STUDY SUMMARY

For the past four years, interns at the USGS have been studying "fossil" and actively eroding bluffs along the Calvert Cliffs in Calvert County, Maryland. The original aim of the study was purely academic. It addressed a classic geomorphologic problem of hill slope evolution. In simple terms we wanted to know how slopes evolved and at what rates. The Calvert Cliffs provided a laboratory where we could measure progressive change in slopes through time due to movement of prominent coastal landforms along the shore. Three types of landforms provided a time framework that would allow us to calculate rates of change in slope development. These landforms were a cuspate foreland (Cove Point) migrating downdrift along the shore and sealing the toes of actively eroding bluffs as it prograded southward over a period of 1700 years. Another was a prograding spit sequence (Flag Ponds) that has been deposited during post colonial times; ostensibly 400 years. Finally we looked to modern harbor structures to round out the study. In short, progressive updrift filling behind an historic harbor structure gave us a near real-time framework for understanding the rates at which near vertical and actively eroding and receding bluffs evolve. Our field trip today is designed to demonstrate our results. Slope evolution is clearly a two-part process. Our actively eroding bluffs show an ongoing process of undercutting by wave action and subsequent sloughing of the bluff face to supply sediment to the beach. Once the toes of these bluffs are protected, however, the next process takes over at a longer time period. In short, slumping and in some cases rotational landslides are the next process in the sequence. We were rather astounded by the rapidity at which slopes changed from actively eroding 70 to 80 degree faces to angles of repose on the order of 30 to 35 degrees. It became clear that we had developed a tool for predicting rates of change for slopes in easily erodable sediments. We can tell you confidently that if you are foolish enough to build or buy a house on a cliff edge with a beautiful view of the Chesapeake Bay, simple toe protection of that cliff is not going to save your bacon. We are going to show you our study sites on the field trip and explain our study rationale along the way. We also hope the field trip will give your all some good ideas. Here is the plan:

The group will meet at the main entrance to the Holiday Inn in Solomons, MD at 8:00 am. We plan to depart at 8:30 am. There will be four stops (Figure 1)  to look at fossil bluffs and ongoing bluff erosion along the Calvert Cliffs. For those attendees who don't mind backtracking, an additional stop is also planned to view recent erosion control measures installed at the Naval Recreation Facility at Solomons, MD. A field trip guide will be available at the meeting. For those stragglers, the mileage guide is included below:

COVE POINT:Welcome to the Cove Point Beach community. Please stay on the path and walk down to the beach. This is private property so please be courteous. The beach we are looking at here is facing south-southeast. This is the maximum fetch direction for southerly summer winds. If we could see thorough the trees behind us to the north, we would see open water to the north-northeast as well. This is the maximum fetch direction for northerly winds. Cove Point is located at the approximate mid point of the Chesapeake Bay along its steep western shore. We are standing on the active beach face of a southward-migrating cuspate foreland. It is made up of concentric beach ridges much like the one we are standing on that descend in elevation to the north and plunge below the present mean sea level at its northern extent. Because of greater wave energy supplied from winter winds across the maximum northerly fetch direction, the northern portion of the foreland is being steadily eroded with sediment transported to the south along its outboard face. Sediment is eventually carried around Cove Point where it blends with sediment derived from bluffs to our south that are brought north by southerly winds and longshore transport.

The results are this beach on the leading edge of landform that is steadily migrating to the south at about 1.5 m/yr. An excellent M.S. thesis by Michael Beardslee of the University of Maryland studied the migration of the foreland. Through a series of vibracores and historic maps he was able to date the migration of this landform. The earliest ¹⁴C age is ca. 1700 BP at the northern edge of the foreland. Thus, we have a progressive record of beachridge formation, erosion, and subsequent formation of a successively new south-facing beachridge. For our purpose here, the migration of the cuspate foreland has preserved a fossil bluff inland from here. The fossil bluff line is continuous from actively eroding bluffs to the north to similarly eroding bluffs to the south. In between is the vegetated fossil bluff line. In effect, it preserves a progressive record of the protection of actively eroding bluffs over a 1700-year period as the foreland moved south and ended wave erosion at the base of the bluffs. The bluffs are clearly now vegetated and are standing at lower angles.

Martha Herzog, now with NOAA, began systematic measurement of slope angles along this fossil bluff line to ascertain progressive changes in slope angle with age that would indicate the rate at which bluffs composed of sediments common to Calvert Cliffs failed and established stable vegetated slopes. Suprisingly there was no progressive change in slope angle. All slopes measured along fossil bluff line had a mean value of 30 degrees. Over 1700 years of progressive protection from wave action slope failure was so rapid that she could not measure it on a century scale. The enclosed figures describe general features. Martha's poster at the poster session provided a complete analysis. So let's move on to see what happens on landforms of lesser age. When leaving this stop, backtrack on Cove Point Road to:

FLAG PONDS NATURE PRESERVE: Like Cove Point, dominant southward longshore transport gave rise to a prograding spit sequence at this location. Unfortunately (or fortunately depending on your view) the spits are now covered by lowland forest and marshes between beachridges. From recent vibracores taken by Peter Vogt from the Naval Research Lab we now know that basal marsh sediment covering the innermost and northernmost beach deposits in this sequence contain Ambrosia pollen for their full depth.

This signifies that the earliest spits here at Flag Ponds were deposited during the colonial period and suggest that the remainder of this spit complex is historic with perhaps a maximum age of 400 years. In terms of our outdoor slope evolution laboratory, Martha Herzog now had a shorter time framework to measure progressive change. Here again, she made consistent measurements of slope angles from oldest slopes in the north to the actively eroding bluffs to the south. Here again, all slopes were a consistent 30 to 35 degrees. The changes occurring in the slopes were more rapid than she could measure. Bluffs failed after protection from waves and subsequent toe stabilization and established low angle vegetated slopes - this time within a 400-year envelope. So let's continue to the next stops and let Inga Clark describe the results of her portion of the study.

FLAG HARBOR MARINA and CALVERT BEACH: Bluffs at Flag Harbor are a continuation of the Calvert Cliffs seen earlier at Flag Ponds. Sediments of Calvert Cliffs are generally characterized by fossiliferous medium to fine sands interbedded with shell-poor silty sands, silts and clays. Due to the dip of strata to south- southeast, only Choptank and St.Mary's Formations of the Chesapeake Group are exposed in the bluffs around Flag Harbor. The Plum Point Member of the Calvert Formation is exposed at the base of bluffs to the north of Calvert Beach (Figure 5).

Under natural conditions, bluffs are subject to wave undercutting, freeze and thaw action and groundwater seepage. These wasting processes maintain slope angles around 70 degrees and relatively constant bluff-retreat rates. Recession rates for this area measured by the Maryland Geological Survey over the 1848-1980 period vary from 0.7 to 1.3 meters per year (personal communication, Lamere Hennessee, Maryland Department of Natural Resources, Maryland Geological Survey).

Development in this area started around 1930's. In late 1940's a pair of harbor structures were constructed in the ravine between Long Beach and Calvert Beach to maintain a dredged channel to Flag Harbor Marina (figure 6). In pace with the residential development along the shoreline several additional smaller groins were built to the north of Flag Harbor between 1950's- 1980's to slow down cliff erosion.

In 1975 a single jetty was put at the mouth of Kings Creek at Calvert Beach. Construction of jetties altered natural pattern of erosion and sediment re-distribution. Downdrift erosion is apparent south of the structures. Updrift deposition behind northern jetty at Flag Harbor and jetty at Kings Creek progressively created beaches along the toes of the bluffs. Protective sand bodies at the toes of the bluffs eliminate undercutting of slopes by wave action. However, slope processes initiated by freeze and thaw action, groundwater seepage and weathering continue to promote recession at the top of the bluff. Rotational slides or slumps displace material from the top of a bluff. Slump and colluvial material accumulate at the base of the bluff and form an "accumulation zone". Eventually, the protected bluffs degrade to form stable, vegetated slopes with angles ca. 30-35 degrees.

Due to the relatively recent construction of jetties at Flag Harbor and Calvert Beach, and, hence, relatively short time period since accumulation of sand bodies, we are able to follow the changes in slope angles of the bluffs and register the time required for eroding bluffs to reach stability. We measured slope angles at intervals northward from the Flag Harbor jetty for a distance of ~1700 meters. A least squares regression of slope angle vs. distance showed increase in angle from south to north. Stable, vegetated slopes closest to jetties are followed by vegetated but slumping slopes where the toes of the bluffs were protected for a shorter period of time. Finally, vegetated slopes give way to steep, actively eroding bluffs to the north of Kings Creek jetty.

The relative time required for the eroding bluffs to reach stability was estimated by interpolating the distance and time for stable slopes to prograde northward since the construction of jetty. Change in slope angle for bluffs north of Flag Harbor is 0.2 degrees per 100 meters while that for the bluffs north of Kings Creek jetty is 5 degrees per 100 meters. Thus, bluffs at the distance of 100-200 meters north of jetty evolve from 70-degree to 35-degree inclinations in about 30 years.

In these settings simple shoreline protection methods such as groins, bulkhead or rip-rap may fix the toe of a slope, but they do not prevent retreat of the bluff face. Even when the toe of the bluff is stabilized, groundwater seepage and freeze/thaw action induce slope failure and lead to the recession of the top of the bluff. Other protective structures aimed to slow down erosion of slope material by overland flow and/or groundwater and decrease slumping ( figure 7) are generally ineffective. Planting vegetation on the slope faces does not decrease retreat at the top of the bluff significantly either. Unless the slope has already reached fairly stable inclinations, the mentioned protective measures do little to prevent deep slips or slumps from occuring.

BIBLIOGRAPHY

Beardslee, M.W., 1997. Evolution of a cuspate foreland: Cove Point, Maryland. Master's Thesis. Department of Geography, University of Maryland, 170p.

Clark, I., Larsen, C.E., and McRae, M., 2002. Historic bluff retreat and stabilization at Flag Harbor, Chesapeake Bay, Maryland. U.S. Geological Survey Open File Report 02-331. http://pubs.usgs.gov/of/2002/of02-331/

Herzog, M., Larsen, C.E., and McRae, M., 2002. Slope evolution at the Calvert Cliffs, Maryland: Measuring the change from eroding bluffs to stable slopes. U.S. Geological Survey Open File Report 02-332.http://pubs.usgs.gov/of/2002/of02-332/

Newell, W.L., and Rader, E.K., 1982. Tectonic control of cyclic sedimentation in the Chesapeake Group of Virginia and Maryland. In: Lyttle, P.T.(Editor), Central Appalachian Geology: NE-SE GSA'82 field trip guidebooks, American Geological Institute, 1-27.

Vokes, H.E., 1961. Geography and geology of Maryland. Bulletin No. 19. Maryland Geological Survey, 243 p.

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature. Any use of trade names is for descriptive purposes only and does not imply endorsement by the USGS.

For questions please contact Curt Larsen or Inga Clark.