Fossil relative dating activity
Fossil relative dating activity - Adult Chat Rooms
by Andrew Snelling Every major, world-recognized city has its unique landmarks and features.Sydney, Australia’s oldest city (settled in 1788) and largest (more than 3.5 million people), and soon to host the 2000 Summer Olympics, is no exception.
The Hawkesbury Sandstone, named after the Hawkesbury River just north of Sydney, dominates the landscape within a 100 km (60 mile) radius of downtown Sydney.A number of Sydney’s old buildings have walls of sandstone blocks.Today, the Hawkesbury Sandstone is mainly used for ornamental purposes.To obtain fresh sandstone, slabs and blocks have to be carefully quarried.It is a flat-lying layer of sandstone, some 20,000 sq. miles) in area and up to 250 metres (820 feet) thick.(which is chemically very similar to window glass, and harder than a steel file), the sandstone is a hard, durable rock which forms prominent cliffs, such as at the entrance to Sydney Harbour and along the nearby coastline.Despite the widespread, spectacular exposures of the Hawkesbury Sandstone, there is a long history of speculation about its origins, going back to Charles Darwin.
Rather than consisting of just one sandstone bed encompassing its total thickness, the Hawkesbury Sandstone is made up of three principal rock types—sheet sandstone, massive sandstone and relatively thin mudstone.
For example, thin repetitive bands sloping at around 20° within the flat-lying sandstone beds (technically known as cross-beds), sometimes up to 6 metres (20 feet) high, would have been produced by huge sandwaves (like sand dunes) swept along by raging water. However, spectacular fossil graveyards have been found in several lenses (lenticular bodies of only limited extent) of mudstone.
Many varieties of fish and even sharks have been discovered in patterns consistent with sudden burial in a catastrophe. The Hawkesbury Sandstone has been assigned a Middle Triassic ‘age’ of around 225–230 million years by most geologists.
This is based on its fossil content, and on its relative position in the sequence of rock layers in the region (the Sydney Basin).
All of these are placed in the context of the long ages timescale commonly assumed by geologists.
Because of its hardness and durability, the Hawkesbury Sandstone not only provides a solid foundation for downtown Sydney’s skyscrapers, but is an excellent building material.