The Shoals of North Beach

History of Fort De Soto and the Surrounding Keys:

Fort De Soto Park is a park operated by Pinellas County, just south of St. Peterburg. The park is operated on five offshore keys; Madelaine, St. Jean, St. Christopher, Bonne Fortune, and the main island, Mullet Key. The keys are connected by either bridge or causeway and is accessible by a toll road from the mainland. Historically, the islands were used for military fortifications and remnants and a historicalmuseum are present on the main island.

The land that would later become Fort De Soto was first inhabited by the Tocobaga Native Americans from about 1000 to 1500 A.D. They lived on Mullet Key and other barrier islands in the area.  In 1529, the Spanish explorer Pánfilo de Narváez investigated the keys after his expedition landed somewhere between St. Pete Beach and Clearwater. Ten years later, in 1539, Hernando De Soto came ashore somewhere near the southern part of Tampa Bay, beginning the conquest of Florida for the Spanish Empire.

It wasn't until 1849, when a detachment of US Army engineers, including Robert E. Lee, walked and surveyed the area. They recommended Mullet and Egmont Keys become fortified as an appropriate site for coastal defense. Both keys could only be reached by boat since they were islands off the mainland. Although no fortifications had yet been built, Union troops were stationed on the two keys during the Civil War to aid in the Union blockade of Tampa Bay in the early 1860s.  Afterwards, the keys were again abandoned by the military until 1882 when military reservations were officially created on the two keys. However, it would be several years before actual permanent construction would begin.

The main military operation on Mullet Key became Fort De Soto in 1900, named for the previously mentioned Spanish explorer Hernando de Soto. The Army post was officially a subpost of Fort Dade, which was constructed on Egmont Key. These posts were to contain artillery and mortars to protect Tampa Bay from any invading forces. Construction of Fort De Soto began in November 1898 and completed in 1906.  The post was active from 1898 to 1910. At least one company was present at Fort De Soto at all times and many times several units were present. Mosquitoes were a constant problem and the post was very hot in the summers.  

In November 1922 the Army announced it would soon close both Forts De Soto and Dade. On May 25, 1923, the forts were officially abandoned, and only one caretaker remained at each post. A number of tropical storms and hurricanes severely damaged the buildings. A few were destroyed, such as the Battery Bigelow in 1932. The Army attempted to sell the post, but there was very little interest. In September of 1938 Pinellas County bought the areas on Mullet Key for $12,500.

What is a Shoal?

In oceanography, geomorphology, and earth sciences, a shoal is a naturally submerged ridge, bank, or bar that consists of, or is covered by, sand or other unconsolidated material, and rises from the bed of a body of water near to the surface. Often it refers to those submerged ridges, banks, or bars that rise near enough to the surface of a body of water as to constitute a danger to navigation. Shoals are also known as sandbanks, sandbars, or gravel bars. Two or more shoals that are either separated by shared troughs or interconnected by past or present sedimentary and hydrographic processes are referred to as a shoal complex.

Shoals are characteristically long and narrow ridges and can develop where a stream, river, or ocean current promotes deposition of sediment and granular material, resulting in localized shallowing, or shoaling, of the water. Marine shoals can also develop by the in-place drowning of barrier islands as the result of episodic sea level rise or by the erosion and submergence of inactive delta lobes.  Shoals can appear as a coastal landform in the sea, where they are classified as a type of ocean bank, or as fluvial landforms in rivers, streams, and lakes.  A shoal–sandbar may seasonally separate a smaller body of water from the sea, such as; Marine lagoons, Brackish water estuaries, or Freshwater seasonal stream at river mouths and deltas.  A sand bar, or shoal, can range considerably in size, from a length of a few yards in a small stream to marine depositions stretching for hundreds of miles along a coastline, often called barrier islands.

Wave shoaling is the process when surface waves move towards shallow water, such as a beach, they slow down, their wave height increases and the distance between waves decreases. This behavior is called shoaling, and the waves are said to shoal. The waves may or may not build to the point where they break, depending on how large they were to begin with, and how steep the slope of the beach is. In particular, waves shoal as they pass over submerged sandbanks or reefs.  Sandbars, or shoals, form where the waves are breaking, because the breaking waves set up a shoreward current with a counter-current along the bottom of the wave.  Sand carried by the offshore moving bottom current is deposited where the current reaches the wave break. Some sandbars may lie further offshore, representing the break point of even larger waves, or the break point at low tide.  In places of re-entrance along a coastline; such as inlets, coves, and bays (Tampa Bay anyone?), sediments carried by a current will fall out where the current dissipates, forming a spit. An area of water isolated behind a large bar is called a lagoon. Over time, lagoons may silt up, becoming salt marshes.  Below is an example diagram of a buildup of sediment to form a spit, with a lagoon forming behind it.

EarthCache Information:

Parking has been provided at the waypoints below, certain times of the year are more busy than others in this area.

The coordinates above take you directly to an example of the lesson above.  There are multiple examples of shoals and sandbars on the North Beach, but the size of the shoals and sandbars may vary depending on low and high tide.

After completing your geological lessons above, please take some time and enjoy these beautiful beaches.  These are some of the best beaches located in the Tampa/St. Pete area.  There is a toll and a parking fee to get down here, but I believe it is worth it.  Also check out the historical Fort De Soto, which is just south of North Beach.  They have some great preserved history and some very cool areas.

Please remember to be careful out on the shoals and sandbars, you can walk on them but you can quickly become stuck on them if the tides happen to change and you are not paying attention.  Also be aware of rip currents if you decide to swim.

EarthCache Logging Requirements:

Please message me the answers to the questions below!

1. Looking out into the Gulf of Mexico, how many different examples of sandbars are visible from the beach at the posted coordinates?  (This answer may vary depending on low or high tides)

2. Using your GPS, or at home on your Google Maps, approximately how big is the largest visible sandbar? (Just give me a linear size, ex. 750' long)

3. Explain how you think these sandbars were formed based on our lesson above.

4. Describe the sediment deposit on the beach.  (Such as fine sand, or more of a gravel mixture with seashells)

5. Since EarthCaches can now have photo requirements, it is required that you have a picture attached to your log from a visit after this EarthCaches publication date.  I would love to see how far West off of the coast of the North Beach on the shoals you can get.  I was able to walk out about 300' from the posted coordinates on connecting shoals on 12/17/2020.

BONUS Location Questions (Not Required):

Depending on the time of year and the low or high tides, there may be a colored deposit on some of the shoals closer to the beach.  What color is this deposit?

According to the sign near the posted coordinates, what is the common name for the grass-like plants that grow on sand dunes?

That's all folks!

Thank you for taking the time to view and hopefully walk around the beautiful North Beach at Fort De Soto Park.