SANDSTONES, PEBBLY SANDSTONES AND CONGLOMERATES
Sandstones, pebbly sandstones and conglomerates can be distinguished from all other rock types by their grainy appearance. Individual grains are visible to the naked eye and, in most cases, the rocks are very gritty to the touch. Grains in sandstones range upward to about two millimeters in diameter (about the diameter of the "lead" in a common pencil) and pebble size in pebbly sandstones and conglomerates ranges up to several inches.
The basic criterion for distinguishing between different kinds of sandstone is the character of the grains. It is difficult, in most cases, to determine the attributes of individual grains, but the properties of the grains influence the color and hardness of the rock and these two characteristics are readily observable. Based on hardness and color, four main types of sandstone can be recognized: (1) gray sandstone, (2) crystallized sandstone, (3) hard sandstone and (4) carbonate cemented sandstone.
Gray Sandstones are a mixture of dark and light grains yielding a rock that is gray in color and usually not as hard as other kinds of sandstone. Hardness may be determined by breaking the core and by examining the edge of the broken core. The edge of gray sandstone core is relatively weak and crumbles either to the touch or with minor impact. Cores of other sandstone types fracture with sharp edges or shatter into small sharp fragments.
Crystallized Sandstones are white and sugary and, in many cases, very hard and brittle. In some examples of this type, the grains are welded together and the cored rock has a smooth, polished appearance (e.g. 23R).
Hard Sandstones have characteristics intermediate between gray sandstones and crystallized sandstones, being light to dark gray in color like gray sandstones, but hard and brittle like crystallized sandstones. Some examples of hard sandstones are specimens 24L, 24R, and upper 28R (this is the broken butt end of a hard sandstone core). This butt end should be compared to lower 28L, which is a mixture of light gray, dark gray and tan grains. It should also be compared to the pure white grains in the crystallized sandstone specimen (e.g. upper 28L).
Carbonate Cemented Sandstones are relatively rare and usually are gray sandstones in which the grains are held together with either calcium or iron carbonate cement. Cements of this kind make the core hard and often produce a brown or yellow stain on the sides of the core (e.g. 26R). Calcium carbonate cement can be detected by effervescence (bubbling) when 10% hydrochloric acid is applied to the rock. Iron carbonates do not effervesce readily but have a deep brown, sugary appearance on the broken butt end of the core (e.g. lower 28R).
The second property for differentiating among sandstones is beddinga property which is visible on the side of the core. There are five typesmassive, crossbedded, streaked with shale, rooted and burrowed. Examples of massive bedding in gray, crystallized, hard and carbonate cemented sandstone are specimens 19L, 22L, 25L, and 26L, respectively.
The most common bedding type in sandstone consists of thin streaks or bands of shale. If the streaks are inclined to the side of the core, the rock is "crossbedded" (e.g. 29L), but if the streaks are at right angles to the side of the core or nearly so, the rock is designated as a sandstone with shale streaks. Specimen 31L is an example of a Gray Sandstone with Shale Streaks and 34L is an example of a Crystallized Sandstone with Shale Streaks. In some cases it is desirable to describe different kinds of shale streaks. The most common type is "rippled", (e.g. 31R) and a less common type is designated "flat" (e.g. 33R).
Rooted and burrowed bedding are less common than other bedding types and both are the result of organic activity on the sediment roots record plant activity and burrows result from the action of burrowing invertebrate organisms (e.g. clams and worms). Both produce a spotted or mottled appearance to the rock (e.g. 36L and 38L), but roots can be distinguished from burrows by the presence of thin films of coaly matter which represent remnants of the plants and which are not present in animal burrows.
Pebbly Sandstones and Conglomerates
Criteria for identifying different kinds of pebbly sandstones and conglomerates are the character of the sand between the pebbles and the kinds of pebbles themselves. The character of the sand between the pebbles is the same as the different kinds of sandstones1) gray (2) crystallized, (3) hard and (4) carbonate cementedbut gray sandstone is the most common. The most common pebble types include coal spars, coal bands (which are made up of smaller coal fragments occurring as distinct layers), shale pebbles, ironstone pebbles, quartz pebbles and rock pebbles. Coal spars (e.g. 39L) are shiny, irregularly shaped pieces of bright coal, whereas coal bands occur as distinct layers (e.g. 39R). Shale pebbles are usually dark gray or black (but not shiny like coal) with frayed or worn edges like specimen 40L. Ironstone pebbles, illustrated on 40R, are usually oval or round and have a distinct orange-brown color. Quartz pebbles are white or light gray and are often best observed when the core is wet like those in specimen 43R. Rock pebbles are usually harder than shale pebbles (compare 41R and 40L) and lack frayed boundaries. Rock pebbles are rare but, when present, may include granite, gneiss, phyllite, slate, chert and volcanics. Pebble types can occur in all possible combinations but usually are found in one of the following groups:
Coal bands, with or without some coal spars
Shale pebbles, with or without some coal and/or quartz pebbles
Ironstone pebbles, with or without some coal and/or quartz pebbles
Shale and ironstone pebbles, with or without some coal and/or quartz pebbles
Rock pebbles, with or without some coal and/or quartz pebbles
Quartz pebbles, with or without some coal
A key for identifying the commonly occurring types of sandstones, pebbly sandstones and conglomerates is given in Table 2.