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Coastal and Marine Geology Program
Origin and Distribution of Beach Rock, Point-of-Rocks, Siesta Key, Florida

West-Central Florida Coastal Studies Home
Open File Report: Second West-Central Florida Coastal Studies Workshop
Chief Scientist
Darren L. Spurgeon, Department of Geology, University of South Florida, Tampa FL
Richard A. Davis, Department of Geology, University of South Florida, Tampa, FL

Although numerous studies have been published regarding the occurrence of beach rock on the east coast of Florida, there have been no significant studies that address the origin and distribution of beach rock on Florida's west coast. Such a study is necessary in understanding what effect, if any, this beach rock has on barrier island development and in understanding sea level conditions during the time period in which this beach rock was deposited and lithified.

Beach rock is beach material that has been cemented within the intertidal zone. It differs from cemented beach deposits in that, by definition, the material must be cemented within the intertidal to be considered beach rock. Therefore, it is an excellent indicator of sea level. Workers, such as Ginsburg (1953) , Russell (1962), and Stoddart and Cann (1965), have conducted numerous studies on beach rock and have argued its origin and cementation, but many agree that its usefulness as a potential sea level indicator cannot be ignored.


Figure 1: Map of the barrier islands along the Sarasota County coastline and Point-of-Rocks study area.

To date, west-central Florida outcrops of beach rock have been found only on Siesta Key in Sarasota County (Figure 1), where it occurs mainly as a quartz sand and molluscan grindstone. According to Cooke (1945), the majority of Sarasota County, however, is underlain by the Hawthorne, the Bone Valley, and the Caloosahatchee formations, with limestone and marl outcrops occurring throughout the county. The beach rock outcrop is not mentioned in that report; however, it has been assumed by Campbell (1985) to be equivalent to the Anastasia formation on the east coast of Florida. The Anastasia is considered to be Pleistocene stage 5e with outcrops extending from St. Augustine south to Boca Raton. Its thickness varies from thinly bedded coarse rock of primarily broken shell fragments (St. Augustine area) to thick, in some places several meters, beds of sand limestone (Palm Beach area). This formation is assumed to be Sangamon in age and to represent a sea level of approximately seven meters above present day.

It is questionable, however, whether the Point-of-Rocks outcrop is equivalent in age to the Anastasia. Knowles (1983) suggested that this west-coast beach rock might be younger than Pleistocene. Nevertheless, amino acid racemization and whole rock C14 age dating techniques will be used on the beach rock at Point-of-Rocks, Dates from these samples should provide part of the data needed to help interpret and apply a paleoenvironment in which the beach rock formed.

The purpose of this two-year study will be to determine the age, the distribution, and the depositional environment of the beach rock at Point-of-Rocks as well as determine any effects it may have had on the development of Siesta Key. Extensive mapping of the exposed outcrop at Point-of-Rocks, probing of the subsurface in adjacent areas for buried beach rock, age determination of representative samples, and comprehensive petrographic thin section analysis will help to accomplish this objective. Furthermore, study of the beach rock on Siesta Key will add to our understanding of the paleoenvironment and sea level history for this area of Florida's west coast.


  • Campbell, Kenneth M., 1985, Geology of Sarasota County, Florida, Florida Geological Survey, Open-File Report 10, 15 pp.
  • Cooke, C. W., 1945, Geology of Florida, Florida State Geological Society, Geological Bulletin 29, p. 1-339.
  • Ginsburg, R. N., 1953, Beach rock in South Florida, Journal of Sedimentary Petrology, v. 23, p. 85-92.
  • Knowles, Stephen C., 1983, Holocene geologic history of Sarasota Bay, Florida, Unpublished M.S. thesis, University of South Florida, p. 128.
  • Russell, Richard J., 1962, Origin of beach rock, Zeitschrift fur Geomorphologic, Neue Folge, v. 6, n. 1, p. 1-16.
  • Stoddart, D. R. and Cann, J. R., 1965, Nature and origin of beach rock, Journal of Sedimentary Petrology, v. 35, n. 1, p. 243-247.

Coastal & Marine Geology Program > St. Petersburg Coastal and Marine Science Center > West-Central Florida Coastal Studies Project > Second West-Central Florida Coastal Studies Workshop > Morphodynamics > Origin and Distribution of Beach Rock, Point-of-Rocks, Siesta Key, Florida U. S. Department of the Interior | U.S. Geological Survey
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Updated May 06, 2013 @ 09:24 AM  (JSS)