ogive

Some valley glaciers carry on their surface a regular pattern of thin dark lines, a series of concentric semicircles or arcs all pointing downhill. Unlike the single dark trace of a medial moraine, the streaks of these chevronlike marks are perpendicular to the flow of the ice, and a set of them might consist of dozens of evenly spaced lines. The dark lines are called ogives, after ogee, an architectural term for a type of curve well known from pointed Gothic arches. Ogives—they bow convexly downhill because the center of a valley glacier is often flowing faster than its sides—frequently turn up downstream from an icefall. The stretching and fracturing of a glacier that occurs at an icefall (where there is a sudden change of elevation), together with the normal seasonal pattern of ice flow—it flows faster in the summer and with less surface melting in the winter—leads to a combination of conditions that actually creates two ogive patterns. The dark ogives mark the heights in an undulating sequence of transverse valleys and ridges, some of which might be seventy-five feet high. The dark/light pattern is called a band ogive, the ridge/valley pattern is called a wave ogive. Alaska’s glacial landscapes are full of ogives. Its many rivers of ice let you know, with these patterns, that despite its stolid appearance the ice is alive, carving its powerful way through the land.

Gretchen Legler