Geology Photos 2: Sedimentary Rock and Structures

Geology Photos 1: Folds, Faults
Geology Photos 2: Sedimentary Rock and Structures
Geology Photos 3: Fossils
Geology Photos 4: Igneous Rocks

Sedimentary Rocks and Structures

These layers of sediment are quartz-rich sandstones of the Lonan Formation, Manx Group. The sedimentary layers would originally have been deposited horizontally but have subsequently been tilted nearly vertical by the Caledonian mountain-building episode. In the foreground, a green-grey, structureless igneous rock has intruded into the sediments. This occurred when the sediments were still buried deep underground and when the igneous rock was still molten.  
Alternating layers of red, pebbly sandstone with laminated sandstone. These are part of the Peel Group and were deposited in a generally arid environment around 390 million years ago and may represent short-lived braided river deposits.
Layers of thick limestone beds alternate with the thin shale partings. The alternations may have formed as a result of regular changes in the orbit of the earth around the sun which affect the earth’s climate and cause different rock types to be deposited. Both rock types are Carboniferous in age (340 million years old).
The red, sub-horizontal pebbly sandstones have been deposited unconformably on top of the steeply dipping layers of the Manx Group at the beginning of the Carboniferous period, 340 million years ago. They were probably deposited by short-lived rivers in a tropical climate after flash-floods.
The pale brown rock in the centre and upper part of the photo is a sandstone of the Dalby Group. The darker rocks below are mudstones. Both rock types were laid down horizontally on the sea bed around 425 million years ago. This photo shows that these layers have been disrupted soon after deposition by the heavy sand sinking into the soft layers of mud beneath it, forming this load structure.
The white speckles within the red Peel sandstones are calcite concretions. They formed by the precipitation of calcium carbonate within soil known as calcrete. This probably occurred in an arid environment around 380 million year ago.