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ROCK AND SEDIMENT DESCRIPTIONS

Before we can begin a meaningful interpretation of the habitat distribution data, we must first examine the different sediment classifications and some of the inherent problems. Much confusion has resulted from a lack of uniformity in sediment names. These are not merely fine points of semantics. Undoubtedly, it is this confused nomenclature which is partly responsible for many of the misconceptions about the relationships between lobster distribution and substrate.

The basic sediment size classification was developed by Wentworth in 1922. It was modified by the Lane committee (Lane et al. 1947). The modified standard lists 5 grades of sand from very fine to very coarse and subdivides gravels into gravels, pebbles, cobbles and boulders (Fig. 39). This is the scheme which most of the lobster researchers have followed when describing habitat.

Unfortunately, the United States Geological Survey (USGS), in order to be consistent with other federal agencies, has chosen to lump all sediment sizes larger than coarse sand into the gravel category. The state geological surveys generally follow the USGS convention. To make matters worse, the USGS has retained the unofficial designation of "bouldery seabed" to designate areas which are predominately boulders. This means that it is necessary to extrapolate (estimate) from the USGS maps the approximate location of the pebbles, cobbles and boulders.

Additional confusion is created with the rock names for sand and gravel mixtures (Fig. 40). At the USGS, a sample is considered a gravel if it contains more than 50% gravel or larger particles by weight. It is considered a sand if it contains more than 50% sand by weight and it is considered a silt or clay if it contains more than 10% by weight of either of these two components.

To clarify the terminology, the traditional standard for Sedimentary Rocks (Pettijohn 1949) provides the following basic definitions:

Boulder - "a detached rock mass, somewhat rounded or otherwise modified by abrasion in transport and larger than a cobble" with a minimum size of 256 mm (about 10 in).

 

Block - "a large angular fragment showing little or no modification by transporting agencies."

 

Cobble - similar to a boulder, but it is restricted in size from 64 mm (~2.5 in) to 256 mm (~10 in)

 

Pebble - "a rock fragment larger than a sand grain or granule and smaller than a cobble, which has been rounded by the action of water wind or glacial ice. It is therefore between 4 mm (~0.15 in) and 64 mm (~2.5 in)".

 

Gravel - the unconsolidated accumulation of pebbles, cobbles and boulders. Later "gravel" replaced the abandoned term "granule" to indicate grains 2 mm (~0.07 in) to 4 mm (~0.15 in) in size.

 

Sand - aggregate of mineral or rock grains greater than 1/16 mm (~0.0025 in) and less than 2 mm (~0.07 in) in diameter.

 

Silt- less than 1/16 mm (~0.0025 in) and greater than 1/256 mm (~0.00015 in).

 

Mud- a somewhat informal term referring to a mixture of silt, clay, and fine sand with sand usually making up less than 20-50% of the sample depending on the amount of clay. Most marine mudís are land-derived having been transported into quiet water primarily by major rivers.

 

Clay - anything less than 1/256 mm (approx. 0.00015 in)