One of the exceptions is shown above. Illinois is a pretty flat state, but outside of Bloomington Tim took this picture of a long, low hill in the distance. This unassuming ridge is a relict of Illinois’ Ice Age past.
As glaciers move, they erode vast amounts of rock which is pushed in front of the glacier. When the glacier reaches its maximum size and then begins to retreat, it leaves a mound of this rock debris, called a terminal moraine (glaciers also form lateral moraines along their sides.) When the glacier is completely melted the moraine remains as a ridge on the landscape, with a characteristic composition of glacially-rounded rock fragments.
The moraine in Tim’s photo was formed during the last major pulse of ice during the Pleistocene Ice Age, about 10,000 to 20,000 years ago. At that time about half of North America was covered with ice, as shown on this map.