With the formation of mountains, the rivers that once flowed uninterrupted from Eurasia to the sea were temporarily obstructed. On the southern slopes, however, new rivers were formed as moist winds rising from the tropical sea were trapped by new-forming mountains. The mountains continued to rise; the inclines became steeper, and the rivers cut deeper and deeper into the land.

Still more mountains were created as the Continental Plates continued crunching against each other. As the Himalayas (the name comes from the Nepali words him, meaning 'snow' and alya, meaning 'home of') rose, they were attacked by the elements - wind, rain, snow, and extreme temperatures. During the ice ages, great glaciers sculpted the mountains, sharpening peaks and deepening valleys. During warm interglacial periods, melt water from the glaciers cut deep chasms and carried large amounts of sand and gravel downhill. The formation of the four major mountain systems was the biggest consequence of all this geological activity.

The story has not ended yet as the Himalayas are still growing. The task of calculating the Himalayas' growth rate is complicated, but one estimate is that they have risen more than 1,200 meters in the past 1 ½ million years. Geologists believe that the Himalayas are now rising at the rate of about 8 to 10 centimeters per year. However, as there are also forces such as erosion wearing the mountains down, the net growth of the Himalayas, geologists believe, amounts to only 2.5 to 5 centimeters over the course of a century.

Geologically, the structure of the Annapurna range is quite similar to that of the Himalayas. In addition to being folded, however, the entire mass was subsequently thrust southward over a foothill zone. In some places, it was overturned completely to form one of the highest mountain peaks in the world.

Annapurna's dramatic crests and ridges were chiseled during the last ice age when they were covered with enormous glaciers. When the glaciers retreated, meltdown water rushing down its steep southern slopes dropped vast quantities of sediment into the Pokhara basin. It was these sediments that gave the basin its flat surface, an unusual feature in the Himalayan region. Glacial debris also dammed some rivers in the basin, creating several lakes of which the largest is Lake Phewa.