![]() A conglomerate with jagged, broken clasts is usually called a breccia, and one that is poorly sorted and without rounded clasts is called a diamictite. These specimens could be called puddingstone. Another name for conglomerate is puddingstone, especially if the large clasts are well rounded and the matrix around them is very fine sand or clay. This type of sedimentary rock forms in a very energetic environment, where rocks are eroded and carried downhill so swiftly that they aren't fully broken down into sand. And a new member of the family, first described from the Moon, is impact breccia.Ĭonglomerate is a sedimentary rock consisting of rounded stones in a fine-grained matrix.Ĭonglomerate could be thought of as a giant sandstone, containing grains of pebble size (greater than 4 millimeters) and cobble size (>64 millimeters). ![]() One created by tectonic activity is a fault breccia. A collapsed breccia forms when rocks are partly dissolved, such as limestone or marble. A volcanic or igneous breccia forms during eruptive activities. A sedimentary breccia arises from things like talus or landslide debris. There are many different ways to make breccia, and usually, geologists add a word to signify the kind of breccia they're talking about. ![]() As a sedimentary rock, breccia is a variety of conglomerate. It is safest to think of brecciation as a process rather than breccia as a rock type. It contains sharp, broken clasts while conglomerate has smooth, round clasts.īreccia, pronounced (BRET-cha), is usually listed under sedimentary rocks, but igneous and metamorphic rocks may become shattered, too. This specimen, from Upper Las Vegas Wash in Nevada, is probably a fault breccia.īreccia is a rock made of smaller rocks, like a conglomerate. It looks like the tarry stuff with which roads are built, but it weighs much less and is softer.īreccia is a rock with sharp angular clasts in a fine-grained groundmass. The specimen in the photo above came from a petroleum seep near McKittrick in the heart of California's oil patch. Now, natural asphalt only has value as a geological specimen. In the 1800s, asphalt deposits were mined for use on city roads, then technology advanced and crude oil became the source for tar, manufactured as a by-product during refining. In the past, asphalt was used as a mineral form of a pitch to seal or waterproof items of clothing or containers. A harder rock with this composition is called a bituminous sandstone or, more informally, tar sand. It has a mild petroleum odor and can be crumbled in the hand with some effort. Its underside is pitch-black, but it weathers to a medium gray. Geologists use the word "asphalt" to refer to what most people call tar, so technically this specimen is asphaltic sand. It flows slowly during warm weather and may be stiff enough to shatter during cold times. Many early roads used mined natural asphalt for pavement.Īsphalt is the heaviest fraction of petroleum, left behind when the more volatile compounds evaporate. The granite that gave rise to it is exposed directly underneath it and is more than a billion years older.īlack, pitchy natural asphalt from a petroleum seep near McKittrick in the heart of California's oil patch.Īsphalt is found in nature wherever crude oil seeps from the ground. This arkose specimen is of late Pennsylvanian age (about 300 million years old) and comes from the Fountain Formation of central Colorado-the same stone that makes up the spectacular outcrops at Red Rocks Park, south of Golden, Colorado. But whereas graywacke forms in a seafloor setting, arkose generally forms on land or near shore specifically from the rapid breakdown of granitic rocks. This type of sedimentary rock is similar to graywacke, which is also a rock laid down near its source. Arkose usually has a reddish color from feldspar, clay, and iron oxides-ingredients that are uncommon in ordinary sandstone. ![]() Its mineral grains are generally angular rather than smooth and rounded, another sign that they have been transported only a short distance from their origin. ![]() This reddish rock is arkose, a young feldspathic sandstone.Īrkose is a raw, coarse-grained sandstone deposited very near its source that consists of quartz and a significant proportion of feldspar.Īrkose is known to be young because of its content of feldspar, a mineral that usually degrades quickly into clay. ![]()
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