Thanks to the Ice Age
by Julia Pearson
At the time of this writing ice and snow is melting into rivulets from the rooftop gutters and is rushing down sidewalks and roads. The polar vortex that slammed into our night- time weather left inches of ice on everything. Snow was shoveled into white mounds that stubbornly clung to the ground, despite temperatures that bounced intermittently above freezing.
It’s been a winter of bitter, frightening cold. The kind of cold that shuns the seasonal Currier and Ives scenes of sleigh rides, snow men, and fields of animal tracks and the imprint of children’s snow angels. Wildlife settled down into secret places. State and local leaders urged everyone off the roads. Skin and lungs both were damaged by the air that was almost metallic in its severe power to hurt.
Swapping stories of past episodes of mad temperature drops became part of the community conversation.
Some Hoosiers recalled signs placed on the roadside by the highway department around 1937 that pictured an outline of the state and marking “Glacial Boundary.” Those signs no longer exist, but they marked the evidence of nature’s impact on our landscape.
Indiana’s geologic history is reconstructed through core sediment analysis, geophysics, and stratigraphy. The first ice sheet entered Indiana sometime before 700,000 years ago directly from what is now Michigan. In general, geologists divide the Ice Age into three great stages, and the glacial deposits they left behind: the Wisconsin Stage, which began about 50,000 years ago and covered nearly two-thirds of Indiana; the Illinoian Stage, which took place from 300,000 to 140,000 years ago; and the pre-Illinoian Stage, which is much less clearly understood. Based on the information provided by the Indiana Geological Survey it is clear that Brown County’s magnificent hills exist because the multiple periods of glaciations in Indiana’s geological history stopped at the northern edge of the county.
Brian Keith, Senior Scientist with the Indiana Geological Survey at Indiana University says the rugged topography of Brown County today is related to two factors. First, there were no extensive glacial deposits to fill in the topography. And second, the nature of the bedrock, called Borden siltstone, which is easily eroded by the natural forces into steep valleys and ridges. The northern part of central Brown County has glacial material from the pre-Wisconsin or older era. Limited glacial material is found along the edges of some deeper valleys of the streams that flow to the west into Monroe County.
Weed Patch Hill in the Brown County State Park is one of the highest elevations in Indiana at 1,058 feet above sea level. It is also the highest point along a serpentine land form called the Knobstone Escarpment that wiggles from northern Brown County southward to the Ohio River. The Knobstone Escarpment marks a transition from erodible shale to more resistant siltstone bedrock, which may have acted as a barrier to the glaciers along the Eastern edge of Brown County. Made up of steep-sided hills and valleys, these rocks of the Mississippian age form the eastern boundary of what is called the Norman Upland composed of bedrock known as the Borden Group. These erosion-resistent, silica-rich siltstone rocks were once part of a vast delta system.
This year the country is preparing for a massive spring thaw and melt-off after the spate of extreme freezes. In geologic time, the active glaciations and the strength and force of running water acted together to erode and shape soil and rock beneath the landscape. Brown County’s beautiful scenery is a result of that activity.
No other environmental factors since the extinction of the dinosaurs sixty-five million years ago can compare to the Ice Age in regards to the profound affect it had on our landscape and the natural world that lives on it. Ground water for 90% of the state’s population today, and nearly all the major aquifers this water comes from, are a direct result of glaciation. The melting ice sheets yielded the major rivers that formed the valleys where humans settled.
This past winter we have learned that climate can be local and personal. The extra school days missed or delayed because of the snow and ice will be keenly felt by Brown County students as they make up for lost time. People will talk about this winter for years to come.