Sanibel Island is considered one of the best shelling beaches in the world. Here on this sub-tropical island are plush, white sandy beaches brimming full of beautiful shells, what are called "the jewels of the gulf." The best time to do this shelling is at LOW TIDE.
Almost six thousand years ago sediments deposited from the mouth of the Caloosahatchee River began to form the island as they emptied into the Gulf of Mexico. This boomerang shaped “sand bar”, due to its perpendicular orientation to the coastline and the currents, continued to grow as shells and sand deposited along the newly formed beaches.
Thanks to the play of the Gulf of Mexico waters and the jutting curve of the island (aptly likened to a crab claw) shells stirred up by faraway winter storms are scooped onto the beaches. Estimates vary, but at least 275 kinds of shells are found here. One of the reasons for these accumulations of shells is the fact that Sanibel is a barrier island which has an east-west orientation when most islands are north-south. Hence, the island is gifted with great sandy beaches and an abundance of shells. It is also due to the fact that Sanibel is "part of a large plateau that extends out into the Gulf of Mexico for miles. It is this plateau that acts like a shelf for seashells to gather.
Following the tides is among some of the most interesting phenomena on our earth. From ancient times there has been tidal observation and discussion about its causes. Pytheas (a Greek geographer and explorer) traveled to the British Isles about 325 BC and seems to be the first to have related spring tides to the phase of the moon. It is something that we all have a general idea about but we are going to look a little deeper into the subject of how and why tides work and why they are important to us and our earth.
For this cache you must visit the location at the posted coordinates at LOW TIDE and make some observations. The posted coords will take you to a public beach access on Sanibel Island along the Gulf of Mexico. A tidal chart can be obtained at the Sanibel Island Visitors center at N 26 27.012 W 82 02.285 or at Tide Link
An ocean tide refers to the cyclic rise and fall of seawater. Tides are caused by slight variations in gravitational attraction between the Earth, the moon and the Sun in geometric relationship with locations on the Earth's surface. Tides are periodic primarily because of the cyclical influence of the Earth's rotation.
The Sun and Moon pull on the Earth, the water, even you! But gravitational attraction depends on distance and mass. For example, you have very little mass and you're very close to the Earth, so the Sun and Moon can't just yank you off the planet. The Sun is extremely massive, but it is an average of 93 million miles (150 million km) from Earth, compared with about 238,866 miles (384,400 km) from here to the Moon. And since the Moon is nearly 400 times closer to our planet, its influence on our oceans is twice as strong as the Sun's.
The moon is the primary factor controlling the temporal rhythm and height of tides. The moon produces two tidal bulges somewhere on the Earth through the effects of gravitational attraction. The height of these tidal bulges is controlled by the moon's gravitational force and the Earth's gravity pulling the water back toward the Earth. At the location on the Earth closest to the moon, seawater is drawn toward the moon because of the greater strength of gravitational attraction. On the opposite side of the Earth, another tidal bulge is produced away from the moon. However, this bulge is due to the fact that at this point on the Earth the force of the moon's gravity is at its weakest. Considering this information, any given point on the Earth's surface should experience two tidal crests and two tidal troughs during each tidal period.
The timing of tidal events is related to the Earth's rotation and the revolution of the moon around the Earth. If the moon was stationary in space, the tidal cycle would be 24 hours long. However, the moon is in motion revolving around the Earth. One revolution takes about 27 days and adds about 50 minutes to the tidal cycle. As a result, the tidal period is 24 hours and 50 minutes in length. The lunar day is longer than the Earth day because the Moon orbits in the same direction the Earth spins. This is analogous to the minute hand on a watch crossing the hour hand at 12:00 and then again at about 1:05 (not at 1:00) because the hour hand is moving in that direction as well.
The second factor controlling tides on the Earth's surface is the Sun's gravity. The height of the average solar tide is about 50% the average lunar tide. At certain times during the moon's revolution around the Earth, the direction of its gravitational attraction is aligned with the Sun's. During these times the two tide producing bodies act together to create the highest and lowest tides of the year. This planetary allignment of the sun, earth, and moon is called Syzygy (siz-eh-gee). These spring tides occur every 14-15 days during full and new moons.(See Below)
When the gravitational pull of the moon and Sun are at right angles to each other, the daily tidal variations on the Earth are at their least. These events are called neap tides and they occur during the first and last quarter of the moon. (See Below)
Tides are most commonly semidiurnal (two high waters and two low waters each day), or diurnal (one tidal cycle per day). The two high waters on a given day are typically not the same height (the daily inequality); these are the higher high water and the lower high water in tide tables. Similarly, the two low waters each day are the higher low water and the lower low water. The daily inequality is not consistent and is generally small when the Moon is over the equator.
The map below shows the geographic distribution of these three tide types on the Earth.
Tides are important to geology because of their ability to transport sediment and carve coastlines. Over a long period, shorelines shift, estuaries and rivers are filled in with sediment, and beaches change from the rise and fall of the tides. This is why there is the constant shifting of the shoreline. In the world of commerce, tides are vitally important to the shipping and fishing industries. Navigators of heavy freighters heading for shallow ports must schedule their arrival for high tide, when the water is deepest. Fishing boats are also subject to the ebb and flow of tides. Predicting these changes is an essential aspect of maritime travel, and highly detailed tide tables are available for almost all coastal areas.
LOGGING REQUIREMENTS:
Please email us the answers to the following questions, DO NOT POST the answers in your log-
1) Is this beach a Semidiurnal Tide Zone, a Diurnal Tide Zone, or a mixed tide and why do you think that?
2) Standing at the waters edge at low tide, how wide (in feet) do you think the inter-tidal zone (distance between the high tide line and the low tide line) is at the posted cordinates?
3) Send us the date and time of your visit, and you must ALSO mention if your visit was closer to the neap tide or spring tide
4) At the beach entrance (waypoint 02) , email us the mile marker number posted here.
5) Please post a picture(Optional) of you and your GPSr at the posted coordinates to show your visit, or of you and a beautiful shell find.
PLEASE NOTE: I will not be sending follow up requests for answers. If you do not complete the requirements,your log will be deleted. In order to claim this Earth Cache, you must follow through with the requirements, not just visit the site.
Some Fun facts about beachsand:
The typical peanut butter and jelly sandwich eaten on the beach contains over 7,000 grains of sand!
Sand from islands or tropical sites has sand that is high in Calcium, since seashells are rich in Calcium and are broken down to form sand there.
Certain beaches and sand dunes create mysterious sounds that scientists still don't completely understand.
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