Surface Disintegration of Granitic Masses

Throughout the drift mantled surface of Iowa, glaciated boulders of crystalline rocks are of common occurrence. They vary in size from a foot to more than fifty feet across. These boulders are rounded more or less, globular in form, though often slightly flattened on one side, sometimes on two. When closely examined, the outside is commonly found to be more or less affected by meteoric agencies, but the interior is fresh and unaltered as a rule. Most of the Iowa boulders are known to be of northern origin, coming from near or beyond the present northern boundary of the United States. In this region the granite, diabase and gabbro areas are usually firm, and but slightly decayed at the surface, the rocks having been planed and scored by glacial action. Passing beyond the limits of the glaciated region an entirely different set of phenomena is presented, and in an area of crystallines, the rocks are decayed for many, often one hundred or more feet below the surface, the bi-products remaining in situ until removed by running water