Old shapes and young ones
Why Are Hills Round and Mountains Pointed?
A rounded hill is usually just a very old mountain. Everything sharp about it has been worn off.
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Step 1 of 5
Two things are happening at once.
Land is being pushed up, and weather is wearing it down. Which one is winning at any moment decides whether the landscape is getting sharper or softer.
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Step 2 of 5
Steep ground wears fastest.
The steeper a slope, the harder rain and gravity work on it. That means sharp shapes are attacked hardest, and a landscape naturally softens over time.
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Step 3 of 5
Water gets into cracks and freezes.
Water in a crack expands when it freezes and levers the rock apart. Repeat it every night for thousands of winters and mountain peaks are taken to pieces.
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Step 4 of 5
Ice carves; rivers round off.
Glaciers carve steep-sided valleys and sharp ridges while they are there. After the ice goes, rivers and soil slowly smooth everything the ice left behind.
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Step 5 of 5
So a rounded landscape is an old one.
Ranges that are still rising are jagged. Rounded hills are usually the stumps of ranges that were once as high as anything on Earth and stopped growing long ago.
Some of the gentlest hills in the world are the roots of a mountain range that was once as tall as the Himalayas.
The short version
Mountains are sharp while they are still rising, and weather rounds them into hills once they stop.
Try it yourself
Leave a heaped spoon of sugar out and mist it with water a few times. The heap loses its edges first.