[Illustration: The Teton Range is a textbook example of fault-block mountain building. The Teton Fault is about 40 miles long. Total vertical displacement was about 30,000 feet. Erosion has removed some 3,000 feet of material from Mount Moran, whose peak now stands about 6,600 feet above the valley floor. Most of the displacement took place with the dropping of the eastern block. The Range’s steep east face eroded faster than the western slope, which still carries some capping sedimentary layers.] Upthrown fault block Gentle western slope Teton Fault Steep eastern face (_Horn-shaped peaks and U-shaped valleys_) Valley floor filled with sediments Down-dropped fault block [Illustration: _As massive Ice Age glaciers flowed through the Teton Range_ (below), _moving ice changed the steep, V-shaped, water-cut valleys_ (above) _into the distinctive U-shaped canyons seen today._] [Illustration: Teton Range today.] Teton Country Lakes [Illustration: Tarn.] [Illustration: Jackson Lake.] With only a brief itinerary in the park you might leave Jackson Hole with a memory of the high peaks and just one lake, much the impression that postcards give. But there are dozens of lakes. Most must be sought off the highway behind fringes of trees or up a short reach of trail. Some nestle in the alpine heights. A checklist of park lakes based on how they were formed includes surprising variety. A few are oxbow lakes, cut off meanders of the Snake River. Two are real oldtimers, Emma Matilda and Two Ocean Lakes, formed about 30,000 years ago as the glaciers melted back. But most are new glistening souvenirs of the latest glacial advance that ended 8,000 years ago. This newest crop is readily identified by the morainal dams that back up each lake. Most easily recognized is the morainal dam of Jenny Lake. Unlike Jackson (large photo), Jenny, Bradley, Taggart, Leigh, and Phelps Lakes mark surfaces gouged by mountain glaciers. The many small ponds dotting the sagebrush flats, such as The Potholes, are not gouge scars, but pits. Here glacial outwash materials surrounded and buried small ice masses that later melted. The technical term for these depression ponds is kettle ponds, but The Potholes were named by a rancher, not a geologist. The Tetons’ highest lakes are called tarns (small photo). Bearing names such as Surprise, Grizzly Bear, Bear-paw, and Rimrock, these are diminutive versions of the glacial lakes at the foot of major canyons. They originate in ice-scoured pockets and are still forming under the small glaciers at the heads of highcountry canyons. The largest and most heavily fished lake is Jackson Lake. Cutthroat trout are native, but lake trout (Mackinaw) were introduced in the 19th century. The lake is 130 meters (425 feet) deep and 26 kilometers (16 miles) long. Jackson, a natural lake, was dammed before the park was established to store more