DEPTH OF FROST AND ICE IX THE TUNDRA. L29 



started would perpetuate itself from year to year until the growth of moss 

 from the sides led to its burial. An origin of this nature seems probable, 

 as the lake basins are due entirely to variations in the surface growth of 

 vegetation and not to inequalities of the substratum of rock or clay on which 

 the humus layer of the tundra rests. The origin and extinction of lakelets 

 is thus a part of the normal growth of the frozen moss-covered plains. 



MOSSY COVERING OF THE WOODED PORTION OF ALASKA. 



Distribution of the Mossy Covering. — The tundra is confined to the vicinity 

 of the coast, where for some reason, probably climatic, trees do not grow. 

 Inland from this belt, however, the mossy covering still continues and 

 occupies a vast area, especially in the lowlands bordering the Yukon and 

 other large rivers. Without exaggeration, it may be stated that the whole 

 of Alaska, excepting the steepest rock slopes and the tops of high mountains, 

 is covered with a dense carpet of moss. 



On the flood-plains of the larger rivers, and generally throughout all the 

 lowlands of Alaska, peaty deposits are forming in the same manner as on the 

 tundra, modified, however, by the growth of arborescent vegetation and by 

 the intrusion of sand and clay in places that are flooded during the high- 

 water stage of the rivers. 



At many localities along the Yukon sections of peaty deposits are exposed 

 often eight or ten feet thick and several miles long. The bluffs where these 

 layers occur are usually from fifteen to twenty feet high and nearly always 

 frozen solid, except where they are too open in texture to retain water. 

 Some of the vegetable layers are interstratified with sand and clay, as already 

 explained ; others at the surface are still increasing in thickness and have a 

 dense forest growing on them. Not infrequently there is a stratum of clear 

 ice interbedded with the layers of peat, sand, and clay. 



Depth of the Frozen Stratum beneath the Moss— -The thickness of the frozen 

 substratum beneath the moss-grown forest has never been determined. The 

 deepest excavations that have been made show that it exceeds twenty-five 



feet. 



At Nulato a well has recently been dug near the river bank through clay 

 and sand to the depth just mentioned, in which the material removed was 

 frozen solid, with the exception of certain dry sandy layers. At Forty-mile 

 creek precisely similar conditions have been revealed by mining operations, 

 the depth reached being also about twenty-five feet.. 



The reason for the great thickness of the frozen layer at these localities 

 seems to be that deposition and freezing went on at the same time Th< 

 certainly seem to be the conditions under which the great thickness of frozen 

 material beneath the tundra and in the flood-plains of the larger rivers of 



