This paper reports an investigation of earth processes in a cold region and uses this information to decipher earth history. The principles developed have applications in the study of alpine regions and areas peripheral to Pleistocene ice caps as well as subpolar regions.
The climate and vegetation in the region of perennially frozen ground are briefly described in order to explain more fully the geologic processes now characteristic of the region and the changes in late Tertiary and Quaternary time. In discussing rock weathering especial attention is given to frost action. Facts obtained from laboratory experiments help explain rock disintegration by frost action and also the movements of rock waste that result from the freezing of water-saturated soils.
Bedrock gravels of stream valleys and coastal plains are buried under thick silt deposits, usually called “muck” by the miners. Extensive mining of these gravels, commonly auriferous, gives a unique opportunity for the study of frozen ground problems and also of deposits in a nonglaciated region through which intercontinental migration of Pleistocene mammals occurred.
Decomposed bedrock, under unaltered gravel of local origin, shows that the climate was conducive to rock decay prior to gravel deposition. The distribution and appearance of gold and other minerals furnishes additional evidence on the weathering of bedrock. The physical and chemical characteristics of the gravels and silts, together with their fossil content, indicate the origin of these deposits as well as the climate and other environmental conditions at the time of deposition. Since the origin of the silts has long been a problem, they were given detailed study both in the field and in the laboratory. Evidence indicates that the creek-valley silts are floodplain deposits formed largely from the local country rock, which, over large areas, is especially susceptible to disintegration by frost action. The gravels were deposited during a cool arid climate, and the silts during a cool humid climate. Deep freezing, accompanied by the formation of ground ice in the silts, occurred subsequently.
Ground ice occurs mostly in the form of layers or lenses and of wedgelike veins that commonly form a polygonal network enclosing silt columns. The veins, in places, join layers above and below to form a cellular structure. These structures have been produced on a small scale in laboratory experiments. Both observation and experiment indicate that the ground ice has been formed by segregation during progressive freezing from the surface downward.
The structure of the silts furnishes evidence of a period of deep thawing accompanied by extensive erosion. Where erosion did not completely remove the thawed silt, caving filled the fissures left by downward melting of ice veins. In favorable places silt was deposited on the eroded surface before refreezing. The period of deep thawing is correlated with the warmest part of one of the interglacial stages. Temperature profiles and the distribution of thawed and frozen ground indicate that in Recent time the temperature has been rising.
The Quaternary history of the nonglaciated area is summarized, and an attempt is made to correlate events in this area with those in the glaciated areas.