sedimentary structure

• Sediments are deposited to form horizontal layers known as beds.
• A bed is a single unbroken accumulation of sediment.
• Beds vary in width from 1cm to tens of metres.
• Each bed is separated by a surface called bedding planes.
• Sedimentary structures are features found on bedding planes or within beds themselves.
• Sedimentary structures can indicate the environment the sediment was deposited in and can also tell you if the bed is the correct way up or upside down.
• Ripple marks form with the movement of sand in high energy conditions.
• Symmetrical ripple marks are from the bi-directional current and are viewed on bedding planes.
• Asymmetrical ripple marks are formed by a uni-directional current.
• Cross bedding is sand grains moved by saltation in a river along the asymmetrical ripple marks.
• Sand dunes behave in the same way, called large scale cross bedding, the whole dune moves in the direction of the wind.
• Dessication cracks form in wet mud, the water evaporates due to heat and the mud contracts, this forms polygonal blocks separated by cracks.
• The cracks are V-shaped and are wider near the surface due to the greater rate of evaporation because it receives more solar energy.
• Salt pseudomorphs form when halite crystals form from the evaporation of a lake, the lake refills and the crystals are disturbed and dissolved, the sediment entering the lake moulds to the shape of the indent of the dissolved crystals.
• Imbricate structure is when the current rolls the pebbles via traction so the flat pebbles are pointing in the direction of the current.
• Flute casts are found at the bases of beds, they form due to the erosion from a turbulent flow.
• load cast and flame structure

  
• flute cast

  
• imbricate structure

  
• salt pseudomorphs

  
• desiccation cracks

  
• graded bedding

  
• cross bedding

  
• ripple marks