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  A
gravity dam is made from concrete or masonry, or sometimes
both. It is called a gravity dam because gravity holds it
down to the ground stopping the water in the reservoir pushing
it over.
 A
cross-section (or slice) through a gravity dam will usually
look roughly triangular.
Gravity dams are suited to sites
with either wide or narrow valleys, but they do need to be
built on sound rock.
There are more than 250 gravity dams
in Britain. Masonry was used in many early dams, as far back
as the 17th Century. However, concrete became more common
from about 1900.
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