Approximately 15,000 years ago, this glacier advanced into Iowa and left behind distinctive surficial deposits that are now referred to as the Des Moines Lobe. The Late-Wisconsinan ice sheet reached its southernmost extent, marked by the terminal Bemis Moraine, around 14,000 years ago. During this time, the Middle Raccoon River was diverted into an ice-marginal position along the advancing edge of the Des Moines Lobe. However, the immediate segment of river within the park itself diverges from the usual ice-marginal position and most likely existed in its present location prior to the ice advance.
The Des Moines Lobe glacier remained active in north central Iowa until approximately 12,000 years ago. This was 5,000 to 8,000 years later than the major Wisconsinan glacial activity northeast of Iowa, and well past the period of maximum glacial cold about 16,000 to 21,000 years ago. The Des Moines Lobe was actually a late surge of glacial ice that occurred during a period of already warming climatic conditions. The ice mass was probably relatively thin in Guthrie County, perhaps only a few hundred feet thick, and may have been even thinner along its outer margins. The ice surge, extending here to the edge of Springbrook State Park, likely was enhanced by lubricating melt waters that flowed at the base of the glacier. Fossil plant material collected from the Saylorville Reservoir area north of Des Moines indicates the presence of a spruce-larch forest in this general vicinity.