As featured in the November/December 2019 Gold Prospectors Magazine
By Tracy Repp
Glaciation is a powerful earth process, characterized by the presence of huge volumes of land-based ice with the ability to completely rework and reshape landscapes. Over the last 3 billion years, there have been about 6 major periods of glaciation, periods that we call ice ages. The most recent ice age began about 34 million years ago (MYA). This ice age, characterized by periods of glacial advancement (i.e. glacial period) and glacial retreat (i.e. interglacial period) as all ice ages are, is considered by glaciologists to be ongoing because at least one permanent ice sheet, the Antarctic, remains. The latest phase of this ice age is called the Quaternary and it began roughly 2.5 MYA. Its last glacial period ended 11,700 years ago, beginning the interglacial period we live in today.
During the most extreme of the Quaternary’s glacial periods, large ice sheets up to 2.5 miles thick formed at the more northern latitudes of North America, Siberia and Europe. The furthest and most recent advance of ice in North America, termed the Last Glacial Maximum (LGM), occurred about 26,500 years ago, and covered the areas shown in Figure 1, below.
The massive ice sheets that form during glacial periods advance by “flowing” downhill and with their immense weight act like bulldozers, scraping over the surface of the earth. With thicknesses of up to 2.5 miles, the weight of this ice as it moves and picks up rock and debris, scours the landscape with incredible ease. Rocks called “erratics” up to 15,000 tons in size, which is about the size of the Brooklyn Bridge, can be plucked from the bedrock and carried hundreds of kilometers away from their source by the advancing ice. These rocks leave behind deep grooves in the bedrock as they are dragged along on the bottom of the ice, kind of like how the grinding of a diamond across a glass surface leaves behind deep cuts and scratches. These grooves are called striations and are indicators of the direction of ice movement.
Sediment and rock carried by glacial ice can either be a jumble of variously sized material called till, which means it was laid down directly by the ice itself, or it can be sorted with the heaviest particles at the bottom and the lightest or finest grained particles at the top, which means it was deposited by meltwater. Knowledge of this type of stratification, or layering, can be very helpful for those looking to sort through glacial deposits, also called moraines, in search of rocks and minerals from far-away places to the north. Typically, moraines are deposited as ridges, marking the edges of where the glaciers once were. Figure 2 is a sketch of the types of moraines that get deposited by glaciers and where.
Figure 3 shows the series of recessional and end moraines of various ages in a portion of the Midwest. Notice how prevalent the moraines are and how rivers over time have cut through them or formed channels between them.
Fortunately for us in the northern United States, our neighbor to the north is rich in gold-bearing ores, which means that gold picked up in Canada can now be found in the U.S. These ores formed in areas like the Canadian Shield, a vast geologic province forming the core of the North American continent, itself formed from a stormy history of volcanism and metamorphism. The Canadian Shield, as shown in Figure 1, begins north of the Great Lakes and extends northward to the Arctic Ocean with Hudson’s Bay at its center. This area is the source of more than 75 percent of Canada’s gold production, which topped about $8.7 billion in 2017 alone. Other gold-bearing ores are present in Canada’s westernmost province of British Columbia, as well as in its northern territories. Rocks from these regions have been found throughout the northern U.S. In fact, nearly all placer gold deposits in the Midwest are glacially derived.
If you want to find gold digging around in moraines though, which contain a mishmash of material from clay to gravel to boulders, don’t forget to keep in mind some of the unique properties of the mineral. The easiest way to find it is often with the knowledge that its heavy mass means that it’s likely to settle out quickly with other heavier material. Finding places where moraines have been reworked by water, which is a sorter of particles, just might be the ticket. This water may be in the form of meltwater from the retreating glacier itself or from various rivers and streams that formed long after the glaciers disappeared from the region. It can be tough trying to find evidence of old meltwater channels long gone but a good sign is to look for areas with gravel. Old stream beds will have a lot of gravel and will be the likeliest place to find gold. A Quaternary geology map, which shows clearly where glacier deposits lie, might also become your best friend when looking for places like these.
In the U.S., glacial gold can be found throughout the Midwest in Indiana, Illinois, Ohio, Michigan, Wisconsin, Iowa, Minnesota, Kansas, Nebraska, Missouri and the Dakotas, as well in a few northeastern states including New York, Pennsylvania and Vermont. Idaho, Montana and Colorado also have glacially derived deposits of placer gold, as does California where valley glaciers still exist.
Stay tuned for my next article, third in this educational series on the occurrence of gold in the U.S., which will focus on the formation of new gold deposits in earthquake-prone zones. I’ll share with you how these types of deposits can form almost in an instant! Happy prospecting!
Tracy Repp is a professional geologist of twenty years and a part-time writer living in Michigan.