Pre-Cambrian Ripple Marks—Cook County Minnesota
Here we see two samples of the Meso-proterozoic (1-1.6 billion years old) Cut Face Creek Sandstone. It is rare to have such an old example of a sedimentary rock preserved at the Earth's surface, and even more rare for the rock to contain preserved ripple marks. This rock marks a disconformity with overlying and underlying basalt flows, which erupted and cooled during a time when the North American continent was rifting apart, beginning the formation of the basin that eventually became the Great Lakes. The sandstone was deposited between major eruptive sequences along that rift.