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Home > Explore! > Geologic Hazards > Landslides > Greenbush Landslide Greenbush Landslide:
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 Greenbush landslide - rotational scarp |  Downed trees on marine clay |
On June 30, 2006 a landslide occurred along the banks of the Penobscot River in Greenbush, Maine. The slide was located along Route 2, approximately 11 miles north of Old Town (Figure 1). The landslide began on June 30, and continued to move and slide until July 2. The resultant landslide, which undercut Route 2, caused this section of roadway to be closed until the river bank could become stabilized, and the roadway section rebuilt.
 Figure 1 |
 Figure 2 |
 Figure 3 |
An initial reconnaissance revealed that the Greenbush landslide was not a unique geologic event in this area. Immediately north of this landslide a similar scarp indicates another landslide of the same type, which occurred in 2002 (Figure 2). Immediately south of this event an old smaller sized scarp is indicated (Figure 3).
 Figure 4 |
The area of the Greenbush landslide lies along the course of the Penobscot River (Figure 1). The course and orientation of the present day Penobscot River is due to its bedrock framework and surficial geological history. Kelley and others (1988) divided the Penobscot River into four distinct regions (Headwater Division, Island Division, Rapids Division, and Tidal Division) on the basis of geological and geomorphic features (Figure 4). The Greenbush landslide lies within the Island Division. |
The Island Division of the Penobscot River is a low-gradient area from Medway down to Old Town. It is characterized by a series of islands and rapids flanked by a wide flood plain. Fine-grained, lower Paleozoic metamorphic rocks of the Vassalboro and Madrid Formations underlay the surficial glacial deposits and outcrop throughout the area (Osberg and others, 1985). Surficial deposits in this portion of the river include eskers, glacial outwash, till, terrace deposits, and modern alluvium, which overlie the glaciomarine Presumpscot Formation.
The Laurentide Ice Sheet began its retreat from the edge of the continental shelf off Maine before 22,100 years ago. By about 16,100 years ago the ice margin in the Penobscot Bay region stood just inland from the present coastline (Borns and others, 2004). For the next 200 years the ice margin fluctuated back and forth, retreating slowly. It then began to retreat rapidly, and by 14,000 years ago the Penobscot lowland was ice free and the margin was near Medway (Hooke and others, 2006).
 Figure 5 |
As the ice retreated inland, vast quantities of sediment were released. The coarser sediments settled out along the ice margin in the form of grounding-line moraines, submarine fans, and massive deltas (Figure 5). The finer grained sediments were carried in suspension and dispersed farther into the sea, where they settled to the bottom to form the Presumpscot Formation. As the ice retreated from the coast, the sea maintained contact with it in lower areas, inundating the lowlands. |
As the ice margin retreated up the East Branch of the Penobscot River, the sea continued to flood in. Submarine fans and deltas formed where subglacial esker-building water emerged from beneath the ice sheet. These fans and deltas were deposited over the marine clay of the Presumpscot Formation. Once the margin retreated above sea level, a proglacial outwash plain or braidplain formed, connecting the ice sheet with the sea. As sea level fell the braidplain was incised, leaving a series of alluvial terraces. It is on one of these terraces that the Greenbush landslide occurred.
Geological mapping in the Greenbush topographic quadrangle, where the Greenbush landslide occurred, has established that a braidplain migrated over and buried Presumpscot silts and clays, which in turn rest on till and bedrock (Hooke and others, 2006).
Some slopes are susceptible to landslides, whereas others are more stable. Many factors contribute to the instability of slopes, but the main controlling factors are the nature of the underlying bedrock and soil, the configuration of the slope, the geometry of the slope, and ground-water conditions.
 Figure 6 |
Once a landslide is triggered, material is transported by various mechanisms including sliding, flowing, and falling (Figure 6). Landslides often occur along planes of weakness that may parallel the hill slope. Soils such as silt and clay are weaker than rock and commonly have complex or multiple planes of weakness. |
Landslides occur in slopes oversteepened by the process of stream erosion or by the activities of man in grading slopes. In many instances oversteepened slopes stand in apparent stability until abnormally high ground-water conditions occur, thus reducing the shear resistance and triggering the landslide. Once a slope in a sensitive soil has been oversteepened by erosion at the toe or by excavation work and the ground-water table is high, the stage is set for a landslide to occur.
Three distinct physical events occur during a landslide: the initial slope failure, the subsequent transport, and the final deposition of the slide materials. Landslides can be triggered by gradual processes such as weathering, or by external mechanisms including:
Local geomorphology was perhaps the most significant natural factor influencing the June 30, 2006 Greenbush slide. The locale of the slide was along a meander on the east side of the Penobscot River (Figure 5). The bluffs were around 20 feet high with a very steep slope.
The primary causes of the Greenbush landslide were undercutting and oversteepening of the slope between the river and Route 2, prolonged rainfall, and overloading of the upper slope.
Heavy spring rainfall, along with a heavy June rainfall of double the normal monthly average, caused the Penobscot River to rise and flow faster. This in turn eroded and undercut the outside meander bend creating a cut-bank, and oversteepened the slope at this location.
As stated above, the heavy rainfall induced increased erosion and oversteepening of the slope. Besides this, the increased rainfall raised the water table which increased the pore water pressure, which in turn reduced the shear resistance in the marine clay causing it to liquefy and slide.
The construction of Route 2 along the bank of the Penobscot River added 8 feet of overburden material and road macadam on top of the underlying substrate. By itself, this overloading probably wouldn't have caused the landslide, but taken together with the rainfall and bank erosion, it produced the Greenbush landslide.
 Figure 7 |
 Figure 8 |
 Figure 9 |
The Greenbush landslide can be technically classified as a rotational earth slide (Figure 7). Figure 8 shows a schematic of a landslide with its major components named, and Figure 9 is an animation of a rotational slide. A rotational slide is one in which the surface of rupture is curved concavely upward and the slide movement is roughly rotational about an axis that is parallel to the ground surface. The next photos show the three (3) main parts of the Greenbush landslide:
 Figure 10 |
 Figure 11 |
 Figure 12 |
On Friday, June 30, 2006 cracks appeared along the west-side shoulder of Route 2 in Greenbush, Maine (Figure 13). Some slumping was also noted. Shortly thereafter a landslide occurred creating a scarp between the toe beams and the river (Figure 14) The toe beams had been driven down to bedrock during the initial construction of Route 2 to prevent shearing from taking place at the Greenbush landslide site (Figure 15). Riverward of these toe beams a major rotational landslide occurred. During the weekend, the area between the road and the toe beams slid right through the beams (Figure 16). The slide was approximately 100 feet across, and an estimated 1500 cubic yards of material was displaced.
 Figure 13 |
 Figure 14 |
 Figure 15 |
 Figure 16 |
The Maine Department of Transportation (MDOT) immediately began remediation of the slide area. They first closed Route 2 and detoured traffic around the slide. The base of the slide was then cleared by removing the trees and slide detritus. They then dug down into the marine clay (Presumpscot Formation) to obtain a stable base to begin the riprap process (Figure 17). MDOT then infilled with large granite blocks to stabilize the slope to prevent further oversteepening (Figure 18). This whole process was estimated to cost between 150,000 and 250,000 dollars.
 Figure 17 |
 Figure 18 |
Since there was a landslide in 2002 directly to the north of this slide, and the areas both north and south show previous and current movement and creep, this area along the Penobscot River needs to be closely monitored to help prevent another large and costly landslide from happening.
Proglacial outwash plain (Braidplain). A broad, gently sloping sheet of sand and gravel deposited by meltwater streams flowing directly in front of a glacier. When an ice margin is retreating at a steady rate there is usually insufficient time for fans to accumulate and a broad and relatively flat outwash plain is formed. As an ice margin retreats, its meltwater will dissect earlier outwash surfaces and flights of terraces may form.
Alluvial Terraces. A stream terrace composed of unconsolidated sediments (including gravel), produced by downcutting of the flood plain by streams.
Cut-bank. A steep bare slope formed on the outside of the curve of a meandering stream or river due to higher water velocity, which causes erosion on outside of curves.
Borns, H.W. Jr., Doner, L.A., Dorion, C.C., Jacobson, G.L., Jr., Kaplan, M.R., Kreutz, K.J., Lowell, T.V., Thompson, W.B., and Weddle, T.K., 2004, The deglaciation of Maine, U.S.A, in Ehlers, J. and Gibbard, P.L., (editors), Quaternary glaciations - extent and chronology, Part II: Elsevier, Amsterdam, p. 89-109.
Hooke, R.L., Kelley, A.R., Borns, H.W., Jr., Jacobson, G., Robinson, B. and Sanger, D., 2006, Glacial and archeological features of the Penobscot Lowland, central Maine: Guidebook for the 69th annual field conference of the Northeastern Friends of the Pleistocene, Stop 7, 36 p.
Kelley, A.R., Kelley, J.T., Belknap, D.F., Sanger, D., 1988, Quaternary stratigraphy and geomorphology of the lower Penobscot Valley, in Field trip guide for the Summer Meeting of the Geological Society of Maine (August 6-7, 1988): Geological Society of Maine, 7 p.
Osberg, P.H., Hussey, A.M. II, and Boone, G.M., 1985, Bedrock geologic map of Maine: Maine Geological Survey, scale 1:500,000.
USGS landslide types and processes, 2004.
Site by Michael E. Foley
Originally published on the web as the December 2006 Site of the Month.
Last updated on December 12, 2006
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