 Over time erosion freed the agates from the solidified lava, which is not as hard as quartz.The creation of the Lake Superior basin reflects the erosive power of continental glaciers that advanced and retreated over Minnesota several times in the past 2 million years.The mile-thick ice sheets easily eroded the sandstone that filled the axis of the rift valley, but encountered more resistance from the igneous rocks forming the flanks of the rift, now the margins of the lake basin.