The sharp ridges separating two glacial valleys are called arêtes.

Here's a bit more about them:

* Formation: Arêtes form as two glaciers erode the landscape on either side of a ridge. The glaciers carve away at the rock, creating steep, narrow valleys. As the glaciers retreat, the sharp ridge between the valleys is left behind.

* Appearance: Arêtes are typically very thin and jagged, with steep, almost vertical sides. They can be quite long and extend for miles.

* Examples: Famous examples of arêtes include the Aiguille du Midi in the French Alps and the Matterhorn in the Swiss Alps.

Let me know if you have any other questions about glacial features!