As the original molten mass of the earth cooled to form a
hard, dense crust, igneous rock was formed.
This contained all the mineral elements to form sand, clays,
silts, chalk, etc.,and under erosion and weathering formed
sediments of these materials.
These sediments under high pressure over a long
length of time created sedimentary rocks,i.e., sandstone,
chalk, limestone, etc. Under the action of extreme heat
(from phenomena such as volcanic activity)
and exceptionally high pressures these rocks could change,
metamorphose, to form metamorphic rocks – limestone
changed to marble, clay deposits metamorphosed to slate.
As the earth continued to cool and shrink it ‘crinkled’ (like
a drying orange) to form hills and mountains, and these
sedimentary and metamorphic rocks were again attacked
by weathering and erosion. Vegetation and forests grew on
some of the sedimentary rocks, and as the forests decayed
they formed layers of peat which were sometimes metamorphosed
to coal and other deposits