Coquina – Surf and Turf Geology EarthCache
Coquina – Surf and Turf Geology
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This Earthcache is in the Washington Oaks State Park - a beautiful place well worth the small entry fee of $5 per vehicle or $2 per pedestrian. Hope that you agree!
Coquina is a naturally occurring sedimentary rock composed of shells, fossilized organic matter, quartz sands and salts, "cemented together" by limestone (calcium carbonate).
Named after the small mollusks it contains, coquina forms as the result of sediment being deposited on beaches by waves and currents. The shells of small sea creatures are moved on to the beach and mixed with quartz sand.
The quartz sand comes from the weathering and erosion of older rocks and sediments inland. The sand is transported into the sea by streams and rivers then moved by currents and waves along the coastline.
Coquina formed as an offshore bar through accumulation of sand and shells when sea levels were higher and today's coast was underwater.
Later, during a glacial period (between 125,000-150,000 years ago), the sea level dropped, leaving the bar exposed to the air and weather. Geologically it is therefore considered a "young limestone", compared with other stones like marble, which are more than one million years old.
The calcium carbonate from the shells once exposed was dissolved in rainwater, which is naturally acidic. The rainwater evaporated and the remaining mixture cemented the loose shell-containing sediment into rock.
It can be soft and quite porous, with a grainy, sponge-like texture and strata that vary in density. The percentages of quartz sand and shell making up the rock vary widely, ranging between nearly pure sand to completely shell.
Once the rock is exposed to weathering it will begin to disintegrate.
Weathering of the coquina will also cause the surface of the rock to blacken and harden.
Coquina is found underneath much of the Atlantic shore of Florida. It was used as building stone in St. Augustine by Spanish explorers at the end of the 1500’s for construction of a settlement and a fort. This started an extended period of the use of the stone, which continued to be used in buildings along the Florida Atlantic coast until into the 1930s.
Indeed coquina could be regarded as Florida’s most important building material and a rock that helped shape the history of the state.
Although covered by sand in most regions, coquina outcroppings and quarries can be found in several Florida counties including here in Flagler, and in Martin and Palm Beach.
Here in Washington Oaks State Park, the rocky outcrops can be clearly seen and show a variety of outcomes from weathering from exposure to the sea and other elements.
You will notice at the coordinates for the cache that the mollusk shells are both whole and fragmented. Some of the shells are quite heavily abraded indicating transport or movement by waves and currents prior to becoming deposited and formed into the rock.
The main fossil mollusk in the coquina along Florida’s eastern coast is a small clam. Other mollusks are present including oysters and a large gastropod or snail.
Along the coast the amount of sand and shell in coquina varies.
While you are looking around the area you may see the fossilized burrows of marine organisms. Also you will see the bowl-shaped depressions on the upper surface of the coquina. These are formed by cabbage palms that once grew on the surface of the coquina. The cabbage palm roots dissolve the coquina forming the bowl.
To claim this cache, please go to the coordinates above and answer the following two questions:
1. Please estimate the proportion of sand to shell in the outcrop at the coordinates shown. A piece of the rock about 8 inches long by 3 inches wide on the East or coast side of the outcrop has been abraded by elements and exposed lighter colored coquina which is less weathered – this may be a good area to look.
Please give your answer as X% sand Y% shell, where X and Y are your estimated values.
2. What is the rough size of the largest fragment (or complete) shell that you can see in the outcrop at the coordinates – please provide the length of the shell. The largest I saw was in the upper layer of the rock outcrop.
3. Now go to the second co-ordinates listed at N29 38.344 and W 081 11.989 and estimate the size of the diameter and depth of the hole that exists in this rock.
4. In your opinion and not essential to answer for the cache, did the hole get created by the cabbage palm referred to above or by natural erosion and weathering or a combination of both?
Feel free to post any photos, but please don't post the answers to your questions. Have fun!
References:
THE CONSERVATION AND PRESERVATION OF COQUINA - A Symposium on Historic Building Material in the Coastal Southeast - Held January 24-26, 2000 at St. Augustine, Florida
THE COQUINA RESOURCES OF FLORIDA’S EAST COAST - Tom Scott, Ph.D., Assistant State Geologist, Florida Geological Survey
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