Natural arch in Precambrian basalt in Minnesota, USA.

Along the northern shore of western Lake Superior are numerous exposures of a lava flow-dominated succession called the North Shore Volcanic Series. This is equivalent to & the same age as the Portage Lake Volcanic Series of northern Michigan's Keweenaw Peninsula (<a href="https://www.flickr.com/photos/jsjgeology/albums/72157632266738191">www.flickr.com/photos/jsjgeology/albums/72157632266738191</a>). The North Shore and Portage Lake successions are ~1.1 billion years old and represent basalt lava flows, plus minor sedimentary rocks, that filled up an ancient rift valley. This old rift is the Lake Superior segment of the Mid-Continent Rift System, a tear in the ancient North American paleocontinent of Laurentia (see: <a href="https://minerals.usgs.gov/science/midcontinent-rift-minerals/images/MRS-map.png" rel="nofollow">minerals.usgs.gov/science/midcontinent-rift-minerals/imag...</a>). Tectonic rifting started along this tear, exactly like the modern-day East African Rift Valley. Laurentia's Mid-Continent Rift System started and then stopped and was subsequently filled and buried. This ancient failed rift is now exposed on either side of Lake Superior in North America's Great Lakes.

The outcrop shown above is in Gooseberry Falls State Park, Minnesota. The rocks are quartz tholeiite basalts - they are part of the Gooseberry River Basalts succession. Three lava flows are exposed at this site.

The opening in the rocks is a natural arch - such features are rarely developed in basalt. This example formed by erosive undercutting along a parting in the basalt lava flow by river water during high flow events (floods). Rock arches that have rivers or streams underneath them are called natural bridges.

Stratigraphy: Gooseberry River Basalts, Upper Southwest Sequence, North Shore Volcanic Series, Keweenawan Supergroup, upper Mesoproterozoic, ~1097-1098 Ma