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Chalk is so common in Cretaceous marine beds that the Cretaceous Period was named for these deposits. The name Cretaceous was derived from Latin creta, meaning chalk. [10] Some deposits of chalk were formed after the Cretaceous. [11] The Chalk Group is a European stratigraphic unit deposited during the late Cretaceous Period.
The White Chalk Subgroup includes what were formerly designated the Middle Chalk and Upper Chalk Formations, together with the Plenus Marls (topmost part of the former Lower Chalk Formation). In the southern province it is divided in the following way (youngest/uppermost at top):
The cliff face, which reaches a height of 350 feet (110 m), owes its striking appearance to its composition of chalk accented by streaks of black flint, deposited during the Late Cretaceous. The cliffs, on both sides of the town of Dover in Kent, stretch for eight miles (13 km). The White Cliffs of Dover form part of the North Downs.
The density of limestone depends on its porosity, which varies from 0.1% for the densest limestone to 40% for chalk. The density correspondingly ranges from 1.5 to 2.7 g/cm 3 . Although relatively soft, with a Mohs hardness of 2 to 4, dense limestone can have a crushing strength of up to 180 MPa . [ 13 ]
Flint, occasionally flintstone, is a sedimentary cryptocrystalline form of the mineral quartz, [1] [2] categorized as the variety of chert that occurs in chalk or marly limestone. Historically, flint was widely used to make stone tools and start fires. Flint occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones.
This part of Scotland largely comprises ancient rocks, from Cambrian and Precambrian times, that were uplifted to form a mountain chain during the later Caledonian orogeny. These foundations are interspersed with many igneous intrusions of more recent age, the remnants of which have formed mountain massifs such as the Cairngorms and Skye Cuillins.
Some chalk mines were extensively large, with passages up to 25 feet (7.6 m) high and 15 feet (4.6 m) wide, their passages taking the form of a Norman arch. Because of chalk's softness, picks and shovels were used to excavate tunnels. Stepped slabs were dug into the chalk, allowing many miners to dig at the same time. Care had to be taken to ...
Between one-half and two-thirds of the world's population live or work in buildings made with clay, often baked into brick, as an essential part of its load-bearing structure. Clay is a very common substance. Shale, formed largely from clay, is the most common sedimentary rock. [9]