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Coastal Landslide with Outrunner Blocks, Bunganuc Bluff
|In late March of 1997, staff at the Maine Geological Survey received a telephone call from a concerned homeowner in the coastal area in Brunswick, Maine, known as Bunganuc. The homeowner reported that a few days earlier, a landslide (Figure 1) occurred on his property at the edge of the coastal bluffs of glacial marine deposits exposed there.
|Several months earlier, a geologist from the Survey had been to the bluff and noted that a new home was under construction, and he left a business card with a worker as a courtesy to the owner, who then called the Survey, inquiring about the possibility of a slide occurring. A discussion followed, the owner wanting to know more about landslides, and what might be done to prevent them. He was aware that they do occur in the Bunganuc area, and it had been just a year earlier that a large landslide took down two homes in Rockland. However, he felt that with enough shore frontage, he was willing to take the risk of building in a landslide-prone area. Following the slide in March, the owner's home measured approximately 100 feet from the edge of the landslide head scarp (Figure 2), and he felt it was important to contact the Survey.
The Maine Geological Survey has published a series of Coastal Landslide Hazards Maps that covers most of Maine's coastal bluffs. The Bunganuc area is shown on the Coastal Landslide Hazard Map of the Freeport quadrangle to be a location of known or interpreted landslides (Figure 3). Moreover, landslides in the area were documented in reports as early as March 1952 (Figure 4) in the Report of the State Geologist 1951-1952. Slides at the bluffs have been known by local people well before the early 1950's. The mudflat, exposed during low tide, is a prime clam and worm-digging locale. The source of the mudflat is the glacial marine clay found in the bluff; it is eroded by the landslide process and is distributed to the mudflat by tidal cycles and wave processes.
The slide of March 1997 is interesting because of how far from the bluff face some of the blocks slid. Figure 5 is a map of the various landslide features including the "outrunner" blocks, the farthest of which traveled 183 feet from the base of the bluff. Figure 6 is a cross-section perspective showing the same features viewed from the side.
The initial blocks that slumped from the bluff were propelled seaward along the slick mudflat surface by a combination of their initial momentum and sliding on a gentle slope with very low friction. The series of photos below shows the landslide scarp, the large mass of slide debris at the base of the bluff, and the outrunner blocks. (These photos were all taken in March 1997.)
More information from the Maine Geological Survey about coastal landslides in Maine:
- Coastal Landslide Hazards Maps (Scale 1:24,000) These color maps show locations of known landslides and areas of potential landslide hazard on bluffs along the Maine coast. The explanation describes factors influencing landslide risk.
- Coastal Bluff Maps (Scale 1:24,000)
These color maps show the shoreline type and relative stability of bluffs along the Maine coast. The slope, shape, and amount of vegetation covering a coastal bluff and the adjacent shoreline are directly related to the susceptibility of the bluff face to ongoing erosion.
- The April 1996 Rockland Landslide, by H.N. Berry IV, S.M. Dickson, J. T. Kelley, D.B. Locke, R.G. Marvinney, W.B. Thompson, T.K. Weddle, R.T. Reynolds, and D.F. Belknap, 1996, 55 p., 13 figs., 3 appendixes. (Catalog No. 96-18)
- Stability of Natural Slopes in the Presumpscot Formation, by S.C. Devin and T.C. Sandford, 1990, 75 p., 17 figs., 5 tables, 5 apps. (Catalog No. 90-24)
- Mineralogy and Pore Water Chemistry of Presumpscot Clays, by L.M. Mayer, 1990, 3 p., 1 table. (Catalog No. 90-23)
- Air Photo Reconnaissance of Slope Failures in the Presumpscot Formation, Cumberland County, Maine, by I.D. Novak, 1990, 4 p., 1 fig., 1 table, map (1:50,000). (Catalog No. 90-22)
- Landslides in the Presumpscot Formation: An Engineering Study,by J. Amos and T.C. Sandford, 1987, 68 p., 22 figs., 7 tables, 3 apps. (Catalog No. 87-4)
- Inventory and Bibliography of Maine Landslides, by I.D. Novak, 1987, 27 p., 2 figs., 3 tables, 1 app., map (1:500,000). (Catalog No. 87-3)
- Living with the Coast of Maine, by J.T. Kelley, A. R. Kelley, and O. H. Pilkey, Sr., 1989, 174 p., 118 figs., 3 app. (Catalog No. COAST)
Website text by Thomas K. Weddle and Henry N. Berry IV. Map and cross-section of March 1997 landslide (Figures 5 and 6) by Berry. 1997 MGS photos by Weddle and John B. Poisson. 2004 MGS photos by Weddle.
Originally published on the web as the December 2004 Site of the Month.
Last updated on January 20, 2011