Bedrock Geology

New Jersey contains a wide variety of physical features produced by complex geological processes that have been going on for millions of years.  These processes have produced a series of landforms that generally trend from the northeast to the southwest.  These landform features reflect the underlying rock type, geologic structure, and geological history and are termed physiographic regions.  In New Jersey we have the Ridge and Valley, Highland, Piedmont, and Inner and Outer Coastal Plains (Fig. 3).  Mountain Lakes is located on the boundary between the Highlands and Piedmont Regions and includes part of each.

Figure 3. New Jersey Landforms

The geologic history of this area begins in the Precambrian Era (Fig. 4, Geologic Time Scale) and continues through the Quaternary period, a time span of over 600 million years.  The oldest rocks in the region are found in the Highlands.  These rocks record a long history of repeated deposition of sediments followed by folding, faulting, and intrusion of molten rock.  These mountain-building processes resulted over time in slowly changing the igneous and sedimentary rocks into metamorphic ones.  The most common of these metamorphic rocks are several types of gneiss, distinguished by their alternate brands of light and dark-colored minerals.

Besides forming the Highlands, the Precambrian rocks also form the “basement” on which the younger sediments of the Piedmont were subsequently deposited.

Figure 4.  Geologic Time Scale

The Piedmont consists of Triassic-Jurassic age sedimentary shales, sandstones, and conglomerates interspersed by lava flows.  These rocks sit directly upon Precambrian rocks with those of Paleozic age (a time span of about 350 million years) missing.  Geologic history indicates that widespread earth movement occurred in the latter part of the Triassic, resulting in the elevation of the Highlands relative to the areas further east.  Subsequently, sediments from the Highlands washed down onto this lower region.  Sedimentation was periodically interrupted by volcanic activity when thick layers (350 to 750 feet) of lava flowed across the surface.  Evidence seems to indicate that when these fissure eruptions took place the sediments were nearly horizontal but were later tilted toward the west.  The most common sedimentary rock of the Piedmont is the Brunswick Shale.  In Mountain Lakes, however, it is mantled with a large amount of glacial drift.

The Ramapo Fault
The Ramapo Fault terminates the Highlands on the east and represents the boundary between this region and the Piedmont as can be seen in the sketch.  The Ramapo Fault is a fracture in the earth that runs about 80 miles from Peekskill, New York, southwest through Mahwah and Oakland in Bergen County, Pompton Lakes in Passaic County, and across Morris County in this vicinity.  The fault terminates in the Peapack-Gladstone area of Somerset County.  Through Mountain Lakes the fault approximates the path currently followed by Conrail (Erie Lackawanna Railroad), but it is difficult to determine its exact location since it has no visible surface trace.

Earthquakes in North New Jersey

The Ramapo Fault has not produced a major earthquake in historic times; however, scientists from Lamont Doherty Geologic Observatory who monitor it constantly have recorded some two dozen small tremors (1.5 to 3.0 on the Richter Scale) in the past five years.

In December 2009, a magnitude-2.0 earthquake rocked the 0.15-square-mile borough of Victory Gardens, one mile southeast of Dover and seven miles northwest of Morristown, according to the United States Geological Survey.  As reported in the NJ Star Ledger, this quake brought the total number of earthquakes in North Jersey in 2009 to at least six: an earlier 3.0-magnitude earthquake near Victory Gardens, followed a couple weeks later by four more quakes near Boonton, Montville, Teterboro Airport in Bergen County, and again in Victory Gardens.

A 3.5-magnitude earthquake that was centered in Milford, Hunterdon County, in 2003 produced about 150 aftershocks in subsequent months. Many, however, were so minor that residents did not feel them, scientists said.

Scientists believe a 5.5-magnitude earthquake could happen in New Jersey every 120 years. Since the last major earthquake to hit New Jersey happened 125 years ago — in 1884 off Sandy Hook, measuring 5.5 on the Richter scale — the state could be due to get hit again.

Pleistocene Glaciation
During the Pleistocene Epoch of the Quaternary there were long periods when large parts of North America were covered by glacial ice.  There were even longer periods called interglacials when the climate was at least as mild as it is today.  Only three of the four major North American ice advances are evident in New Jersey.  The last glacier, termed the Wisconsin, retreated from this area about 15,000 years ago, leaving behind a greatly modified landscape.

The maximum thickness of the ice sheet in New Jersey has never been determined, but rough estimates place it at between 1500 and 2500 feet.  Its general direction of movement was from north to south, carrying with it a great deal of material scoured from the land.  This rock debris, termed glacial drift, ranged in size from clay and silt up to huge boulders.

When the ice started to melt back as fast as it was advancing, the front of the ice became stationary for a time.  At this point the sediments and rock was released from the melting ice and dumped in a pile at the front of the ice.  Thus it formed what is called a terminal moraine. The Wisconsin terminal moraine in New Jersey consists of a low irregular ridge of glacial material averaging a mile wide and ranging in height from 25 to 300 feet.  This curving ridge marks the southern extent of glaciation in the state.  Across Morris County the moraine extends roughly from Chatham to Hackettstown.  This may explain the 363 feet of unconsolidated deposits that were encountered before reaching bedrock when Rte. 46 Well No. 5 was drilled.

The pre-glacial drainage was completely disrupted by the glaciers; and when the ice began to melt, new drainage patterns were formed.  This process was delayed somewhat by the formation of a huge melt water lake called Glacial Lake Passaic.  This lake extended from the Watchung Mountains on the east to the Highlands on the west and at its maximum size was about 30 miles long and up to 240 feet deep.  The Piedmont portion of Mountain Lakes was covered by this lake.

Please proceed to the MCPRIMA to map the bedrock within the Borough.