After the flood and during the Ice Age, land bridges appeared that connected the Middle East with all parts of the world. The waters in the ocean may have been fully frozen, allowing people to cross over during the ice age . Between Alaska and Russia is the Bering Strait, and most likely all people who live on the North and South American continents crossed over in that area. Another plausible theory is that people built boats and sailed to America (remember that Noah built a huge ark, so that knowledge could have been passed on to his relatives).
During the Flood, the “fountains of the great deep” (Gen. 7:11) opened up. This is generally understood to be underwater volcanic eruptions. The volcanoes would have released a large amount of smoke, ash, and particles into the atmosphere, preventing any sunlight from entering, and the heat emitted in the oceans would have also warmed the waters. Movements of the Earth’s crust would release heat to warm the water. Evaporation of the warm water would have caused a great accumulation of snow, especially since the air was cooler and the sun couldn’t melt the snow. The lack of sunlight from the smoke would make the continents cooler and the snow wouldn’t melt until the volcano smoke went away thus allowing the sun to heat the earth again. Yes, there was one ice age, and it lasted for several hundred years, but it only covered the northern parts of the European and American continents. For more information and pictures, see the following link: http://www.answersingenesis.org/articles/nab/where-does-ice-age-fit