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National Geologic Map Database

Geologic Unit: Shell Rock

Usage:

Shell Rock Formation of Cedar Valley Group (IA)
Shell Rock Formation (IA)
Shell Rock Limestone (IA)


Subunits:

(alphabetical) all in IA: Mason City Dolomite Member, Nora Member, Rock Grove Member.


Geologic age:

early Late Devonian (Frasnian)


Type section, locality, area and/or origin of name:

Typical development along Shell Rock River between Rockford and Nora Springs and on northward, also about Mason City and to the north and northwest. Named from Shell Rock River (Thomas, 1924).
Reference section (subsurface): FM-4 core, in sec. 31, T. 96 N., R. 18 W., Floyd Co., IA (Witzke and others, 1988).


AAPG geologic province:

Iowa shelf


For more information, please contact Nancy Stamm, Geologic Names Committee Secretary.

Asterisk (*) indicates usage by the U.S. Geological Survey. Other usages by state geological surveys.

"No current usage" (†) implies that a name has been abandoned or has fallen into disuse. Former usage and, if known, replacement name given in parentheses ( ).

Slash (/) indicates name does not conform with nomenclatural guidelines (CSN, 1933; ACSN, 1961, 1970; NACSN, 1983, 2005). This may be explained within brackets ([ ]).