28 Smithsonian Exploration in Alaska in 1904 



valleys widen into basin-like expanses there are typical lacustrine 

 deposits. They are made up of unconsolidated grayish colored 

 sands and clays sometimes underlain by gravel beds which appear 

 to be of contemporaneous age. These silts may usually be dif- 

 ferentiated from the Recent alluvium by their lighter color, which 

 also suggests their fluvio-glacial origin. As recognized and de- 

 scribed by observers the Pleistocene occurs as deposits of gravels, 

 sands, and clays distributed throughout the drainage basins. In 

 Alaska, as in Siberia, no inter-Glacial deposits appear to occur, and 

 the relations of the fluviatile and lacustrine deposits to the glacial 

 accumulations of the mountains, beyond the general fact that most 

 of the materials of the former are the down-stream residual of the 

 latter, cannot be stated at present. 



3. derivation, nature, distribution 



The events that lead up to their deposition appear to have been 

 a period of general erosion, for the unconformable underlying sur- 

 faces of the older rocks are swept clean, and then a time of land de- 

 pression making transportation and deposition the chief work of the 

 rivers. This subsidence diminished the flow of the drainage, re- 

 ducing the current to such an extent as to form lakes where there 

 was room for them. Fluvial deposits were laid down along the 

 valleys and lacustrine silts in the expanded basins. Some of the 

 drainage courses were obstructed at intervals by elevations of the 

 older strata and others by flows of basalt that appear to rest directly 

 on the eroded surfaces of the older rocks, thus demonstrating their 

 age, though relatively young, to be older than the silts. In the 

 basins thus formed the finer silts were laid down, while in some 

 cases there appear to have been areas of relatively quiet water 

 caused by the channels through the extensive flood plains clogging 

 with sediment. Thus at the time the " Yukon Silts " and " Kowak 

 Clays " were forming, Alaska, for the most part, was a low-lying 

 country, characterized by enlarged rivers with slow drainage and 

 many lakes. The supply of water was abundant and the flood plains 

 extensive. The present valleys were the drainage channels for the 

 volumes of water that flowed from the great glaciers to the south 

 and east and from the local glaciers within its limits. All these 

 waters were highly charged with silt and the finer sediment was 

 carried far beyond the glaciated areas where today they lie deposited 

 in former lake basins and along the vallevs. 



The silts are from fifty to two hundred or more feet in thickness 

 and at the " Palisades " on the Yukon and in the Coleen Basin on the 



