FLOOD-PLAIN DEPOSITS OF THE YUKON. ] -_> | 



especially interesting region for such study. This portion of the Porcupine 

 flows through a low, densely forested region, which is an extension of the 

 lowlands of the Yukon already described. Its course is extremely tortuous, 

 and in fact forms a continuous series of gracefully sweeping curves. In its 

 meanderings it cuts away the banks on its concave side, and deposits the 

 material removed lower down on its convex side. In this way a marked 

 contrast in the character of its banks has been produced. On the outer 

 curves the banks are precipitous, owing to the undercutting of the river. 

 They are uniformly about twenty feet high, and densely covered with fully 

 grown spruce trees. The river has cut a swath through the forest and left 

 the trees standing on its border as the grain stands beside the path of the 

 reaper. 



On the inner curves the banks are low and gently sloping, and near the 

 water are bare of vegetation. Proceeding up the shelving shore, one comes 

 first to coarse grasses and yellowish-green Equisetums. Beyond this belt is a 

 growth of young willows, which iucrease in height away from the river, and 

 soon form a dense growth thirty or forty feet high. Mingled with the willows 

 and replacing them on the landward side are clumps of alders and groves of 

 poplars. Beyond this belt lies the unexplored spruce forest, which stretches 

 away for miles and densely covers the land to and beyond the distant hills. 



The immediate border of the river on the convex curves is formed of 

 current-bedded gravels. Going up the beach one comes to sand banks, 

 which in their turn pass beneath deposits of fine silt. These are the flood- 

 plain deposits of the river, and are arranged in a definite sequence resulting 

 from their mode of deposition. The gravels are deposited by the swift waters 

 along the border of the main channel, while the finer superimposed strata 

 are spread out by the slack water on the margin of the stream during its 

 flood stages. 



Fresh-water shells were frequently observed in the finer deposits. Cross- 

 bedding, common in all the strata, is best defined in the coarse deposits. At 

 times the sand and silt layers are finely laminated, and may closely resemble 

 lacustral deposits. In one instance a layer of coarse sand more than twelve 

 feet thick was observed. Though deposited by the river it was homogeneous 

 throughout, and did not exhibit a single line of stratification or cross-beddiDg. 



As the river slowly changes its course by taking from one bank and 

 depositing on the other, the sheets of debris it spreads out are increased by 

 additions to their margins, preserving at the same time their order of super- 

 position. 



Within the forest there is a dense growth of mosses ami lichens, decaying 

 beneath while growing above. This process superimposes a layer of peal 

 on the deposits spread out by the river. The soil is every where frozen at a 

 depth of about a foot below the surface. 



