Читать книгу Exploring the Solar System - Peter Bond - Страница 81

As over‐deepened, U‐shaped valleys, gouged rock outcrops, and plains strewn with unsorted boulders or pulverized rocks show, large areas of Earth were once covered with sheets of moving ice. It is clear that, from time to time, extended periods marked by long, cold winters allowed a huge build‐up of snow and ice. Under such ice age conditions, Earth's average temperature may drop by 5°C and polar ice sheets can advance over much of northern Eurasia and North America, covering up to 30% of the planet's land surface.