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A glacier is a huge block of ice that flows very slowly over land. Glaciers are formed in the North Pole and South Pole and in high mountains. The cold climate and low temperatures in these areas help large amounts of ice to collect and turn into ice blocks.

Glaciers begin to form when snow falls during the winter months. The snow builds up in layers and these layers get heavy because of their weight. Gradually, the ice becomes so thick that it begins to move because of its own weight and starts moving down the mountains. This happens due to the Law7 of Gravity. As the ice moves down the slopes mountains, it start melting slowly due to the heat of the Earth.

As glaciers move downwards, they also change the shape of the land they move on.

Most glaciers flow very slowly and move less than 30 centimetres a day. The various parts of a glacier move at different speeds. The central and upper part of a glacier flow the fastest. The sides and bottom move more slowly because they rub against the walls and floor of the valley.

The world's most famous glaciers are in Europe. They lie in the French and Swiss Alps.

They are Mer de Glace and the Aletsch Glacier. There are glaciers in North America as well, such as the Malaspina Glacier in Alaska.

The _____ and _____ part of a glacier flows the fastest.

A)  bottom, side                 

B)  central, upper

C)  centre, side                   

D)  upper, side

E)  None of these