Question

During cold periods called ____________________, large parts of Earth’s surface were covered with sheets of ice.

Answer 1

Ice age i think because it sound like it

Answer 2

During cold periods called 'ice age', large parts of Earth’s surface were covered with sheets of ice.

Explanation:

Ice age refers to the coldest period on earth.

The ice age was the period in which ice glaciers and sheets expand across the earth. That's why it is known as the cold periods.

The complete statement is: During cold periods called ice age, large parts of Earth’s surface were covered with sheets of ice.