The size of the oolite reflects the time they have had exposed to the water before they were covered with later sediment. The oolites are commonly found in large current bedding structures that resemble sand dunes. Strong intertidal currents wash the 'seeds' around on the seabed, where they accumulate layers of chemically precipitated calcite from the supersaturated water. The mechanism of formation starts with a small fragment of sediment acting as a 'seed', e.g. They are usually formed in warm, supersaturated, shallow, highly agitated marine water intertidal environments, though some are formed in inland lakes. Oolitic hematite occurs at Red Mountain near Birmingham, Alabama, along with oolitic limestone. Dolomitic and chert ooids are most likely the result of the replacement of the original texture in limestone. The term oolith can refer to oolite or individual ooids.Ooids are most commonly composed of calcium carbonate (calcite or aragonite), but can be composed of phosphate, clays, chert, dolomite or iron minerals, including hematite. Strictly, oolites consist of ooids of diameter 0.25–2 mm rocks composed of ooids larger than 2 mm are called pisolites. The name derives from the Ancient Greek word for egg. The sediments left behind from Lake Idaho are known as the Chalk Hills and Glenns Ferry Formation.Oolite or oölite (egg stone) is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. Captured by the Snake River, the waters drained out in a massive flood that gouged Hell's Canyon. Geologist think that 2 to 4 million years ago, water from melting glaciers caused Lake Idaho to overflow to the west. Thousands of feet of sediment were deposited on the Lake's bottom over its 6.5 million years of existence, interrupted at times with layers of basalt and volcanic ash (tuff) from eruptions of adjacent volcanoes. Some 200 miles long and 35 miles wide, Lake Idaho drained south into Nevada. Now a valley, the Plain became a basin for Lake Idaho. Here, the earth's crust was pulled apart, northeast to southwest, and was stretched thin like taffy.Īs crustal extension progressed between 11 and 9 million years ago, the Owyhee Mountains and the Boise Front responded by rising to their present height along faults bordering the rift. Few other lands in Idaho support such a rich suite of rare species in such a small areaĮxtending across southwest Idaho between the Owyhee Mountains and Boise Front is the broad valley of the western Snake River Plain.Įvidence indicates that the Plain began as a continental rift about 12 million years ago. The physical and chemical properties of the Shoofly Oolite provide the foundation for the unique set of plants and fossils found here. Rivers and fans at the base of the adjacent mountain slopes later deposited a veneer of alluvium over the lake deposits. ![]() Siltstone, deposited by lake waters, forms the stratum above it. In some places, the upper surface of the oolite has been sculpted into hummocks, small arches, and intriguing shapes.īeach sands of varying thickness underlie the oolite. Small, isolated deposits are exposed discontinuously across the 40 miles between this spot and Murphy. Wave action that varied with the seasons, the weather, and the types of sediment in the water washed the ooids back and forth in the shallows on the southwestern side of Lake Idaho, depositing them from 2 to 40 feet thick on steeper benches near the shore.Įrosion carried away softer siltstone and volcanic tuffs but left the more resistant oolite to weather above the mudflats. Most other examples of ooid formation and deposition are found in wave-agitated sea waters or on the beds of much saltier lakes. ![]() The Shoofly Oolite is one of the largest freshwater lakebed oolites known in the world. Oolite is sedimentary limestone composed of tiny ooids, which form when calcium carbonate precipitates in concentric layers around individual grains of sand. The natural sculpture garden is a section of the Glenns Ferry Formation called the Shoofly Oolite.
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