(side one)



Almost 1.2 billion years ago molten rock called magma from deep in the Earth flowed up between two adjoining tectonic plates them located under present day eastern Minnesota. Through a process called rifting some magma flowed underground into Minnehaha County where it intruded into fractures in the overlying quartzite. While still below the surface of the earth it slowly cooled and hardened to form a dark-colored, coarse-grained rock.

Subsequent uplift of the Earth's crust and erosion


(side two)

combined to expose, and erode away most of the large body of solidified magma. Only scattered remnants remain today.

Geologists named the hardened magma "Corson Diabase" because outcrops of the rock were first found north of Corson, S. Dak., along Split Rock Creek.

In the early 1900s a quarry was opened in Lien park to obtain Corson Diabase. Some was used to build the black facade on the north side of the original Washington High School. Diabase similar to that found at this site has also been found in other county locations.

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