feet higher than modern sea level; thus there are very broad areas of very low relief, and large areas that are dominated by wetlands. This lowest estuarine terrace is made up of sediments of the Kent Island, Nassawadox, or Tabb formations.
So much for a brief overview of the story on land. The story under the Bay is even more interesting. I won't say as much about it, because the readings provide a clearer picture of this part of our story.
Consider that, especially during the Pleistocene, sea level was much lower than today. Channels were incised deeply into the bottom of what we now think of as the floor of Chesapeake Bay. The main channel system was that of the Susquehanna River. But during the subsequent high stand of sea level, much of that channel would eventually be filled in by sediment. Furthermore, the sediment would show a gradual transition, with rising sea level (visible in the sediments closer to the surface), from fluvial gravels and sands to estuarine fine sands and muds. This is exactly the kind of sequence revealed in the sediments brought up from boreholes that were drilled for the pilings of the Chesapeake Bay bridge.
Consider the next low stand. At the seaward end of the Bay, as the water drains out and sea level declines, fluvial incision begins again and cuts a new channel headward, dissecting the landscape that had just been covered by marine and/or estuarine sediment. Does the new channel re-occupy the location of the old one? Or, if the old one was completely obliterated, do we form a new channel in a different place? What evidence do we have to answer this question, and what is the historical sequence of events interpreted from this evidence? This is the primary tale told by one of the papers in the assigned readings, and that paper (Colman and others, 1990) includes several figures illustrating both cross-sections and a plan-view map with three distinct sets of Pleistocene paleochannels representing several successive locations of the Susquehanna River channel during glacial low stands. We will have more to say about this in class. We will also discuss the Pleistocene history of climate and sea level in more detail.
Holocene sea-level rise
We live in the Holocene, which began about 10,000 years ago. It started with a rapidly rising sea level; the rate of rise gradually tapered off around 7 or 8,000 years ago. Sea level has continued to rise since then, although at a lower rate. This rate appeared to have slowed down considerably within the last 2000 to 3000 years; yet modern rates in the Chesapeake Bay area seem to show considerable acceleration in recent sea-level rise. The prospect for continued acceleration as a result of global warming seems quite certain; the only question is how rapid the acceleration will be. An increase of 4-7 times the current rate of sea-level rise seems entirely feasible. We will discuss actual rates in class, as well as some of the differences between global ("eustatic") and local rates of sea-level rise.
How do we even know anything about the rate of sea-level rise? You need to know (a) the age of an artifact or deposit; and (b) the elevation of sea level at the time it was deposited. The most popular dating tool is radiocarbon, which works primarily on samples containing organic matter. But even if we can date the organic matter, how do we find organic matter that can be clearly identified as representative of sea level?
We'll discuss this in class as well.
Once we have developed a sea-level rise curve, we can start to recognize stages in the progressive growth of Chesapeake Bay under the influence of the advancing ocean as it invades the land. We can estimate, for example, the approximate times when the head of tide reached various locations as the invading sea moved up the Susquehanna River valley.
Estuarine geomorphology: coastal modification of a fluvial landscape
The next important question is this: how is the upland fluvial landscape transformed into the estuarine landscape? What are the major processes and what are the telltale signs by which we recognize their influence?
We will explore this in class, and we will also look at some maps for the visible evidence in today's landscape. (I will try to post web links to some of these maps so that you can examine them in detail for yourself. In case you're not aware of it, virtually all of the USGS topographic maps for the U.S. are now accessible online.)
What are the major processes?
Before we review them, please note that an estuary is by definition a zone of mixing between freshwater and marine influence. Therefore you will find different things happening in different parts of the system at any given time. The marine portions closer to the mouth will be more dominated by wind, waves, tidal currents, and transport of sediment along the shoreline. They will face a broader stretch of open water and deeper water, so that there is more opportunity for a lot of wave energy to strike the shoreline. By contrast, the headward portions are really tidal rivers; even if they have reversing tidal currents, most of the phenomena we have just discussed will be less influential. At the same time, because of proximity to the head of tide, there will be larger amounts of sediment derived from the river, and deposition rates may be considerably higher. The intermediate zone is just that: it shares some characteristics of both but is not quite dominated by either. We will find that this zonation applies reasonably well to Chesapeake Bay.
But now think about what happens as sea level rises. The head of tide migrates upstream or up-Bay, and therefore the different zones also migrate: a place along the river may start out dominated by the river, but may gradually be inundated by rising sea level, and eventually that same location may be at a point midway down the estuary, where the environmental conditions are very different. Thus, in a sense, the lower portions of the estuary may be in a "later" stage of development than the headward portions. Of course, if sea-level rise slows down or if there is so much sediment coming in that the whole estuary starts to fill in, then the process may actually reverse itself: a delta forms in the estuary and the river reasserts itself by extending its channel back downstream. This really hasn't happened in Chesapeake Bay, except in a few tidewater tributaries that were affected by excessive soil erosion and sedimentation during the period of intensive agriculture. We will return to this subject later.
Back to the processes:
1) Coastal inundation of the river valley, followed by gradual drowning of the side slopes, eventually inundating even the low divides between adjacent valleys. I refer to the net result of this as "dismembering the drainage network". I will show you some examples from a paper on the paleogeography of Delaware Bay. When we look at nautical charts and maps showing part of Chesapeake Bay, you should be able to reconstruct this same process in your own mind. Dismembering of the drainage network eventually results in a series of tidal creeks all emptying along a common coastline rather than forming integrated parts of a dendritic drainage system, because the downstream trunk of the system has been completely drowned. Just before the drainage divide is completely drowned, it forms a peninsula between tidal creeks; then, as the low spots on the peninsula are inundated, it separates into islands; finally, even the highest parts are inundated, but the remnant patches of island may be occupied by tidal marsh.
2) Growth of tidal marsh occurs as rising sea level inundates low, relatively flat surfaces. There are two main locations where this may happen: in broad flat terrace areas that are drowned, water may spread out and marshes may grow in a broad swath. This is common on the lower eastern shore. A subsidiary example would occur as marsh starts to grow on the last remnant of a drainage divide as it is inundated, or around the fringes of an island as it is being drowned by rising sea level. Note, however, that in many areas with limited supply of sediment, the marsh cannot keep growing fast enough to stay above water as sea level continues to rise. We are currently losing substantial areas of marsh on the Eastern Shore as a result of this phenomenon.
The second example occurs in a tributary valley. As rising sea level inundates the valley, the zone of contact between sea level and the upland stream is one where the stream loses its gravitational energy, drops its sediment, and aggrades its bed. The shallow zone at this location may be rapidly colonized by marsh grasses, and you will find linear patches of marsh grass along this part of the channel system in many small tidal tributaries. With increasing sea level, the marsh may spread over the local floodplain but is still largely confined to the valley floor. Later on, you may find patches of marsh exposed along the open coast by erosion.
3) Shore erosion, largely under the influence of wind-driven waves, tends to straighten shorelines, cutting back protruding headlands and filling in indentations or embayments. An irregular shoreline exposed to enough wave energy will eventually be straightened in this way owing to the effects of wave refraction. We will discuss this process in class and illustrate the result with some examples.
The result is that, after first inundating the fluvial landscape and dismembering the drainage network, we now truncate it: cut back the peninsulas, gradually erasing the fingerlike extensions of the valleys and turning them into short stubs of valleys with flat, relatively straight ends. If we erode far enough back into the valley, we may also expose patches of marsh along the shoreline.
4) Coastal sediment transport occurs primarily under the influence of waves, which rarely hit the shoreline at a perpendicular angle; the result is that there is usually some component of wave activity oriented parallel to the shoreline, and this causes longshore drift. The migration of sand bodies, often derived from winnowing and sorting of the debris left behind by shore erosion, creates spits and bars that may extend out into open water. Bars may grow across the mouths of small- to medium-size tidal creeks, thereby creating a continuous straight shoreline even where indentations were originally present. The inundated portions of smaller tidal creeks may be isolated by these bars to become salt ponds, which eventually will fill in with marsh grass and peat. If the creek is a little bit larger, there may be enough of a tidal current to maintain an inlet through which the tide can enter on the rising portion of the tidal cycle and drain out into the open estuary on the falling portion of the tidal cycle.
5) Most of what we have discussed up to this point is localized in particular parts of the estuarine system. But deposition of sediment, which will be discussed as a separate topic in a few weeks, also has a profound effect on estuarine geomorphology. Estuaries have a tendency to trap virtually all of the sediment that enters them, for reasons to be discussed later. Therefore they have a limited lifespan before they fill in; the faster sea level rises and the smaller the amount of sediment entering the system, the longer the estuary will survive. In the next couple of centuries, Chesapeake Bay may actually grow slightly larger than it is now. But eventually either sea level will stop rising or sedimentation will overtake other processes and gradually fill in the Bay floor. Most likely this will occur in a headward area that will become a delta, and the delta will gradually grow (or "prograde") downbay until most of the Bay is filled and the estuary becomes a shallow combination of wetland and floodplain. If you go to some of the tributaries of the Bay, such as the Jug Bay portion of the Patuxent River, you can see systems that already look like this.
The end result of this is to create an estuarine fill. The next time sea level falls again, this bay bottom or estuarine fill will be trenched and the remaining portion will be left behind as an estuarine terrace, comparable to the estuarine terrace that makes up most of the lower eastern shore today.
Here are some web links to mapped information:
nautical charts for Maryland
Historical nautical charts for Virginia
Topographic maps of Chesapeake Bay area