This document summarizes key information about rock textures, weathering processes, erosion and sediment transport, and the formation of sedimentary rocks. It describes how sandstone has rounded grains making it permeable, while granite has interlocking crystals making it impermeable. It explains different types of weathering including chemical, physical from temperature changes and freezing, and biological from plants and animals. Weathered rock pieces are eroded and transported by water, wind or ice, undergoing abrasion, and deposited in layers as sedimentary rocks. Fossils can form and evidence in the rocks like grain size and rounding shows the deposition environment.

1. 7H Summary Sheets
Rock textures
Rocks are made of grains. Each grain is made of a chemical
called a mineral. The texture of a rock is a description of the
size and shape of the grains.
Sandstone has rounded grains. It is permeable, because water
can get into gaps between the grains.
Granite has interlocking grains.
The interlocking grains are sometimes
called crystals. Rocks with interlocking
grains are not porous.
7
H Weathering
Rocks can be worn away. This is called weathering.
Chemical weathering happens when rain water reacts
with minerals in the rock. Rain water is slightly acidic,
because it contains dissolved gases.
Physical weathering can happen in different ways. The minerals in a rock expand if the rock gets
hot, and contract if it cools. These changes in size can produce strong forces. If the rock is heated
and cooled over and over again, the forces can make cracks in the rock. This is called onion-skin
weathering.
Physical weathering can also happen if water gets into a crack in the rock and freezes. Water
expands when it turns into ice, and makes the crack wider. This kind of physical weathering is
called freeze–thaw action.
Biological weathering is when rocks are broken up or worn away by plants and animals. For
example, plant roots can grow into cracks in rocks and make the cracks bigger.
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2. 7H Summary Sheets (continued)
Erosion and transport
Weathered pieces of rock fall to the bottom of cliffs. This
movement of bits of rock is called erosion. The bits of rock
can be transported away by flowing water, wind and ice
(glaciers). Pieces of rock bump into each other while they are
being transported, and bits get knocked off them. This is called
abrasion. The bits of rock carried by a river are called sediment.
Forming sedimentary rocks
Fast-moving water can move larger pieces of rock than slow-moving water. Rivers slow down
when they flow into a lake or the sea. The slow-moving water cannot carry all of the sediment, so
some of it is deposited on the bottom. Sediments often form layers. Layers of sediment can also
form when sea water evaporates and leaves salts behind. 7
Sometimes dead plants or animals fall to the bottom of the sea. If their remains get covered by H
other sediments they may form fossils. When a dead organism forms a fossil, its form can still be
seen because its hard parts have been turned into stone. Fossils can help geologists find out how
rocks were formed.
If a lot of plant material is buried at once, it may turn into coal. When tiny sea plants and animals
get buried they sometimes turn into oil or natural gas. These are all fossil fuels.
The higher layers squash the lower layers, squeezing out the water from the gaps between the
grains of sediment (compaction). Dissolved minerals in the water can crystallise in the gaps as a
‘glue’ that sticks the grains together (cementation).
Evidence in the rocks
Geologists can explain how sedimentary rocks formed using evidence in the rocks.
Rounded grains in the rock show that the sediments must have been transported for a long time,
because abrasion has smoothed them. Sharp-edged grains show that the grains were probably not
transported for very long.
Small grains show that the grains layers of
sedimentary rock
were deposited by wind (which
can only move very small particles),
or by slow-moving water. Rocks
with large grains, or with a mixture
of grain sizes (such as
conglomerate) must have been
deposited by fast-moving water.
Layers in the rock show that the
river that deposited them
changed speed from time to time.
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