Boulders are strewn throughout the forests of the Sourlands.
Originally published in “Seeing the Sourlands” 2019.
Rock Road, West Rock Road, Rock Brook, Rocktown, Stony Brook, and Stony Brook Road—looking at a map of the Sourlands region sends a message. This is a place where rocks play a prominent role. To understand how it happened—how the forests of the Sourlands became strewn with boulders—it is necessary to delve into the region’s geologic history.
For many millions of years what is now central New Jersey was beneath the ocean. During that period sediment from nearby land washed into the ocean, settled to the ocean floor and became compacted over many more millions of years into sandstone and shale. Eventually the area rose above the water and then about 250 million years ago the tectonic plate that is now Africa and the tectonic plate that is now North America pulled apart, creating rifts in both plates that were filled by volcanic magma from deep in the earth. Two kinds of rock were created by this magma. If it pierced the surface, allowing it to cool rapidly, the magma formed basalt rock. (The term “basalt” has nothing to do with “salt.” The word basalt is derived from a Latin word meaning very hard stone.) If the magma did not reach the surface it cooled more slowly, allowing mineral crystals to grow to visible size. This stone is called diabase rock. Aside from the size of the crystals, diabase and basalt are the same thing.
The basalt and diabase are much harder than the soils and sedimentary sandstone and shale that was around it and therefore they have been more resistant to the 200—or so—million years of erosion that has taken place in our region. That long of a period for erosion, along with that long of a period of a freeze/thaw cycle, has brought some of the diabase to the surface and fractured much of it into boulder sized pieces. The result is that the Sourlands have a mantle of diabase at a shallow depth and a plethora of diabase and basalt boulders strewn about the landscape.
All of this stone—the sandstone, the shale, the diabase and the basalt—has great commercial value. The sandstone and shale have been exploited for constructing buildings, (Some of the region’s brown sandstone was quarried and carried to New York City via the D&R Canal to make those beloved brownstone townhouses.) The diabase and basalt are principally used in the manufacture of asphalt and are therefore vital to road construction and maintenance.
The basalt and diabase are the main reason why the Sourland region has remained undeveloped compared with the rest of central New Jersey. It is almost impossible for large-scale residential development to get the water that is needed, or to safely dispose of septic wastes. Further, having boulders strewn about the landscape made the Sourlands an undesirable place for farms. In the eighteenth century the original forests were almost all cut for fuel, for charcoal making or for pastures. Livestock could be pastured in meadows strewn with rocks even if it was impossible to plow for crop fields. By the mid-twentieth century most of the pastures were abandoned because the small-scale farms of the Sourlands could not compete with large-scale farms in landscapes more suitable for farming.
The abandonment of the Sourland region by farmers allowed the emergence of a natural landscape that is now seventy to a hundred years old. The Sourlands is now the third largest forested area in New Jersey (after the Pinelands and the northwest). Today it is a refuge for plants and wildlife and for people who want to escape the developed world without driving great distances. Unfortunately, recent technological advances have made it more feasible to have residential development in the Sourlands, threatening the sanctuary with development.
By Jim Amon
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