Beaches of the Gulf Coast

3. Beach Materials, Structures, and Sources

3 Beach Materials, Structures, and Sources T HE most common term associated with the beach is sand. True, most beaches are predominantly sand, but there are many other kinds of materials that can also be present in large amounts at some locations. In fact, the term sand denotes only grain size; it tells us nothing about the composition of the particles. Sand can be composed of a wide range of minerals. This chapter discusses the range of materials that constitute beaches: their textures, composition, and origins. This information will give us a much more comprehensive appreciation of the beach environment. Beach Textures Sand is a particle that is between 0.0625 mm and 2.00 mm, or about 1/16 inch. This range of particle size is part of a comprehensive size classification called the Wentworth Grain Size Scale (table 3.1). Some of the terms for grainsize categories in this classification are quite recognizable, but they also have specific quantitative definitions. For example, the term boulder has a specific definition: any particle between 256 mm and 1048 mm or about 10 inches and larger. The terms cobble, pebble, silt, clay, and mud also have specific quantitative size ranges. Beaches can be composed of boulders, cobbles, or sand (figure 3.1). Two grain-size terms warrant special explanation. One is gravel, which refers to all grain sizes larger than sand (>2.0 mm). Most of the time we think of pebble-sized gravel, but the term is not restricted to that category. The other commonly used term is mud, a mixture of silt and clay. It is important to remember that these two commonly used terms have specific definitions according to particle size (figure 3.2). 47 48 General CharaCTerisTiCs and dynaMiCs of BeaChes We typically speak about beach sediments by using their mean or average grain size. When a beach sediment is described as medium sand, the average grain size is in that range. This can be expressed in millimeters, but that approach is a little messy in terms of the very large range of numbers. In 1934 a professor at Northwestern University, William Krumbein, suggested using the – log2 as a better way to express grain size. That means every size category is doubled or half of the adjacent one.This approach provided grain size in singledigit numbers called φ units. Another important textural aspect of beach sediments is their sorting. This is a quantitative expression of the uniformity of grain size at a particular location or in a specific sample of beach material. In a statistical sense, it is the Ta B l e 3 . 1 . W E N T W O RT H G R A I N S I Z E S C A L E mm ø units Size class 2048 –11 Very large 1024 –10 Large 512 –9 Medium Boulders 256 –8 Small 128 –7 Large Cobbles 64 –6 Small 32 –5 Very coarse 16 –4 Coarse 8 –3 Medium Pebbles 4 –2 Fine 2 –1 Very fine Granules 1 0 Very coarse 1/2 +1 500 µm Coarse 1/4 +2 250 µm Medium Sand 1/8 +3 125 µm Fine 1/16 +4 62 µm Very fine 1/32 +5 31 µm Very coarse 1/64 +6 16 µm Coarse 1/128 +7 8 µm Medium Silt 1/256 +8 4 µm Fine 1/512 +9 2 µm Very fine Clay G R AV E L M U D 49 MaTerials, sTrUCTUres, and soUrCes standard deviation of grain-size measurements. People who study sediments conduct grain-size analyses and generally plot their distribution on graphs, either in bar graph form or as a linear curve (figure 3.3). If the distribution is narrow and the curve is narrow and steep, the sediment is well sorted(figure 3.4). When the distribution is wide, the sediment is poorly sorted. Many beaches along the Gulf of Mexico are a combination of two types of materials: typically shell gravel and sand. This grain-size situation is called Figure 3.1. (a) Boulder beach in the Bay of Fundy, Canada. (b) Cobble beach on the island of Tahiti. (c) A sand beach on the Florida coast. (a) (b) (c) 50 General CharaCTerisTiCs and dynaMiCs of BeaChes bimodal because each of the two types has a mode, or the most common grain size (figure 3.5). Each may be well sorted, but the sample as a whole...