DEPARTMENT OF AGRICULTURE, CONSERVATION AND FORESTRY Geological Survey Robert G. Marvinney, State Geologist

OPEN-FILE NO. 20-9

Title: Maine 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. 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 ...... 8 ...... 9 Glacial till...... 9 Glaciolacustrine deposits ...... 10 Glaciomarine mud (The Presumpscot Formation) ...... 11 ...... 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, , 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, , 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, , 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/

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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 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 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 , or huge, square blocks of unaltered clay. In one place may been seen a portion of an old wood , with a large pile of 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.

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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 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 (1600s to 1950), perhaps going unnoticed due to the sparse population, but several of the publications referenced here allude to Native Americans having witnessed landslides prior to Euro-American settlement. Thus, landslides are not a recent phenomenon and technology would confirm this in the 21st century.

Modern landslides (1950 to present) tend to be relatively small (≤1 acre/0.5 hectare) rotational slumps along river cut banks or coastal bluffs. Two size exceptions are the 1983 Gorham landslide along the Stroudwater River (about 7 acres/3 hectares) and the 1996 Rockland Harbor landslide (about 3.5 acres/1 hectare; Amos and Sandford, 1987; Berry and others, 1996). Most of these landslides can be attributed to periods of consistent but not record-breaking moisture, and/or human disturbance. Causative factors will be discussed in more detail in Section 2. Instead of creating a static list of landslides in this guide, MGS has created updatable databases of landslide information that readers can view and sort to learn more: • Maine Coastal Landslides • Maine Inland Landslides Points and Extents

The Lidar Revolution Lidar stands for Light Detection and Ranging, and it is essentially a laser scan of the earth’s surface from an airplane. The collected data can be processed to remove trees and vegetation, resulting in bare earth . The State of Maine began acquiring lidar data in the early 2000s and the entire state should be covered by 2020. Lidar data has many uses, but it has certainly revolutionized geologic mapping in areas with heavy forest cover such as Maine, allowing scientists to view the landscape in fantastic detail (see Thompson (2011) for more details). Topographic hillshade imagery generated from lidar data recently revealed hundreds of previously unknown landslides that were obscured by vegetation and was used to delineate these new features and update Maine’s landslide database (Fig. 2). Scientists can identify landslides from lidar imagery based on their unique topography (covered in Section 3), which makes them stand out from Maine’s glacially sculpted and smoothed landscape.

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Figure 2. The Swan Pond Brook Valley in Biddeford and Arundel, Maine. Top: Traditional reconnaissance information such as air photos and topographic contours suggest a typical stream valley. Bottom: Lidar hillshade imagery of the same area shows that the valley contains many landslides (outlined in red) that were hidden by tree cover, and now stand out due to their “rumpled” topography. Maps: MGS. Due to a lack of prior knowledge, it was assumed that these newly uncovered landslides must be prehistoric (before 1600s). In 2016, the Maine Geological Survey (MGS) began working on a project in cooperation with the Maine Emergency Management Agency (MEMA) to determine the ages of a subset of these landslides and in turn, what may have caused these events. Prior to this study, the only prehistoric landslide examined in Maine was the Bramhall landslide, which occurred around 13,500 years ago in the area now known as Portland’s Western Promenade (Thompson and others, 2011). This landslide was recognized by scientists in the 1800s because they likened the topography of the area to the modern landslides of their time, such as the events along the Presumpscot and Stroudwater Rivers (Morse, 1869). Early topographic maps of Portland Harbor also depicted the characteristic hummocky ridges of a landslide (Fig. 3), and Hitchcock (1874) mentions sticks and leaves buried 14 feet (4 meters) deep in the area found during railroad construction. Urban development has altered most of the landslide feature, but that development also uncovered trees and other vegetation that were buried by the landslide and then used for radiocarbon dating of the event (Fig. 3).

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Figure 3. The Bramhall Landslide, Western Promenade in Portland, Maine. Lidar hillshade imagery (left map) shows that urban development has mostly obscured the “hummocky” landslide ridges that were visible on an 1862 map (center map; U.S. Coast Survey, 1862). In 2007, a large construction project uncovered trees and other vegetation (right photo) that had been buried and killed by the landslide about 13,500 years ago. Lidar map and photo: MGS. Twenty-eight inland landslide sites were chosen for the MGS/MEMA study from the new lidar-delineated inventory. All these sites are in southern Maine, where inland landslide and population densities are highest (Fig. 4). The age of each landslide was determined by finding vegetation that had been caught up in or buried and killed by the landslide (Fig. 3 and 5). The age of the vegetation when it died was determined by radiocarbon analysis, which then provided an estimate of when the landslide occurred. (See Spigel (2019) for a detailed description of this process.) Due to the older age of the Bramhall landslide, it was originally hypothesized that most of the newly uncovered landslides were similar in age and had occurred soon after the end of the last Ice Age when the landscape was likely unstable. The new study found that landslides have been occurring in southern Maine throughout the post-glacial epoch known as the Holocene, and many in the study were only hundreds of years old. One site was determined to be the previously mentioned 1849 landslide – its exact location within a series of landslides was previously unknown. Clusters of multiple landslides around 4,000, 3,300, and 600 years ago could be related to a significant event such as a large or climate shift. The 600-year cluster could be related to a known climate shift from the warmer, drier Medieval Climate Anomaly (spanning ~1,000-700 years ago) to the cooler, wetter Little Ice Age (spanning ~420-250 years ago). Since modern and historical landslides are usually related to wet conditions, it follows that prehistoric landslides may have been caused by similar factors. However, more research needs to be done to determine if prehistoric landslide clusters are related to climate or earthquakes. To view the locations and ages of prehistoric landslides, see the Maine Inland Landslide database.

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Map Date: January 2019

Figure 4. Map of Maine inland landslide locations (as of January 2019, delineated from lidar, air photos, and historical accounts) in relation to population density. Most landslides have occurred in the most populated area of the state, which is related to the local geology (Presumpscot Formation) described in Section 2. The Presumpscot Formation may occur at low elevations (such as valley bottoms) south of the marine limit (dark blue line). For coastal landslide locations, see the Coastal Landslides database. Map: MGS.

Figure 5. Auger sample of a soil that was buried (about 6 ft/1.8 m deep) by a prehistoric landslide in Arundel, Maine. The dark brown area was once the and had many -preserved plant fragments that were used for radiocarbon dating of the event. Photo: MGS.

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What causes mass wasting? There are many natural and human factors that may cause mass wasting. Sometimes the factors are singular and clear, such as a large earthquake or a deluge rainstorm that suddenly adds water weight to a slope. But mass wasting events can also be the result of a combination of factors that come together at the right place and time, making it difficult to pinpoint a single cause. Therefore, it is important to consider many aspects of a location and how they might change over time when determining mass wasting susceptibility.

Undermining a slope When the toe or base of a slope is removed or undermined and left unsupported, mass wasting may occur. Humans may undermine slopes when developing the landscape, especially along roads where slope cuts may be needed to straighten or flatten a roadway (Fig. 6). In Maine, natural processes such as stream bank and coastal bluff erosion may undermine slopes and make them susceptible to mass wasting (Fig. 7). Sudden water level drops after flooding or due to human manipulation (such as reservoir drawdowns) can also leave river banks and coastal bluffs unsupported. When water-saturated banks that were once draining to a higher water table are exposed (and often freshly eroded due to high flows), there is a lot of water pressure (hydraulic head) in the bank until water can drain to the new lower level. Thus, slumping and sliding is common soon after flood waters have receded. Reservoir drawdown during the Fort Halifax dam removal in Winslow was blamed for slumping and sliding along the in 2008 and 2010.

Figure 6. A major road cut on U.S. Route 1 at the Penobscot Narrows Bridge in Prospect, Maine. Fractures in the rock make it susceptible to mass wasting so bolts, cables, and mesh are required to stabilize the area and prevent rockfalls. Photo: MGS (taken from observatory tower).

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Figure 7. Left: A cut bank along Sandy Stream in Unity, Maine just after a spring flood event. High flow has eroded and steepened the bank, reducing its stability. Right top: An eroding coastal bluff in Brunswick, Maine where slumping has occurred in the past. Right bottom: The coastal bluff erosion cycle. All images: MGS. Adding weight Humans can physically add weight to slopes by building structures, roads, etc., but water is the most common way in which weight is added to slopes in Maine. During rain and snowmelt events water may flow over the land surface (runoff) into streams, lakes, and the ocean, or it may seep into the ground, moving slowly through the surficial materials and adding weight. Water weighs about 8.3 pounds per gallon or about 1 kilogram per liter. According to the USGS (1988), a storm in which one inch of rain falls on one acre of land translates to about 27,154 gallons (102,789 liters) of water and about 226,600 pounds (102,784 kilograms or 113 tons). One inch of snow per acre translates to about 2,715 gallons (10,279 liters) and about 22,660 pounds (10,278 kilograms). Humans can add water weight through irrigation, drainage pipes, and septic system leach fields. If the weight added to a slope exceeds its resistance to gravity, mass wasting can occur.

Reducing shear strength Shear strength is the ability of a material to resist movement by gravity. Factors such as the internal and of an unconsolidated earth material will influence its shear strength. Internal friction is related to particle shape – for example, angular, jagged grains can lock together to maintain a steeper slope than smooth, round sand grains. and boulder rip-rap that is placed on slopes for protection is often very angular for this reason. Cohesion exists when chemical bonds or surface water tension help to hold sediments together. Adding water to

8 Maine Landslide Guide earth materials not only adds weight, it also affects shear strength. For example, if you add some water to sand at the beach, this will increase its cohesion via water surface tension, allowing you to mold it into a sandcastle. If you keep adding water to the sand, eventually it becomes saturated, increasing pressure in the pores (spaces between grains) and eliminating contact between sand grains, reducing internal friction so the castle collapses. Thus, water can act like a glue, holding particles together, but large amounts of water can act like a lubricant (Fig. 8).

Figure 8. Illustration of dry sand grains (left) and saturated with water (right). The dry grains have contact with each other, creating friction and stability. Graphic: MGS. Geology Maine’s landscape is generally considered to be rocky and rugged. Bedrock (a.k.a. ledge) can certainly be moved by mass wasting processes, but most mass wasting events occur in the more easily disturbed unconsolidated surficial materials that overlie bedrock. Maine’s surficial materials that comprise various surficial geology units were deposited by glacial processes during the last Ice Age, which have since been modified by weathering, fluvial erosion and transport, and mass wasting. General descriptions of surficial geology units that are frequently associated with mass wasting are listed below. For more detailed descriptions of Maine’s surficial geology, see Thompson (2015). It is important to note that characteristics of the units described here vary such that mass wasting susceptibility within these units also varies. For example, areas mapped as the Presumpscot Formation could be anything from 20 feet of hard weathered clayey to a thin weathered crust over 100 feet of plastic clayey silt. This is certainly justification for site-specific geotechnical investigations when working in areas with generally susceptible sediments.

Glacial till Glacial till is a mixture of sediments, from clay to boulders, that was deposited by during the last Ice Age and is probably the most common surficial material in Maine. The wide variety of particle sizes leads to greater internal friction and higher shear strength than in other glacial deposits. Till on the up-ice side of hills (generally north to northwest facing) is often lodgement till or “hardpan,” which was plastered on the hillside and compacted by glacial ice, making these areas less susceptible to mass wasting. But till can also be loose and vary in sediment composition, so while it is generally more stable than other surficial materials, it is not immune to mass wasting (Fig. 9).

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Figure 9. Lidar hillshade image of landslides comprised of glacial till (circled in red) on the southeast or down-ice side of a hill in Wilton, Maine. Till on down-ice slopes tends to be less compact, making it more susceptible to mass wasting. Map: MGS. Glaciolacustrine deposits During the end of the last Ice Age, the retreating ice sheet or glacial deposits such as till often blocked valleys creating short-lived lakes, especially in western Maine (Fig. 10). Fine sediments, such as clay, silt, and sand settled out in the bottom of these lakes, similar to the muck we find on the bottom of modern lakes. After the lakes drained, modern streams occupied these valleys, eroding and exposing the former lake (glaciolacustrine) sediments. These finer sediments can be more susceptible to mass wasting due to lower internal friction and higher water retention.

Figure 10. Lidar hillshade image of landslides in the East Branch Valley in Byron, Maine (circled in red). At the end of the last Ice Age, this valley was likely dammed by the retreating ice sheet creating a pro-glacial lake that filled with sandy glaciolacustrine deposits, which are more susceptible to mass wasting. Map:MGS.

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Glaciomarine mud (The Presumpscot Formation) The Presumpscot Formation is a glaciomarine mud comprised of silt, clay, and sand that was deposited at the bottom of ocean waters that covered parts of southern Maine at the end of the last Ice Age (Fig. 11). The weight of the ice sheet had depressed the earth’s surface, which allowed the ocean to inundate areas as the retreated. Many different marine deposits accumulated and were eventually lifted above sea level as the earth’s surface rebounded. (For more information and illustrations of this process, see Thompson (2015) and Loiselle (2003).) Informally referred to as “blue clay,” the Presumpscot Formation is typically found at lower elevations, such as valley bottoms, in areas south of the late-glacial ocean boundary known as the “marine limit.” While Presumpscot Formation characteristics vary, it is generally defined as a “sensitive clay” meaning that it can deform and flow when disturbed or left unsupported leading to unstable . Most landslides in Maine are related to the Presumpscot Formation, making it a very significant part of Maine’s geology (Fig. 4).

Figure 11. A Presumpscot Formation exposure in Gardiner, Maine. When exposed to water and air, Presumpscot Formation can weather into a hard, tan crust (top), but at depth it can be blue-gray and stiff to very soft (bottom). Photo: MGS. Mud that flows may seem counterintuitive – usually mud is sticky and can be molded, but the Presumpscot Formation is different due to its formation process. The small particles that comprise Presumpscot Formation are known as “glacial flour” and are the product of glacial scouring and erosion. Meltwater streams carried glacial flour to the ocean where positively charged salts in the water attracted the negatively charged flour particles in a process called flocculation. Tiny clumps or “flocs” of flour settled to the ocean floor to become Presumpscot Formation in a “house of cards” structure (Fig. 12). After the mud was lifted above sea level by rebound, fresh groundwater slowly replaced the salty water that held the particles together, weakening the “house of cards” structure. If the mud is disturbed by shaking or other movement, it can liquefy, flow, and collapse into a more stable structure (Fig. 12).

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Figure 12. Left: Platy silt and clay with some sand in a “house of cards” structure (flocculated). Note that particle sizes are greatly exaggerated here – silt and clay particles are 0.06 mm or less in diameter. Right: More stable structure after “house of cards” collapse. Graphic: MGS. Another factor related to geology is orientation of features such as fractures and layers within units in relation to slope orientation. Vertical fractures allow water in, which can slowly break materials apart with freeze-thaw action in addition to the many impacts of water already mentioned. If fractures, layering within units, or boundaries between units are oriented in the same direction as a slope, these create natural failure planes on which the materials can move and may increase mass wasting susceptibility (Fig. 13).

Figure 13. Illustration of different geologic layers in a hillside (side view). Left: The layers are parallel to the hillslope so boundaries between the layers can act as a natural sliding plane, increasing mass wasting susceptibility. Right: Layers are almost perpendicular to the hillslope, reducing the likelihood of movement. Graphic: MGS. Earthquakes Seismic shaking loosens materials on a slope, but it can also cause liquefaction of sensitive clays like the Presumpscot Formation, leading to landslides. Liquefaction occurs when water- saturated sediments experience a sudden increase in water pressure, which leads to a sudden loss in shear strength (see shear strength explanation). Water is often expelled from the sediments, causing them to compact, leading to landslides or sunken areas. Maine does experience earthquakes but they are generally low magnitude (≤ 4), mostly resulting in rockfalls (Fig. 14). Earthquake research by the USGS indicates that stronger events are possible in Maine – they may happen so infrequently that humans have yet to document one and its impact (Wheeler, 2016). As mentioned in Section 1, Maine’s prehistoric landslides may indicate stronger earthquake events in the past.

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Figure 14. An earthquake-induced road cut rockfall in Acadia National Park (2006). The large number of fractures in the rock outcrop pictured here made it more susceptible to mass wasting. Photo: National Park Service. Extreme storm events Extreme storm events, such as nor’easters, hurricanes, or intense thunderstorms, during which large amounts of water are rapidly added to the landscape can trigger landslides. Water can affect a slope in many ways, but an intense storm may also undermine slopes with wave action or high velocity stream flow, adding to the landslide risk (Fig. 15). For example, the 2007 “Patriot’s Day Storm” was a nor’easter that resulted in flooding, erosion, and mass wasting (Foley, 2007).

Figure 15. Small landslide in Brunswick, Maine related to the 2007 “Patriot's Day Storm.” Photo: MGS.

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Mass Wasting Types and Maine Examples There are many types of mass wasting and definitions of their characteristics vary worldwide. This section describes the most common types of mass wasting in Maine and are generally aligned with the definitions set by the U.S. Geological Survey in Highland and Bobrowsky (2008).

Creep Creep is the gradual downslope movement (< 1 inch or 2 cm per year) of soil and other surficial materials due to freeze-thaw action and is common on slopes statewide (Fig. 16). Creep does not pose a direct risk to human life, but it can impact infrastructure over time by tilting fences and other structures that were not properly driven below the frost line. In some cases, creep may indicate an unstable slope prone to other types of mass wasting, but this is not always a reliable indicator. Creep may be identified on a slope by curved tree trunks, tilted fences, utility poles, and retention walls, cracks in pavement, or soil ripples (Fig. 17).

Figure 16. The effect of soil creep on sediments in a slope. When water in the pores between sediments freezes, the slope expands outward slightly (1→2). When the ice thaws, gravity pulls the sediments straight down, moving them slightly downslope (3→4). Graphic: MGS.

Figure 17. Left: A more extreme example of slope expansion or heaving due to ice (on Big Moose Mountain near Greenville, Maine). Right: Curved tree trunk due to creep in Baxter State Park, Maine. Photos: K. Spigel.

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Rockfall A rockfall is the sudden and rapid downslope movement of rocks. The rocks may bounce and break into smaller pieces as they move and mobilize other materials. Falls tend to continue until they reach an obstruction or flatter topography (Fig. 18). Freeze-thaw action tends to slowly loosen rock blocks from slopes along pre-existing fractures until they fall, but earthquakes may also trigger rockfalls (Fig. 14). Rockfalls may occur statewide in areas with steep slopes and exposed bedrock (natural or human made) but are most likely in the mountainous western and central regions of the state (Oxford, Franklin, Somerset, and Aroostook counties; Fig. 19).

Figure 18. Side view diagram of a rockfall. Left: Fractures in the rock (black lines) make it more susceptible to mass wasting. Right: Freeze-thaw action or shaking knocks rock blocks loose, which fall and break into smaller pieces. Graphic: MGS.

Figure 19. Photo of a suspected rockfall on the southeast side of Bald Mountain in Newry, Maine (circled in red). Rocks tumbled from the steep bare slope, destroying trees and mobilizing materials below. Photo: MGS.

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Landslides A landslide is the downslope movement of earth materials (due to gravity) along a surface of rupture (shear plane). Landslides may start with slow movement (inches or centimeters to feet or meters per day) that ends in very rapid movement (feet or meters per second), or they may happen very rapidly without warning. There are many different types of landslides, and sometimes an individual landslide can have the characteristics of multiple types. When assessing a landslide, it is best to categorize it as the type it most resembles since a perfect match is unlikely. Most landslides have a few characteristics in common, which are outlined below and in Fig. 20:

• Scarp: The often-steep slope that marks the upper extent of a landslide. • Slide blocks: Earth materials that move as a single unit during the landslide. • Crater: Depression created by the landslide. • Shear plane/rupture surface: The surface on which a landslide moves; the boundary between the crater and underlying undisturbed materials. • Toe: The lower extent of a landslide, often has little structure. When landslides occur in coastal bluffs or river corridors, the toe is often reworked or removed by wave action or stream flow.

The most common types of landslides in Maine are described in the following pages.

Figure 20. Side view of a landslide with common features. Graphic: MGS.

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Rotational Slide/Slump A rotational landslide is the down and outward movement of earth materials along a curved plane (Fig. 21). Small rotational landslides (< 0.25 acre/0.1 hectare) are sometimes called “slumps.” This type of landslide is common along river channels and coastal bluffs and may be triggered by undermining the base of a slope, adding weight to a slope, wet conditions, an earthquake, or a combination of these factors (Fig. 22).

Figure 21. Side view of a slope before (left) and after (right) a rotational landslide. Graphic: MGS.

Figure 22. Rotational landslide examples. Left: Side view of a 2006 rotational landslide along U.S. Route 2 and the in Greenbush, Maine. The curved rupture plane is very clear, and the rotational movement has pitched trees on the slide block back towards the scarp. Photo: Maine DOT. Right: Slide blocks from a 2005 rotational landslide along the in , Maine. Most of the trees have been pitched back towards the scarp (left, out of view) by the rotational movement. Photo: MGS.

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Translational A translational landslide is the downslope movement of earth materials along a plane with little to no rotational movement (Fig. 23). This type of landslide is more common in areas where thin surficial deposits overlie bedrock (such as in mountainous areas) and may be triggered by undermining the base of a slope, adding weight to a slope, wet conditions, an earthquake, or a combination of these factors (Fig. 24).

Figure 23. Side view of a slope before (left) and after (right) a translational landslide. In this example, the sliding has occurred along the boundary between two geologic units (yellow and gray layers). Graphic: MGS.

Figure 24. A translational landslide on Mount Hittie in Grafton, Maine (circled in red) that occurred in 1990. The slide may have started as a rock fall (narrow top area) that mobilized materials below. A thin layer of glacial till then slid over bedrock, leaving “stripes” down the slope known as slickensides. Photo: MGS.

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Flow A flow is the downslope movement of water-saturated earth materials. There is little structure to a flow, with materials often moving as a slurry (Fig. 25 and 26). This type of landslide requires wet conditions but may occur in combination with undermining the base of a slope, adding weight to a slope, or an earthquake. Flows are often confused with gullies and vice versa. In a gully, sediments are picked up and carried downslope by flowing water, not by gravity alone. Gullies often originate in areas of concentrated surface runoff, such as a culvert or drain pipe outlet, and their craters are more v-shaped than bowl-shaped (Fig. 27). It is important to recognize the difference, as flows tend to be one event while gullies can remain active, resulting in long-term erosion problems.

Figure 25. Side view of a slope before (left) and after (right) a flow slide. Graphic: MGS.

Figure 26. Left: Lidar hillshade image of a prehistoric flow landslide (circled in red) along the Stroudwater River in Westbrook, Maine. Flow landslide craters tend to be relatively flat with few (if any) slide block ridges. The toe of this landslide emptied into the Stroudwater River and has long since been reworked by the river. Map: MGS. Right: View of a flow in Chelsea, Maine (circa 1977), looking towards the scarp. Photo: MGS.

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Figure 27. Photo of a gully head in Hollis, Maine. Photo: MGS. Spread Spread landslides occur when a stronger earth material layer breaks apart and moves along and/or sinks into a weaker/softer underlying layer (Fig. 28 and 29). This type of landslide requires unstable earth materials at depth (such as the Presumpscot Formation) and may be triggered by undermining the base of a slope, adding weight to a slope, wet conditions, an earthquake, or a combination of these factors.

Figure 28. View of a slope before (left) and after (right) a spread landslide. Graphic: MGS.

Figure 29. Left: Lidar hillshade image of a spread (circled in red) along the in Standish, Maine. The slide blocks could almost be put back together to create the original slope. Map: MGS. Right: Side view of spread slide blocks in an unusual landslide in Norridgewock, Maine. A resting on Presumpscot Formation mud failed in 1989 due to nearby construction, resulting in a spread landslide comprised of garbage. Photo: MGS.

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Retrogressive Landslides Any landslide may be “retrogressive” if it begins with movement at the base of a slope and then proceeds back into the slope, instead of the entire slope moving all at once. For example, larger landslides often begin with the movement of one slide block at the base of a slope. If the slide block remains intact and does not travel far, it can buttress the upslope area and halt further movement. If the slide block disintegrates and/or travels far from the slope, the freshly exposed area is left unsupported and movement can continue in a positive feedback known as “retrogression.” Retrogression continues until the landslide materials are blocked or the material source is exhausted. Indeed, the sizes of landslides in Maine may be more related to the occurrence of retrogression than the magnitude or severity of the causative factors. The 1996 Rockland Harbor landslide is a good modern example of a retrogressive landslide (Fig. 30).

Figure 30. Oblique aerial image of the 1996 Rockland Harbor landslide in Rockland, Maine, modified from Berry and others (1996). The first slide block fell and disintegrated to become the toe, leaving the slope unsupported and allowing retrogressive movement until the slide blocks buttressed the slope.

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Summary What types of mass wasting occur in Maine? • Soil creep is the most common type of mass wasting in Maine but also the least destructive. • Rockfalls may occur in steep areas with exposed bedrock. • Many types of landslides are possible in Maine including flows, spreads, translational slides, and rotational slumps/slides which are most common in recent times.

Where are landslides most likely to occur in Maine? • River corridors (especially cut banks) and coastal bluffs with active erosion (i.e. unstable banks with exposed surficial materials). However, areas that are not actively eroding can still fail or activate quickly after significant events like intense storms or sudden snowpack melt. • Areas with unconsolidated surficial materials – especially the Presumpscot Formation (Fig. 31). • Landslide hazard and susceptibility maps exist for some areas of Maine and can be accessed through the Maine Geological Survey website.

Figure 31. Presumpscot Formation in landslides. Left: In the toe (gray area) of a 2005 landslide along the Merriland River in Wells, Maine. Right: Soft Presumpscot Formation removed from a 2006 landslide site along the Penobscot River in Greenbush, Maine. Photos: MGS.

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When are landslides most likely to occur in Maine? • Spring and early summer due to snowpack melt and/or spring rains. However, prolonged wet conditions may lead to landslides at any time of the year. • After a significant water drop that leaves bluffs and river banks unsupported, especially if freshly eroded and/or waterlogged (Fig. 32).

Figure 32. A bank of the in Kennebunk, Maine. Water flow regulation at a downstream dam has resulted in a rapid water level drop (top to bottom of light area marked with red arrow). Water under pressure in the bank attempts to drain to the new lower elevation, creating springs and areas of weakness in the bank (circled in red). Photo: MGS. What can be done to help prevent landslides and most importantly, lower the risk to human life and property? • Development in coastal bluff and river corridor areas should be avoided. A river corridor landslide poses an even higher risk if the toe blocks the river channel, leading to flooding upstream of the site and flash flooding downstream once the area is breached. See Brooks and others (1994) for photos of this process after a glaciomarine mud landslide in eastern . Flooding and other storm damage unrelated to landslides is also likely in bluff and floodplain areas, providing an added incentive to develop elsewhere. • If development must take place in susceptible areas, underlying geological materials and their engineering characteristics should be investigated, especially if the Presumpscot Formation is present, and necessary adjustments or accommodations planned (Fig. 33). For example, areas may need to be drained and compressed prior to building, or footings may need to be placed below the Presumpscot Formation in a more stable geologic unit.

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Figure 33. Soft Presumpscot Formation being removed from a construction site in Portland, Maine. Photo: MGS. • Manage water on slopes. Pay attention to drainage outlets, leaking infrastructure, septic systems, and irrigation that can add water weight, reduce shear strength, and create eroded/weak spots (Fig. 34).

Figure 34. Slope drain near the 1996 Rockland Harbor landslide. The outlet is protected from erosion with rip-rap and vegetation. Photo: MGS. • Manage vegetation on slopes.* Healthy plant roots can help stabilize a slope and take up water. However, plants that are dead, unhealthy, or have shallow root systems may just be adding weight to the slope without benefit. The Cumberland County Soil and Water Conservation District (2017) has a slope stabilization planting guide aimed at coastal areas, but many ideas and plant suggestions could also be used for inland slopes.

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• Avoid undermining slopes.* If slope cuts are required, protect the area appropriately (vegetation, retaining structures, rip-rap, etc.). • Adjust slopes.* In highly susceptible areas, sometimes risk can be lowered by and reducing slope angles. • Monitor slopes. Slopes should be monitored for signs of movement such as cracks at the top of the slope, creep, and unusual settling or movement in nearby roads and buildings (Fig. 35).

Figure 35. Left: Cracks in the top of a slope in Brunswick, Maine indicate movement. Center: Cracks in the of the house in left photo indicate movement. Right: Cracks in the pavement indicate slope movement in Greenbush, Maine. Photos: MGS.

* NOTE: Placement of rip-rap and barriers and removing/displacing soil and/or vegetation in areas adjacent to coastal and inland waters or wetlands may be subject to permitting by the Maine Department of Environmental Protection (DEP) under the Natural Resources Protection Act (NRPA). Contact the DEP and your town office prior to altering slopes and vegetation to avoid fines. See Slovinsky (2011) for more tips on slope management in coastal areas.

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References Amos, J., and Sandford, T.C., 1987, Landslides in the Presumpscot Formation, Southern Maine: Maine Geological Survey Open-File Report 87-4, 68 p. https://digitalmaine.com/mgs_publications/179/ Berry, H.N., IV, Dickson, S.M., Kelley, J.T., Locke, D.B., Marvinney, R.G., Thompson, W.B., Weddle, T.K., Reynolds, R.T., and Belknap, D.F., 1996, The April 1996 Rockland landslide: Maine Geological Survey Open-File Report 96-18, 55 p. https://digitalmaine.com/mgs_publications/213/ Bradbury, C., 1837, History of Kennebunkport from its First Discovery by Bartholomew Gosnold, Kennebunk, Maine: Kennebunk, Maine, J.K. Remich, 301 p. Brooks, G.R., Aylsworth, J.M., Evans, S.G., and Lawrence, D.E., 1994, The Lemieux landslide of June 20, 1993, South Nation Valley, southeastern Ontario – a photographic record, Geological Survey of Canada Miscellaneous Report 56, 20 p. https://doi.org/10.4095/193534 Cumberland County Soil and Water Conservation District, 2017, Coastal Planting Guide, https://cumberlandswcd.org/site/wp-content/uploads/2018/02/Attachment-E1-Maine-Coastal- Planting-Guide-November-2017-For-Electronic-View-Release-Version-1.1.pdf (accessed March 2, 2020). Bourne, E.E., 1875, The History of Wells and Kennebunk: Portland, Maine, B. Thurston and Co., 797 p. Foley, M.E., 2007, Brunswick, Maine Patriots' Day 2007 Landslide: Maine Geological Survey, Circular GFL-126, 17 p. http://digitalmaine.com/mgs_publications/417 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 Hitchcock, E., 1836, Sketch of the geology of Portland [Me.] and its vicinity: Boston Journal of Natural History, v. 1, p. 306-347. Hitchcock, C.H., 1874, The geology of Portland: American Association for the Advancement of Science Proceedings, v. 22, p. 163-175. Loiselle, M., 2003, Simplified surficial geologic map of Maine: Maine Geological Survey, map, scale 1:2,000,000. http://digitalmaine.com/mgs_maps/1968 Maine DEP, 2000, A Homeowner’s Guide to Environmental Laws Affecting Shorefront Property in Maine’s Organized Towns, Augusta, ME: Department of Environmental Protection, 44 p. Morse, E.S., 1869, On the landslides in the vicinity of Portland, Maine: Boston Society of Natural History Proceedings, v. 12, p. 235-244.

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Novak, I.D., 1987, Geology of the September 1983 landslide at Gorham, Maine, in Andrews, D. W., Thompson, W. B., Sandford, T. C., and Novak, I. D., eds., Geologic and geotechnical characteristics of the Presumpscot Formation, Maine's glaciomarine 'clay': unpublished proceedings of a symposium sponsored by the Maine Geological Survey, Morrison , University of Maine, and University of Southern Maine, March 20, 1987, variously paginated, 14 p. Remich, D., 1911, History of Kennebunk from its Earliest Settlement to 1890, Portland, Maine: Lakeside Press Co., 542 p. Rowe, M.B., ed., 1952, Highlights of Westbrook History: Westbrook, Maine, Westbrook Women’s Club, 237 p. 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/ Spigel, L.J., 2019, Determining the Ages of Maine’s Prehistoric Landslides: Maine Geological Survey, Circular GFL-245, 13 p. https://digitalmaine.com/mgs_publications/587 Thompson, W.B., 2011, Lidar Imagery Reveals Maine's Land Surface in Unprecedented Detail: Maine Geological Survey, Geologic Facts and Localities, Circular GFL-175, 13 p. http://digitalmaine.com/mgs_publications/465 Thompson, W.B., Griggs, C.B., Miller, N.G., Nelson, R.E., Weddle, T.K., and Kilian, T.M., 2011, Associated terrestrial and marine fossils in the late-glacial Presumpscot Formation, southern Maine, USA, and the marine reservoir effect on radiocarbon ages: Quaternary Research, v. 75, p. 552-565. U.S. Coast Survey, 1862, Portland Harbor, map, scale 1:20,000. https://historicalcharts.noaa.gov/historicals/preview/image/325-00-1862 USGS, 1988, Rain, a water resource, USGS Pamphlet. https://www.usgs.gov/special- topic/water-science-school/science/rain-and-precipitation?qt-science_center_objects=0#qt- science_center_objects

Wheeler, R.L., 2016, Maximum magnitude (Mmax) in the central and eastern for the 2014 U. S. Geological Survey Hazard Model: Bulletin of the Seismological Society of America, v. 106 p. 2154-2167.

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