Image: LakeAtnaDeposits

Description: The bluffs near the confluence of the Gakona and Copper Rivers expose nearly 300 ft of the Quaternary lacustrine, alluvial, and glacial deposits that fill the Copper River Basin. Most of the section seen in this view consists of finely laminated to indistinctly bedded sand, silt, and clay, with or without coarser material, that was deposited in glacial Lake Atna. A few beds contain organic material that has been dated by radiocarbon-age techniques. Outcrops in the lower left foreground (glove for scale) consist of alternating thin beds of coarser and fi ner grained material. The fractured, lighter colored beds above them are varved silt and clay. Beyond the scattered trees, the middle part of the section includes interbedded diamicton, silt, and sand. Some of the poorly sorted diamicton beds were probably deposited by slumping of deposits on the bottom of the lake, but other diamicton beds could represent the advance of glaciers into the lake. The upper 30 ft of the bluff consists of windblown sand and silt, some of which still is accumulating today. The entire section below the windblown material was deposited during the last major glaciation in Alaska between about 58,000 and 9,400 years ago, as dated here, and indicates rapid erosion of the mountains that surround the Copper River Basin.
Title: LakeAtnaDeposits
Credit: Image: https://www.nps.gov/wrst/learn/nature/upload/A-Geologic-Guide-color-version-for-web.pdf, page 79 Caption: Same source, page 78
Author: Henry R. Schmoll
Usage Terms: Public domain
License: Public domain
Attribution Required?: No