How do sedimentary layers start out

When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation. The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form. The crystals stick the pieces of rock together.

Each are formed in a different ways. Sedimentary rocks contain rounded grains in layers. Chalk, shale and limestone are examples of sedimentary rocks. This activity is a very simple demonstration of how how sedimentary rocks are formed. Sedimentary rocks are formed when layers of sediments build up over time and harden to form a rock with many layers. Imagine a river flowing over stones breaking them down, carrying them along until they settle.

Picture a layer of sand building up over the stones, followed by layers of mud, more rocks or gravel and sand. Build up the sandwich layer by layer, add some sliced tomatoes or even raisins and chocolate chips to resemble larger rocks.

The sedimentary sandwich is a very basic demonstration of how sedimentary rocks form but remember the real process can take millions of years! Layers of broken rock transported from rivers build up at the bottom of the ocean and build up in layers called sediments. This is known as sedimentation. Bedding can form in almost any depositional environment. Cross-bedding is bedding that contains angled layers and forms when sediments are deposited by flowing water or wind. Some examples are shown in Figures 6.

Cross-beds in streams tend to be on the scale of centimetres to tens of centimetres, while those in aeolian wind deposited sediments can be on the scale of metres to several metres. Cross-beds form as sediments are deposited on the leading edge of an advancing ripple or dune.

Each layer is related to a different ripple that advances in the flow direction, and is partially eroded by the following ripple Figure 6. Cross-bedding is a very important sedimentary structure to recognize because it can provide information on the direction of current flows and, when analyzed in detail, on other features like the rate of flow and the amount of sediment available.

Graded bedding is characterized by a gradation in grain size from bottom to top within a single bed. Some graded beds are reversed coarser at the top , and this normally results from deposition by a fast-moving debris flow see Chapter Most graded beds form in a submarine-fan environment see Figure 6.

Ripples , which are associated with the formation of cross-bedding, may be preserved on the surfaces of sedimentary beds. Ripples can also help to determine flow direction as they tend to have their steepest surface facing down flow.