Slump Blocks and Slope Failure on the Navesink Highlands:
A map of the slump block zone has two sheets:
A graphic illustration of slump block stages is here: Slump Block Stages.
Geological studies, as well as experience in construction, in the courtroom and in the councils of local government, testify to the unique fragility of the slopes of the Navesink Highlands and the risks of tampering with them. Documentation of the risk and its occurrences goes as far back as two centuries and as recently as 1999.
A key problem of slope fragility that is unique to the area has been so clearly established that it has a specific geological name: slump blocking. The main study of the phenomenon was done in 1974 by James P. Minard for the U.S. Geological Survey (USGS) and is the source of geological information in this text. Slump blocking is a fundamental issue for the high elevation area of Atlantic Highlands, Highlands and the Navesink section of
At the outset, it is important to be clear about what slump blocking is not. It is not erosion -- the wearing away of the earth's surface by natural processes such as water flows. It is not a landslide -- the sliding of a mass of loosened rocks or earth down along the surface of a hillside or slope' In erosion and landslides, materials at higher elevations become. loosened from their setting and tumble or slide or wash down to lower elevations.
In contrast, slump blocking is not mere surface movement. It means that an entire block of land slips downward' The soil in the displaced block remains intact, in one piece, undisturbed. The top of the block ends up being lower than its headlands; the toe of the block now rests at a lower point of the slope, all soil in the block between top and toe has moved down with them; and all parts of the block stay part of the block, in the same formation and sequence of layers as before. There is also rotational movement of the block which tilts its upper surface inward. Slump blocks can be quite large. some in this area measure up to 590 feet wide and 2,950 long and drop vertically as much as 85 feet. A 1986 report on slump blocking at Mt Mitchill compares the phenomenon to the simple image of "a slab of gelatin slipping halfway off a tilted plate", (Asbury Park Press, January 12, 1986).
Thus, while the issues of geology and slump blocking relate to generalized concerns about protecting steep slopes, they are more fundamental and far-reaching than ordinary slope instability. The attached graphic exhibit shows geological and historical stages of actual slump block movement and secondary slumping along
Minard's paper on slump blocking described the geological composition and structure (physiography) of the Navesink Highlands slopes as unlike that of any other coastal area in
For about 4 miles, the Navesink Highlands ridge is punctuated by 14 slump block lines which Minard mapped. Included are seven locations where Minard says slumping has definitely occurred, and seven where it "probably has occurred." It has taken place mainly at elevations between 100 and 200 feet above mean sea level along the steeper northern side of the ridge. Because this side faces the bay and prized water views, it has been far more disturbed by construction than the inland-facing gentler southern slopes, which in
Beyond mapping these zones, Minard's USGS report recommended actions to avoid triggering slump blocking. The report calls for "careful thought, planning, investigations, tests, and analyses" before construction in this entire "area of geological hazards." Precautions should include "avoidance of the removal of material from the toes of possibly critical slopes, prevention of excessive water infiltration in the ground in critical areas, and avoidance. of excessive loading on upper surfaces in these areas" (Minard 1974, page 23).
The case of
The earliest recorded slump block event in the Navesink Highlands was in April l782. A newspaper report about it in a
On the ridge of the mountains, commonly called Navisink hills, in Monmouth County, East Jersey, a considerable quantity of land, some say 40 acres, gave way...and sunk directly down a considerable depth; forming a cavity equal in circumference, at bottom, to the void space above. The tops of the trees, that sunk with the soil, and which were mostly of considerable bulk are now nearly level with the edges of the remaining ground. Round this again the earth opens, in one continuous fissure, a foot or more in breadth, for a
considerable distance; and as is conjectured, from its present appearance, will shortly go down also....
A later report noted that "the noise was heard for a distance of several miles" and that the location, known then as Greenland Bank, was "the highest point of the highlands," which was named Mt.
The case of
Far-reaching measures for protection of
* First was the creation of a
Any vibratory energy such as pile-driving or use of heavy earth-moving equipment should be a mater of concern. Developers and local authorities should be guided by the recommendations in the Minard study. The possibility of a slump is ever-present and erosion left unchecked can become a monster.
The faults that occurred and the further risk required a major change in the County route known as
* Second, efforts began in 1986 to protect a l3-acre triangle of beach, dune, wetland and cliff along the bayshore coast directly below
peninsula between the Ocean and Keyport.)
* Third, between the beachfront park at the foot of
The case of
Minard's 1974 USGS report mapped one slump block in the area below
In April 1976, Judge Andrew A. Salvest decided for the homeowners. He concluded that the pipe blocked natural water drainage on the lower slope and backed up water into soil blocks, which then slumped of their own increased weight. The Judge's decision said that "the damming effect" created by the pipeline was "a proximate cause" for the "sliding and damage to the property owners' buildings"
(Russell et al, Tosi et al, and Day et al. Dockets no. L-30624-72, L36710-72 and L41330-73, p.6).
The case of the "
In the late 1990s, a developer proposed a large construction project on 24 acres in
Following several years of lawsuits and State agency hearings aimed at stopping the project, the project was not approved by the Middletown Planning Board in Marsh 1999, based partly on a geologist's testimony about the risks of slump blocking at the site. The developer's plan would have taken two of the risks that Minard warned against. It would have cut away 1.35 acres of steep slope at the foot of
A second developer acquired the property in 1998 and by October 2000 reduced the project to 180 units and its acreage (including parking and a stormwater structure) to less than a third of the tract. The remaining two-thirds, including most of the steep slope area, was preserved as open space -- eight acres acquired by Atlantic Highlands as an extension of the Lenape Woods Nature Preserve, and eight acres acquired by Middletown to construct ballfields for shared use with Atlantic Highlands.
The case of
On January 3, 1999, residents around
Hopkin was later quoted in the Asbury Park Press as stating that "This is not an erosion issue. It was a slope failure....Anyone can see that that area has subsided." After geological and engineering studies, the remedy was to build a wall 300 feet long and 1l feet high and to re-grade the hillside to a more gradual slope. Highlands Borough bonded $500,000 for the cost of this construction.
From Paul D. Boyd
Historian, Atlantic Highlands Historical Society
Chair, Environmental Commission of Atlantic Highlands
(1) Quoted material in the above section is from Barber and Howe's Historical Collections of the State of
Borough of Atlantic Highlands | 100 First Avenue, Atlantic Highlands, NJ 07716 | Phone: 732-291-1444 Fax: 732-291-9725