Permafrost; an incredible natural feature of the far northern reaches of the world. In Yukon-Charley Rivers, the natural existence of permafrost is most evident in the low-lying areas near the Yukon River.
It is here where beautiful examples of Holocene epoch* ice wedges, which are not at all common in Interior Alaska, have formed since the last ice age in younger sediments (in this case, river sediments), under conditions resembling modern conditions. Most of the ice wedges that bedevil road and structure building in Interior Alaska are older, Pleistocene wedges that formed in a colder, older climate. In the upper Yukon valley, it has been just cold enough to form wedges in more-or-less modern times, but only on ideal sites such as these flat lowlands along the Yukon River.
During very cold winter weather, the frozen ground contracts and creates deep cracks that fill with snowmelt water the following spring. This water then freezes in the crack and never melts, because the ground stays frozen year-round (that's what 'permafrost' is). After many cycles of cracking and freezing in the same place, the wedge can get a foot or more wide, like the ice wedges shown in the photos along the Yukon River, causing the formation of polygon-shaped areas on the ground surface that are visible from the air and satellite imagery.
* 11,700 years ago to present time
NPS Digital Image