DEPARTMENT OF AGRICULTURE, CONSERVATION AND FORESTRY Maine Geological Survey Robert G. Marvinney, State Geologist OPEN-FILE NO. 20-9 Title: Maine Landslide Guide Author: Lindsay J. Spigel Date: March 2020 Contents: 27 p. report Recommended Citation: Spigel, Lindsay J., 2020, Maine Landslide Guide: Maine Geological Survey, Open-File Report 20-9, 27 p., 35 figs. Maine Geological Survey Open-File No. 20-9 Maine Landslide Guide Lindsay J. Spigel Maine Geological Survey Maine Department of Agriculture, Conservation, and Forestry Open-File No. 20-9 Image: Lidar hillshade and aerial imagery of Maine’s largest prehistoric landslide complex in Hollis, Maine. Landslides have occurred in this area from about 4,000 to 600 years ago. Maine Geological Survey Open-File No. 20-9 Table of Contents Introduction ..................................................................................................................................... 1 Purpose ........................................................................................................................................ 1 Historical Overview .................................................................................................................... 2 The Lidar Revolution .................................................................................................................. 3 What causes mass wasting? ............................................................................................................ 7 Undermining a slope ................................................................................................................... 7 Adding weight ............................................................................................................................. 8 Reducing shear strength .............................................................................................................. 8 Geology ....................................................................................................................................... 9 Glacial till................................................................................................................................ 9 Glaciolacustrine deposits ...................................................................................................... 10 Glaciomarine mud (The Presumpscot Formation) ................................................................ 11 Earthquakes ............................................................................................................................... 12 Extreme storm events ................................................................................................................ 13 Mass Wasting Types and Maine Examples .................................................................................. 14 Creep ......................................................................................................................................... 14 Rockfall ..................................................................................................................................... 15 Landslides ................................................................................................................................. 16 Rotational Slide/Slump ......................................................................................................... 17 Translational ......................................................................................................................... 18 Flow ...................................................................................................................................... 19 Spread ................................................................................................................................... 20 Retrogressive Landslides ...................................................................................................... 21 Summary ....................................................................................................................................... 22 What types of mass wasting occur in Maine? ........................................................................... 22 Where are landslides most likely to occur in Maine? ............................................................... 22 When are landslides most likely to occur in Maine? ................................................................ 23 What can be done to help prevent landslides and most importantly, lower the risk to human life and property? ...................................................................................................................... 23 References ..................................................................................................................................... 26 Maine Landslide Guide Introduction Purpose Mass wasting is the downslope movement of earth materials under the force of gravity. Landslides are just one of many mass wasting types, and definitions of their characteristics vary worldwide. The purpose of this guide is to provide Maine citizens with introductory information about the types of mass wasting that may occur in Maine and their causative factors. Historical and modern landslides in Maine have fortunately not resulted in any loss of life, but these events have destroyed homes, roads, and other property. As Maine’s population grows, it is important for residents and land use planners to understand when and where landslides are most likely to occur and to plan accordingly. This guide is not intended to be a site-specific resource or a substitute for detailed investigations of areas where landslide planning or remediation might be necessary. If readers would like additional information about landslides, landslide mitigation, and geology related to mass wasting in Maine, the following free resources are an excellent place to start: Highland, L.M., and Bobrowsky, P., 2008, The landslide handbook - a guide to understanding landslides: Reston, Virginia, U.S. Geological Survey Circular 1325, 129 p. https://pubs.usgs.gov/circ/1325/pdf/C1325_508.pdf Slovinsky, P., 2011, Maine Coastal Property Owner’s Guide to Erosion, Flooding, and Other Hazards (MSG-TR-11-01), Orono, ME: Maine Sea Grant College Program, 85 p. https://digitalmaine.com/geo_docs/126/ Thompson, W. B., 2015, Surficial geology handbook for southern Maine: Maine Geological Survey, Bulletin 44, 97 p. https://digitalmaine.com/mgs_publications/2/ 1 Lindsay J. Spigel Historical Overview Landslides are a known hazard in Maine, especially in southern Maine where events have been documented from the late Ice Age (about 14,000 years ago) to modern times (e.g. Morse, 1869; Novak, 1987; Berry and others, 1996; Thompson and others, 2011). The following is a quote from an account of the largest documented landslide in Maine (about 38 acres/12 hectares), which occurred in Westbrook on the north bank of the Presumpscot River and gives an idea of the destruction a landslide may cause (Morse, 1869, p. 237-238; Fig. 1): “On the 22nd day of November, 1868, another land-slide occurred on the north bank of the Presumpscot River… This slide was much greater in extent than those already spoken of. The bed of the river, some two hundred feet in width, was filled for nearly half a mile with the debris. The contour of the sunken area is quite different from the other slides… As one looks into this chasm from the banks above, the appearance is startling. On a large portion of the sunken area, the trees stand nearly vertical, but here and there occur long ridges of soil bearing upon them trees, inclining at various angles, many of the trees prostrate, and the intervals between the ridges filled with the light, upturned, plastic clay, or huge, square blocks of unaltered clay. In one place may been seen a portion of an old wood road, with a large pile of cut wood, but little disturbed. Looking toward the river from the sunken area, the sight is singularly wild, for here the masses of earth have been forced out, the ridges of earth crowding upon each other, and trees and shrubs are broken, bent and turned in every direction.” Figure 1. View of the 1868 Westbrook landslide crater from near the Presumpscot River, looking roughly north. Photo: Westbrook Historical Society. 2 Maine Landslide Guide The 1868 event was not the first to be witnessed along the Presumpscot River – another smaller landslide had occurred on May 5, 1831 just downstream of the 1868 landslide location near what is now the U.S. Route 302 bridge (Hitchcock, 1836). The oldest documented Maine landslide is thought to have occurred in late June or early July of 1670 along the Kennebunk River at Durrell’s Bridge in Kennebunk Landing (Bradbury, 1837; Bourne, 1875; Remich, 1911). Witnesses referred to the landslide as “the wonder,” and noted blue clay with marine shells exposed by the event that partially dammed the river. Another landslide occurred in this same area on June 11, 1834, destroying part of Durrell’s Bridge and blocking the river again (Bradbury, 1837; Remich, 1911). The remnants of these landslides have long since been modified by the rivers and humans. Landslides were also documented along the Stroudwater River, where a relatively large event (about 7 acres/3 hectares) occurred on June 5, 1849, and a smaller landslide occurred in 1873 where Spring Street crosses the Stroudwater River in Westbrook (Morse, 1869; Rowe, 1952). Many other landslides may have occurred in Maine during historical times