Small deposits of Neoproterozoic ironstone in the New Jersey Highlands are hosted by the Chestnut Hill Formation, a terrestrial sequence of siliciclastic rocks, sparsely preserved felsic and mafic volcanic and tuffaceous rocks, and thin limestone metamorphosed at greenschist-facies conditions. Sediments of the Chestnut Hill Formation were deposited in alluvial, fluvial, and lacustrine environments in a series of fault-bounded subbasins along the Iapetan eastern Laurentian margin. Ironstone occurs mainly in the upper part of the sequence in sandstones, quartzites, fine-grained tuffs, tuffaceous sediments, and carbonate-bearing beds. Ore is massive to banded and contains the assemblage hematite ± magnetite, which is locally associated with tourmaline and Fe silicates + sericite + calcite + chlorite ± quartz. Ironstone alternates with clastic bands, and sedimentary structures in ore bands and clastic bands are consistent with alternating chemical and clastic sedimentation deposited synchronously. Chestnut Hill rocks exhibit geochemical compositions that are dissimilar to typical sedimentary and volcanic rocks. They display evidence for two stages of post-diagenetic alteration. The first stage involved widespread potassium metasomatism, which produced increased values of K, Ba, and Rb that are not correlated with increased Fe or other hydrothermal elements. The metasomatizing fluid may have been basinal water heated during emplacement of Chestnut Hill volcanic rocks. The second stage produced alteration of Chestnut Hill rocks, and also Mesoproterozoic rocks along the footwall contact of the deposits, by hydrothermal fluids likely from a volcanogenic source. The ironstone deposits were formed by hydrothermal processes related to extension during formation of continental rift subbasins in the New Jersey Highlands. Iron was sourced from Fe-rich Mesoproterozoic rocks at depth, where it was leached by hydrothermal fluids that migrated upward along extensional faults. Iron and other metals were precipitated in permeable basin sediments and chemically favorable volcanic rocks, as well as precipitated directly as chemical sediment.

New U-Pb sensitive high-resolution ion microprobe (SHRIMP) ages from zircon and monazite document a 350 m.y. geologic evolution for the New Jersey Highlands. Two pulses of calc-alkaline magmatism that include the Wanaque tonalitic gneiss (1366 ± 9 Ma and 1363 ± 17 Ma) and Losee Suite tonalitic gneiss (1282 ± 7 Ma), dacitic gneiss (1254 ± 5 Ma), and dioritic gneiss (1248 ± 12 Ma) represent the southern continuation of eastern Laurentian margin arc activity. Supracrustal paragneisses, marble, and cogenetic metavolcanic rocks were deposited in a backarc basin inboard of the Losee arc. Ages of 1299 ± 8 Ma to 1240 ± 17 Ma for rhyolitic gneisses provide lower and upper limits, respectively, for the age of the supracrustal succession. Inherited cores in zircon grains from supracrustal rhyolitic gneiss and from Losee Suite rocks yield overlapping ages of 1.39–1.30 Ga and indicate proximity to an older arc source temporally equivalent to the Wanaque tonalitic gneiss. Location of the backarc inboard of the Losee arc implies a northwest-dipping subduction zone at this time beneath the eastern Laurentian margin. A-type granite magmatism of the Byram and Lake Hopatcong intrusive suites at 1188 ± 6 Ma to 1182 ± 11 Ma followed termination of arc and backarc magmatism and documents a change to decompression melting of delaminated lithospheric mantle by upwelling asthenospheric mantle. Waning stages of A-type granite magmatism include clinopyroxene granite (1027 ± 6 Ma) and postorogenic Mount Eve Granite (1019 ± 4 Ma). Overgrowths on zircon and monazite give ages of 1045–1024 Ma, fixing the timing of granulite-facies metamorphism in the New Jersey Highlands; other overgrowth ages of 996–989 Ma reflect the thermal effects of postorogenic felsic magmatism and hydrothermal activity associated with regional U–Th–rare earth element (REE) mineralization.

In contrast to results of regional soil radon studies in unglaciated areas, bedrock geology shows no correlation with radon concentrations in glacial soils overlying the Green Pond outlier and Reservoir fault zone, New Jersey Highlands. Total gamma radiation and uranium concentrations in the Paleozoic sedimentary rocks of the Green Pond outlier are generally lower than in the Precambrian gneisses of the Reading Prong to the west. The sedimentary bedrock shows average gamma radiation of 220 c/s (136 to 323 c/s) and uranium concentrations of 0.5 to 0.6 ppm, whereas gamma radiation from the Grenville gneisses averages 284 c/s (240 to 576 c/s) and average uranium concentrations are 1.2 to 2.2 ppm. Rare pegmatites that occur along the Reservoir fault zone yield anomalously high average gamma radiation of 2,018 c/s (1,949 to 3,495 c/s) and average uranium concentrations of 28.5 ppm. Radon concentrations in the glacial soil cover exhibited similar averages with wide ranges regardless of underlying bedrock geology. No appreciable difference was found between soil radon concentrations over Paleozoic sedimentary units, the pegmatites, fault zone, or Precambrian gneisses. Radon from soil over the Paleozoic sedimentary bedrock averaged 518 pCi/L (237 to 2,695 pCi/L), whereas it averaged 527 pCi/L (200 to 1,872 pCi/L) in soil over the Grenville gneisses. The Green Pond outlier and the Reservoir fault zone are blanketed by the Wisconsin-age glacial cover and contain recessional deposits that are proximal to the terminal moraine. All soil radon was sampled in the glacial cover. The glacial sediments contain erratics primarily composed of lithologies in the area but also of exotic rock types. Because uranium concentrations in the erratics and matrix are highly variable, soil radon of individual samples is governed by the local uranium concentrations. Other possible reasons for the lack of correlation between bedrock and soil radon are the high permeability of the glacial soil that permits radon diffusion, atmospheric dilution, and variations in cover thickness.