When we think of hidden New Jersey, we have come to consider the historic buildings, the out-of-the-way natural areas or long departed personalities that graced, or still grace the Garden State. However, perhaps the most hidden aspect of the state is the bedrock that sits many yards below our feet as we explore the “surface” Jersey. Geologic history has literally shaped our state with much of the area north of the Raritan Bay made up of rock that dates back to a time that predated the dinosaurs, while the southern part of the state consists of sediments that were deposited long afterward.
|Green Pond conglomerate in place on Green Pond Road in Rockaway|
A famous example of these out of place rocks (known as glacial erratics) is Tripod Rock in Morris County. A frequently found glacial erratic in the Montville and Boonton area is a distictively attractive rock known as Green Pond conglomerate. It's part of a larger rock formation that stretches from the New York State Thruway southwestward all the way down to Route 80 in Morris County. Alternately, it's known as Schunemunk puddingstone after another of its locations, on New York's Schunemunk Mountain. The United States Geologic Survey website has a page dedicated to Green Pond Conglomerate so I have no problem claiming this rock formation in the name of New Jersey.
A conglomerate is a rock that consists of a matrix that has pieces or fragments of other rock (known as “clasts”) mixed in. “Puddingstone” is a more colloquial expression that refers to this uneven mixture of rocks. In the case of Green Pond Conglomerate, the matrix is a reddish purple siltstone with white quartz fragments as its clasts. What I had found out however, is that these erratics are not far from home. They originated as a formation in Green Pond, a section of Rockaway Township. The matrix consisted of a reddish silt or clay that eventually hardened into rock during the Silurian period about 420 million to 445 million years ago. There the rock sat minding its own business until about 50,000 years ago when the Wisconsin Glacier slid across what is now New Jersey to slice off chunks of our hero to deposit the fragments across northern Morris County.
Over the years, Green Pond Conglomerate has been prized for its decorative qualities and has been used to construct stone walls and other landscaping and construction features on many residential and commercial properties. However attractive it looks as part of these structures, I recently had an interest in finding it where it originated so Sue and I took a ride to Green Pond to find the mother lode. Turning off Route 23 in Newfoundland onto (what else?) Green Pond Road, we could easily see the imposing ridge of Bearfort Mountain to our west. The ridge paralleled Green Pond Road, as we traveled south, so whenever we saw a side road to our right we turned in hopes of getting closer to the ridge that must have been the source of the conglomerate. We soon found ourselves in the Green Pond, a lake community that sits on the shore of… you guessed it: Green Pond.
Luckily, we found a small dirt parking lot that serves a trailhead. A friendly homeowner was seeing to her flower garden and was happy to let us know that the conglomerate could be found in the area. Unfortunately, we weren’t dressed for hiking so we passed on a trip along the trail but we could see a characteristic color to the ridge in that area. Even better, however was when we continued south on Green Pond Road and found a large outcropping of the conglomerate evident as a result of the cut that was made to construct the road, itself. We had found the origin of the rock formation that bears the name of one of New Jersey’s own communities. It was certainly worth the drive.