From Ohio History Central
Kettles are small glacial lakes that formed when a large block of ice was separated from the main retreating glacier and buried in sand and gravel outwash. When the ice block melted, a depression was left that filled with water, thus creating a small pond or lake. Many kettle lakes in Ohio have filled in with sediment and vegetation since they were formed and are now boggy or swampy areas. Sometimes these low areas are excavated for a farm pond and the skeleton of a mastodon or other Ice Age animal is found in the sediment.