The April 1996 Rockland Landslide
|
Plate 1
Panoramas of the 1973 and 1996 Landslides, Rockland, Maine
Catastrophic landslides occurred in adjacent coves of Rockland Harbor, Maine, in 1973 and in 1996. Despite the long time between the landslides, their marked similarity indicates that such catastrophic landslides are recurring events in this region, and not isolated ones. In both cases the landslides occurred in embayments on the north shore of Rockland Harbor, which has a scalloped shoreline (location map). Bedrock outcrops are found only on the points of land between embayments. The shoreline has steep bluffs 30 to 40 feet high of Pleistocene glacial-marine clay. The combination of a steep slope and a high bluff seems to have been sufficient to cause the clay to fail under its own weight. Continual small slumps on the bluff face and gradual marine erosion at the base of the bluff may have set the stage for the catastrophic failures.
The panoramic views illustrate the immense scale of the landslides. The 1996 event destroyed two houses and came within a few feet of Samoset Road. The details indicated on the photos reveal how the events happened and demonstrate how closely the two landslides compare. In this type of landslide, known as a retrogressive slump, the main movement involves the weak clay at depth flowing out away from the bluff onto the mud flat. Under such circumstances the weak, saturated clay flows easily, like a fluid. This fluidized material with small chunks of broken clay can be seen in the outer lobe, or foot, of the landslide. As the lower part of the mass moves, the upper part breaks into large blocks that move down and out with the flow. These blocks can be seen in the middle area of the slides. Many of the blocks have moved hundreds of feet, with trees, shrubs, and grass intact. Blocks near the bluff face move first, followed in succession by blocks farther and farther inland. In this process, the individual blocks are commonly tipped backward toward the head of the slide. The 1973 slide shows examples of such rotated blocks. The sliding blocks may also push the fluidized clay ahead of them into ridges. By the time the movement reaches the head of the landslide, the pile of blocks in front inhibits the lateral movement, and the motion is more of a vertical dropping of blocks into a jumble of debris. Notice that the turf and other vegetation does not prevent this type of catastrophic landslide from occurring.
Panorama of the January 25, 1973 landslide
(Photography by Arthur M. Hussey, II)
|
Photo 1 |
Annotated Photo 1 |
| |
|
| |
|
Panorama of the April 16, 1996 landslide
(Photography by John B. Poisson)
|
|
|
Contents
Introduction
Description
Other slides
Factors
Potential
Action
Summary
References
Appendix A
Appendix B
Appendix C
Plate 1
Last updated on October 6, 2005
