Indiana’s Timeless Tales – Pre-History to 1781
Chapter title – Illinoisan Glacier Boundary
Illinoisan Glacier Boundary
Visitors to Washington County on south central Indiana will find this marker placed by the Indiana Historical Bureau.
Title of Marker:
Illinoisan Glacier Boundary
NE corner of SR 135 & Lick Skillet Road, 8 miles north of Salem (Washington County, Indiana)
Erected 1995 Indiana Historical Bureau
Marker ID #:
Nearby is the boundary of the Illinoisan Glacier, which covered all but approximately 6,250 square miles in south, central area of Indiana. Most of Indiana’s topography was affected by four separate glacial advancements during Pleistocene epoch, circa one million years ago.
Brief History By the Author:
The Pleistocene Age began roughly two million years ago and ended about 10,000 years ago. During this vast period, at least three episodes of extensive glaciations covered most of what is now Indiana. These glacial events are called the Pre-Illinoisan, Illinoisan, and the Wisconsinan Ages.
The Ice Ages
The Pre-Illinoisan began about 1,200,000 years ago and ended about 550,000 years ago. An interglacial period followed that lasted several thousand years. The Illinoisan began approximately 350,000 years ago and lasted about 50,000 years. Another interglacial period followed this glacial event, followed by the last glacial period, the Wisconsin, which began about 150,000 years ago and ended approximately 10,000 to 12,000 years ago. A period of global warming has produced the climate we know today.
These glaciers created two vastly different landscapes in Indiana. The northern two thirds comprise what geologists call the Tipton Till. Glaciers covered this area during all four glacial events. The glaciers probably never touched the southern third. A hilly, heavily forested land still bears the marks of the vast water runoff that occurred when the Ice Age finally ended around 10,000 to 12,000 years ago. If the glaciers had never formed, all of Indiana would probably look like the southern third of the state.
The Glaciers Form
The Huron-Erie Lobe is the glacier that covered Indiana during the last glacial event. Scientists estimate that the average temperature of the earth was about six to twelve degrees Celsius colder than it is now. Sometime about two million years ago, Earth’s climate cooled. Over vast regions of what is now Canada and North America the temperature dropped below freezing and remained there thorough the year. Snow fell and did not melt. More layers of snow covered this un-melted snow, building up layer after layer of snow. This weight of the accumulated snow turned the snow to ice. The ice formed layers up to two miles thick in the Great Lakes region. Over central Indiana, the glaciers were probably a mile thick. This gradually diminished as the ice reached its margins.
The pressure deep in the ice field caused the ice to become almost fluid in its movements. The ice flowed over the landscape, carving out rivers and lakes. It also created hills and the dune area around Lake Michigan. The weight of the ice sheet created the Great Lakes basin, and then filled that basin with melt water when the temperatures warmed and the ice melted. Geologist estimate that the ice moved about a foot a day, first advancing, and then retreating. Always grinding the terrain beneath it and changing it.
Most of the southern portion of the state had glaciers at different times; however, there is a segment in the south central region that has never, as far as scientists can tell, ever had glaciers. During the last episode, the boundary was a ragged line from approximately Terre Haute in the West to Brookville in the east. Below that, the older Karst topography of caves, sinkholes, knobs and disappearing steams that are not found in the northern areas
The glaciers’ presence created the two basic landscapes we find today in Indiana. The northern two thirds of the state that the glaciers covered consists of a flat landscape that geologist refer to as the Tipton Till Plain, covering the bedrock. As the glaciers advanced and retreated over the eons, they carried dirt, rocks and other debris with them. When the last glaciers melted, they dropped this dirt and rock mixture right where they were. Geologists refer to four basic types of deposits left by the glaciers as till, outwash, Lacustrine and Silt.
The Four Types
Sand, silt, and clay combine with gravel and boulders are the main components fo glacial till. Till was deposited directly by the glacier and has remained largely in the same location. As the glaciers melted, the melt water formed layers of outwash. Heavier components like gravel and rock were deposited first. The silt, sand and clay particles were carried greater distances by the flowing melt water. The glaciers had carved out depressions in the landscape, which formed the many lakes found in northern Indiana. The silts deposited at the bottoms of these lakes are called Lacustrine. Winds carried the finer materials, called silt, and deposited them further away. These silt layers, called loess, were blown mostly from the Wabash and White River valleys. Near the river valleys, this loess sometimes formed thick layers.
Glaciers have never covered the southern one third of the state, as far as geologists can tell. This region has some of Indiana’s most ancient soils and terrain. Most of the state’s bedrock layer consists of limestone, dolostone, sandstone, and shale. Much of southern Indiana is under laid with limestone. Much of the southern area consists of Karst landscape. In this type of landscape acidic groundwater flows through the limestone bedrock, dissolving it. This action over time creates sinkholes in the surface, underground caverns and disappearing streams. One predominant feature of south central Indiana is the Knobstone Escarpment
Geologists call the knobs the Knobstone Escarpment. They include some of Indiana’s most rugged terrain. It stretches from Brown County State Park in the north to the Ohio River. Elevations range from 360 feet near the mouth of the Wabash River to Weed Patch Hill, which has an elevation of 1,056 feet above sea level. This hill is in Brown County State Park and is the third highest area in Indiana.
Much of the limestone that Indiana is famous for is also found in the southern part of the state. Indiana’s limestone deposits formed during the Ordovician period, about 1.5 million years ago when the land that is now Indiana lay near the tropics, covered with a warm, shallow sea. This sea was rich with marine organisms, such as brachiopods, bryozoans, trilobites, and corals. These organisms died and settled on the bottom of this sea. Through Continental drift, this land migrated north and around 40 million years ago, this sea dried up. Geologic forces lifted the land mass out of the sea. The limestone deposits became covered with sediment over the ages. Glaciers scoured the countryside during the Ice Age, exposing some of this rock.
Oolitic Limestone is made up of particles called ooliths. These small, carbonate particles are composed of concentric rings of calcium carbonate. Sand or shell fragments rolled around on the floor of this warm, shallow sea collecting a layer of limestone. The rocks consistent structure allows it to be easily sculpted or carved. The stone is almost perfect building material.
The Quarries of Indiana
Indiana’s quarries produce rock known by many names, Indiana Limestone, Indiana Oolitic Limestone, Bedford Oolitic Limestone, and Bedford Rock. The limestone belt that produces this high quality stone encompasses most of Monroe and Lawrence Counties. Limestone of lesser quality underlies much of the rest of central and east central Indiana. Hoosiers began quarrying limestone during the middle of the Eighteenth Century. Indiana has been at the forefront of limestone production. Limestone from Indiana has been the preferred building material for many buildings from New York to Washington DC and other places. The Empire State Building has Indiana limestone as a major component of its structure.
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