The ridge in the center, was likely caused by glaciers occupying Green Lakes Valley on the left and Isabelle Valley to the right. These glaciers formed at the head of valleys in what are called cirques. Cirques are bowl shaped depressions scoured into the head of the valley. Alpine glaciers move outward from the cirque to occupy the stream valley.

Figure 5.4 illustrates a mountainous region prior to glaciation. Notice that the stream valleys, especially the tributary valleys have a V-shape to them. As snow accumulates at the head of river valleys mountain glaciers form and move down slope occupying the valleys.

Moving towards lower elevations, they scrape away at the valley sides removing and transporting material down slope. As parallel glaciers erode the sides of the interfluve from opposite sides, the upland narrows into the characteristic sharp knife edge - like shaped ridge.

As the ice recedes, the arete is exposed as shown in the diagram of a landscape after glaciation. Notice the shape of the stream valleys after they have been altered by glacial erosion. They now have the characteristic U-shape.

Figure 5.7 Aerial photo of a portion of the Niwot Ridge arete

Figure 5.8 Topographic map of a portion of the Niwot Ridge arete.

Notice how the ridge is depicted with elongate, enclosed contour lines.