The Franklin-Sterling area is located in Sussex County, New Jersey, and Orange County, New York, along the western border of the New Jersey Highlands. A geologic study of the Precambrian rocks of the area which include metasedimentary, igneous, and probable metavolcanic types was undertaken by geologists of The New Jersey Zinc Company. The marble and paragneisses have been divided into stratigraphic units which are, from oldest to youngest, the Hamburgh Mountain gneisses, the Franklin marble band, the Cork Hill gneiss zone, the Wildcat marble band, and the Pochuck Mountain gneiss series. Some of the Precambrian igneous rocks were probably intruded during deformation as sills and phacoliths, whereas the granites and pegmatites were intruded as discordant bodies during or after the last stages of deformation. The Precambrian rocks have been metamorphosed to a high grade but show signs of subsequent hydrothermal metamorphism.
The major structures in the area are isoclinal, overturned synclines and anticlines formed during Precambrian time. Two sets of minor cross-folds were superimposed on the major folds. Paleozoic folding is apparent only in the Paleozoic rocks. The most recent fault movements were post-Ordovician but may have been controlled by Precambrian faulting. Two sets of cross-faults are associated with the major faults.