Rondout Formation

Dolostone, limestone; Binnewater Sandstone; High Falls Shale; Warwarsing Limestone; Decker Limestone; Bossardville Limestone; Poxono Island Formation-shale, dolostone.
State New York
Name Rondout Formation
Geologic age Upper Silurian
Lithologic constituents
Sedimentary > Carbonate > LimestoneWarwarsing Limestone; Decker Limestone; Bossardville Limestone
Sedimentary > Clastic > Mudstone > ShaleHigh Falls Shale, Poxono Island Formation-shale, dolostone.
Sedimentary > Carbonate > Dolostone
Sedimentary > Clastic > SiltstoneDecker Limestone, Poxono Island Formation
Sedimentary > Clastic > SandstoneBinnewater Sandstone
Comments part of Undifferentiated Silurian Rocks I, 0-400 ft. (0-120 m). Secondary unit descriptions from USGS Geologic Names lexicon (ref. NY046): The Binnewater Sandstone in southeastern New York consists of interbedded, thin- to medium-bedded, cross-bedded, rippled, light-gray, quartzose sandstone, and partly mudcracked, green and gray silty shale, dolomite, and limestone. Lower contact is gradational with the High Falls Shale and the upper contact is gradational with the Rondout Formation. Some earlier reports have mistaken the Poxono Island Formation as the Binnewater Sandstone. Unit is of Late Silurian age. The High Falls Shale consists of a 55 foot thick lower unit of red and green shale and green dolomite, a 12 foot thick medial unit of green, calcareous dolomite with a bed of dark-gray, laminated dolomite at the base, and a 25 foot thick upper unit of gray, green, and red calcareous and silty shale with some thin dolomite beds. The basal contact of the High Falls is placed at the base of the first red bed above the uppermost quartzite or conglomerate of the Shawangunk Formation and the upper contact is gradational with the overlying Binnewater Sandstone. Unit ranges from 46 to 100 feet thick. The High Falls Shale has been confused by some earlier workers as being equivalent to the Bloomsburg Red Beds. Rondout and Decker Formations mapped undivided, but discussed separately in text. Decker is light- to medium-gray, calcareous quartz siltstone and sandstone, locally a fine-pebble conglomerate. Locally interbedded with medium-gray, medium to coarse-grained, thin- to medium-bedded limestone and very fine-grained dolomite. Total thickness approximately 72 ft. Lower contact with Bossardville Limestone is gradational. Age is Late Silurian. Bossardville Limestone mapped in northwestern NJ. Composed of medium-gray to medium-dark-gray, very fine-grained, argillaceous limestone and limestone, weathers medium bluish gray. Unit is thin-bedded, laminated to ribbon-textured. Desiccation columns and cracks occur in southwestern part of map area. Total thickness is approximately 100 ft. Lower contact with Poxono Island Formation is gradational and placed at top of uppermost dolomite. Underlies Decker Formation. Age is Late Silurian. The Poxono Island Formation is about 500 feet thick at Port Jervis, NY, and gradually thins to the north toward Ellenville where it is about 300 feet thick. Consists of laminated, platy, thin-bedded, greenish-gray calcareous siltstone, dolostone, and light-greenish-gray shale. Between Accord and High Falls, NY the Poxono Island grades laterally into the Binnewater Sandstone. The Poxono Island overlies the Bloomsburg Red Beds or the High View Tongue of the Shawangunk Formation and underlies the Bossardville Limestone or Rondout Formation. Unit is of Late Silurian age.

Fisher, D.W., Isachsen, Y.W., and Rickard, L.V., 1970, Geologic Map of New York State, consisting of 5 sheets: Niagara, Finger Lakes, Hudson-Mohawk, Adirondack, and Lower Hudson, New York State Museum and Science Service, Map and Chart Series No. 15, scale 1:250,000.

USGS Geologic Names lexicon found at:

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Counties Orange - Sullivan - Ulster