General Information: Washington
Valley Geology
The First and Second Watchung Mountains dominate the topography of the Washington Valley
area. These mountains, along with the Third Watchung Mountain located north of
Bridgewater, are the result of a series of three huge lava flows separated by periods of
sedimentation, tilting, and erosion.
The lava cooled quickly, creating an igneous rock with a fine, or aphantic, texture
(crystals less than 1/32” in size) called basalt. When igneous rock is allowed to
cool slowly it forms larger crystals. The crystals in basalt are not visible to the
unaided eye. But, you can see them easily in a piece of granite, which is a rock formed
below ground where it retains its heat much longer than the basalt formed on the surface.
The difference between gabbro and basalt is the fine texture of lava, which cools quickly
on the surface and magma, which cools slowly underground. In between the lava flows
sediments covered the area. With time and pressure these sediments formed the red shales
and sandstones of the Brunswick Formation. Shale is formed from mud rather than sand, and
is produced mainly by compaction. Red shales generally result from oxides contained in the
sediment, which often originated near an arid region. The shale beds between the layers of
basalt may be as much as one hundred feet thick. Where the shale is in contact with the
basalt it was sometimes baked, or metamorphosed, into slate.
This process occurred in the late Triassic period. The Triassic was the first period of
the Mesozoic Era (the age of dinosaurs) and ended 208 million years ago as the
supercontinent Pangea began to break up.
This huge layer cake slowly deformed into a bowl shape, lower in the center and higher at
the edges, known as the Watchung Basin. This tilted the layers dip toward the north. The
First and Second Watchung Mountains are the edges of two of these nested bowls of basalt
and the Middle Brook follows the shale filled gap between them. The present topography
results from the ability of the basalt to resist erosion better than the shale. The gentle
slope (dip) of the northern side of the First Watchung, south of the brook, reveals the
present tilt of the surface of the lava. The undermining and fall of the eroded edge of
the second lava flow controls the steeper southern side of the Second Watchung Mountain,
north of Washington Valley Road.
Washington Valley was south of the furthest extent of the Wisconsin glacier. However,
older glaciers, the Kansan and Illinoian, did reach into Bridgewater. But few traces of
them remain. The Second Watchung Mountain was the southern shore of Lake Passaic, which
was the largest glacial lake in New Jersey and extended across Morris County to Passaic
County.
There was also glacial lake Hackensack to the east.
The First Watchung Mountain has a series of old mines where a seam of 75% pure copper ore
reaches the surface. The mines dip toward the north following the seam. If this area were
not so valuable for residential use, modern miners would strip mine the area to access the
copper.
