The Beaches of Toledo Bend
Beaches are a common feature of a coastline. Beaches are made up of eroded material that has been transported from elsewhere and deposited by the sea or water.
Constructive waves help to build up beaches. The material found on a beach (i.e. sand or shingle) depends on the geology of the area and wave energy.
A beach is formed when deposited materials carried by waves are accumulated in a zone along the coast. The materials on the beach vary in size from fine sand to pebbles. The waves sort these materials according to the weight. The finer sand deposited nearer the sea and the coarser materials further inland.
What is the Sand made of?
On many beaches, most of the sand (not including seashells) is made of the minerals quartz and feldspar. These grains ultimately came from igneous and metamorphic rocks that typically are very old. Quartz, the most common mineral, is composed of silicon dioxide, while feldspar, the second most common mineral, is made up of sodium, calcium, or potassium combined with silica. Quartz is the most common mineral in many beaches because it is very hard and durable, and survives both transport by rivers to the coast and reworking by waves better than any other common mineral.
Beach Color
Beaches come in many colors: white, black, tan, yellow, red, and more. Basically, beach material is small pieces of rocks that have eroded and have been carried to the beach via the wind and water. The geology of the surrounding area determines the color of those rocks
What Determines the Size of Sand or Pebbles on a Beach?
Two things determine the size of sand on beaches: the energy (height) of the waves, and the size of the material furnished to the beach. Other things being equal, the higher the average waves on a beach, the coarser the sand. This is because higher waves tend to move the smaller grains offshore to quieter water. Dune sand is consistently finer grained than the adjacent beaches even though the dune sand is derived from winds blowing across the beach. The difference in size is due to the fact that winds generally only pick up and carries the finer grains of beach sand and leaves the heavier, larger sand grains behind.
Grain size determines the slope of the beach. Beaches with finer grain sizes tend to be flatter with gentler slopes (between the high and low tide line) than beaches with coarser grains. The reason for this has to do with the ability of sand to absorb water (porosity and permeability) from wave swash. Fine sand absorbs relatively little water and most of the water that flows up the beach also flows back down the beach. This backwash tends to move sand in a seaward direction and thus flattens the beach. Coarse sand or gravel on a beach absorbs much more of the wave swash and more water moves up the beach when a wave breaks than moves back down toward the sea
Natural erosion and accretion
Beaches are changed in shape chiefly by the movement of water and wind. Any weather event that is associated with turbid or fast flowing water, or high winds will erode exposed beaches.
Longshore currents will tend to replenish beach sediments and repair storm damage. Tidal waterways generally change the shape of their adjacent beaches by small degrees with every tidal cycle.
Over time these changes can become substantial leading to significant changes in the size and location of the beach.
To Log this Earthcache:
At the posted coordinates you will see an information sign about the history of Toledo Bend and the total area. You can also get much-needed information from the description.
You will need to visit three locations along the beach
Wave point 1: N 31 29.275
W 093 42.466
Answer these three questions:
- What is the size and color of the sand here?
- Are the minerals igneous or metamorphic?
- Estimate how far you are from the water.
Wave point 2: N 31 29.269
W 093 42.455
Answer these three questions:
- What is the size and color of the sand here?
- Are the minerals igneous or metamorphic?
- Is the sand being deposited or eroded? Explain what Earth process do you think is at work here.
Wave point 3: N 31 29.254
W 093 42.446
Answer these three questions:
- What is the size and color of the sand here?
- Are the minerals igneous or metamorphic?
- Look to the west – do you see a pattern in the water/sand? Describe it and the process that is used to form it.
Final Questions & Optional Picture
- How are the minerals found at wave point one different than wave point three?
- Describe how you think these minerals arrived at their locations.
- Optional, take a picture at wave point three.
Sources
http://www.bbc.co.uk/schools/gcsebitesize/geography/coasts/depositional_landforms_rev1.shtml
http://coastalcare.org/educate/beach-basics/
http://www.seafriends.org.nz/indepth/index.htm