Glaciation of the Coeur d'Alene District, Idaho

Abstract

Cirques and glaciated valleys are present on northern slopes of peaks exceeding 5500 feet in elevation in the Coeur d9Alene District, Idaho. Glacially striated rock surfaces at the tops of cirque headwalls and on the divides above indicate that ice carapaces were present when the cirques were overfilled during glacial maxima. Freshness of landforms and deposits indicates a Pinedale age for the last widespread glaciation. Small ice masses may have developed within the most sheltered cirques during Temple Lake time. Till of Bull Lake age is present on benches and spurs near the head of the South Fork valley. Striated cobbles in coarse gravel as much as 1250 feet above present drainage levels are believed to be evidence of Buffalo glaciation. Following deposition of the gravel, streams eroded laterally before rejuvenation resulted in excavation of new valleys and exhuming of former valleys. It is possible to identify cirques that were occupied during one, two, or more glacial episodes. Continental ice of Spokane (Wisconsin) age impounded Glacial Lake Coeur d9Alene to an elevation of about 2600 feet. Water level was stabilized by a bedrock spillway at Setters on the western side of the lake. Numerous boulders of granite, plucked from bedrock near Pend Oreille Lake, were rafted into the district on ice bergs. Positions of these erratics indicate that postglacial erosion has been extremely slight.