Dredge No. 4 National Historic Site of Canada
Geology:
The Klondike gold field forms an area roughly 2,080 square kilometres (800 square miles) in size, extending from the Klondike River on the north to the Indian River on the south. Its east-west limits are Flat and Dominion creeks and the Yukon River.
One of the principal characteristics of Klondike gold is that it occurs in a free or placer state. Because of gold's high specific gravity it is found on or in bedrock. The gold bearing rocks of the region date from the Cambrian or Silurian age (400,000,000 B.C.) and are composed primarily of light coloured sericite schists of igneous origin. During the late Pliocene period (2,000,000 B.C.) these rocks were eroded into gold-bearing gravels which settled in the beds of ancient streams. After these gravels were deposited there was an uplifting of the land from north to south which accelerated the velocity of the streams flowing north into the Klondike River. These streams rapidly deepened their channels. This deepening, coupled with erosion, resulted in a concentration of gold-bearing gravels on bedrock of the existing creek beds and valleys and explains why some creek claims on Bonanza and Eldorado creeks were such rich producers. The effects of this geological evolution were that gold was deposited and subsequently found at three distinct ground channels, located 30 - 91 m ( 100 to 300 feet) above the present creek beds; on benches, which were fragments of old valley bottoms formed at an intermediate stage of erosion, and in the beds of existing creeks.
The Klondike was one of the few areas of Canada that benefited from the ice age. Because it was located in a semi-arid zone the Klondike escaped the scourge of glaciation; hence the gold bearing gravels were not removed by glacial ice or buried under glacial accumulations. However, the region was subjected to extreme cold. This led to the formation of permafrost or permanently frozen ground. While permafrost added to the burden of mining it did have a singularly desirable result. By arresting weathering of the rock, it retarded the natural transportation of gold and its dispersal into quantities that would have precluded extraction. But geology and climate were important not only for the role they played in the formation and deposition of the gold bearing gravels - they also dictated the methods that would be required to extract them.