It's Hudson's Fault ! EarthCache

The listed coords will bring you to the Hanley Roadcut in Hudson, WI. This upthrusted wall of limestone you will be passing through is the end result of ancient plate tectonics which nearly divided this continent in half.

One billion years ago, the Earth's crust split across part of the North American continent. This tear or rift, known as the Midcontinent Rift System, was marked by ongoing volcanic eruptions and heavy lava flows through the length of the rifting. This expanding continental division extended for 950 miles from what is now Lake Superior to Oklahoma, and was on its way to becoming a full-fledged ocean when the process halted.

Crustal movement that first pulled the continent apart then reversed and began to push it back together. This compression forced the dense basaltic rocks upward, producing a range of mountains along the axis of the rift. The thick lake deposits were eroded off the steadily uplifting mountain range and were redeposited in a series of deep sedimentary basins along its flanks. In some areas, the sediments were almost completely removed from the uplifting central block (known as a horst) and the basaltic volcanic rocks were exposed. In other areas, the basaltic rock remains buried up to 5,500 feet, and only older layers of the sedimentary rock are exposed due to erosion.

There were two arms of the Midcontinental Rift. The eastern and the western arms. The eastern arm has greatly influenced the geology of this region.

The fault line near Hudson is part of the Hastings - Lake Owen Fault, and it runs southwest to northeast. It’s course can be traced from Hastings, MN, across the Mississippi River, proceeding just south of Afton, across the St. Croix River, then passing through Hudson, WI. The fault line crosses the Willow River at the Falls, and continues to proceed northeast toward Lake Owen and beyond. There is a second fault line which runs parallel approximately 8.9 kilometres north and is named the Cottage Grove Fault. The opposing pressures of these fault lines have created a “Reverse Fault” and have caused the layers of bedrock inbetween to be thrust upward. This raised elongated mass of rock is known as a “horst” and this particular land mass between these two faults is known as the Hudson-Afton Horst.

The upper layers of sedimentation of the elevated landmass become unstable and are easily eroded away with time, thus exposing the much older layers of bedrock situated in a higher elevation in relation to the surrounding rock.

This roadcut is an uplifted block of Cambrian Limestone. On the east and west side, where the land surface is lower in elevation, is Ordovician sandstones, a softer, more erodible rock. The reason the limestone is higher is because the sandstone is softer and is removed quicker than the limestone leaving the limestone in higher elevation. If one were to dig at the east or west slopes of the fault, they could potentially find the actual fault zone, but as the fault is currently exposed, the "fault line" is probably not very recognizable as weathering has disturbed it.

The following tasks are required to log this EarthCache:
Please forward the following in an e-mail:

1. The width of this uplifted limestone formation, and reference your unit of measure and methodology (did you walk, bike or drive, etc?);

2. At Waypoint 1 and take an elevation reading;

3. Posting a photograph of yourself or your GPS at this site is an optional requirement. Posting interesting photographs of this geological anomaly to educate other visitors to this EarthCache and to document your experience here are appreciated.