Campgaw Mountain - Geology

Geology

Campgaw Mountain formed as molten rock extruded onto the surface 185 million years ago. At that time the mountain occupied the northern section of an active rift valley running through northern and central New Jersey and part of Pennsylvania. After the rift failed in the early Jurassic, the mountain was elevated as erosion removed the sandstone and shale surrounding the basalt lava flows of Campgaw Mountain.

The main ridge of the mountain is primarily composed of Preakness Basalt, being an extension of Second Watchung Mountain, but the northeast corner of the mountain is composed of Orange Mountain Basalt, as the ridge of First Watchung Mountain emerges from surrounding glacial sediments along the north side of main ridge. The combined ridges of First and Second Watchung Mountain make a slight turn to the west before terminating at the Ramapo Fault, marking the northernmost limit of the Watchung Mountains.

Campgaw Mountain appears somewhat detached from the rest of the Watchungs because it occupies a local syncline (downfold) that is itself somewhat detached from the main Watchung syncline by an intervening anticline (upfold). The effect of this is that, when traveling north from the Preakness Range and Goffle Hill, the ridges of First and Second Watchung Mountain seem to slowly dip back into the Earth before reemerging again as Campgaw Mountain. Despite the apparent gap at the surface, Campgaw Mountain is firmly linked to the rest of the Watchungs by the continuity of the ridges through the anticline.