The further back into the earth's past you go, the less material exists and the harder it becomes to reconstruct the paleogeography. This makes understanding the creation of the cratons, the stable cores of continents, especially difficult. Although parts
The further back into the earth's past you go, the less material exists and the harder it becomes to reconstruct the paleogeography. This makes understanding the creation of the cratons, the stable cores of continents, especially difficult. Although parts of the North American craton were cleaned of covering sediments by glaciation, some parts were further buried. However, techniques such as magnetic and gravity mapping allow us to glimpse what lies under covers of sediment and rock.
Above is a isostatic gravity map of Iowa. Locations of higher gravitational acceleration (recall g=mG/r2), are locations of denser rocks (oceanic rock), locations of lower gravitational acceleration are less dense rocks (continental). On the above map, red is higher acceleration and blue is lower acceleration.
The most obvious feature is the three "stripes" across central Iowa, consisting of a central high gravity stripe, and surrounding lower g