Geologic Trip, Lake Tahoe Area
The photograph looks across Emerald Bay toward Lake Tahoe, just beyond the inlet to the bay. Lake Tahoe lies in a down-dropped block that was formed during the Plio-Pleistocene uplift of the Sierra Nevada. While the mountains were being uplifted, the Tahoe block was dropped about 5,000 feet along a series of large faults that splayed and opened to the north. This fault-bound valley was drained by a north-flowing river. Later, the north part of this valley was filled with a thick pile of volcanic rocks that dammed the river valley. The waters of Lake Tahoe are held in place by this dam. These volcanic rocks extend from Tahoe City and Incline Village north to Sierraville.
Emerald Bay was formed by a glacier during the last glacial episode. The glacier began in the Desolation Wilderness ice field. When the glacier reached Lake Tahoe, it crossed the large boundary fault on the west side of the Tahoe basin and abruptly entered the lake. Upon entering the lake, the glacier began to melt and dump its load of glacial debris along the shoreline. When the glacier retreated, it left the large lateral moraine that now forms the southeast side of Emerald Bay and the recessional moraine that nearly closes the mouth of the bay. . Fannette Island in the center of the bay is an outcrop of granite that rises 150’ from the water.