The ground surface of Marion County and the materials that underlie it are the results of the last glaciation of central Indiana. Marion County is part of the physiographic province, a geographic region with a distinct type of landscape, landforms, rock type, and evolutionary history, called the Tipton Till Plain, which is named after Tipton County, two counties north of Marion County. It is underlain by glacial deposits known as till or drift, which is unsorted material composed of clay and boulders of intermediate sizes directly deposited by glacial ice. Overall, the surface slopes gently toward channeling most of the drainage into the river.
Most of this surface is a level moraine, or glacial deposits, with higher elevations of ridges and hills. The topographic relief at any given section is about 10 to 30 feet. Perhaps the most prominent topographic features in this generally flat landscape are several isolated hills or short ridges that rise about 100 to 130 feet above the surface.
These isolated hills are glacial landforms called kames, which are deposits of sand and gravel that were entrapped in pockets along the margin of a glacier that once covered this area or in crevasses within the glacier. The best-known of these is Strawberry Hill, where the poet James Whitcomb Riley is buried in , but other kames are present in the southern half of the county.
The relief of the steep valley walls of the major streams looks rugged but is at most only about 100 feet or slightly higher. The major valleys of Marion County were formed by glacial streams of meltwater as the glaciers retreated.
In Marion County, the surface elevation of the lowest point is about 650 feet above sea level, and the surface elevation of the highest point is about 900 feet above sea level. The two points are on opposite sides of the county. The lowest point is on the Johnson County line where the southward-flowing White River leaves Marion County. The highest point is on a ridge in the northwestern corner of the county just west of where the southward-flowing Eagle Creek enters Marion County. It is not, as is commonly believed, Strawberry Hill, which rises to only 840 feet. at the center of Indianapolis is at an elevation of about 715 feet above sea level.
Although the total relief, or the difference between the lowest and highest points, for the 403 square miles of Marion County as a whole is about 250 feet, the local relief for any particular part of the county is considerably lower. Thus, the topographic impression one sees from an aerial view is a generally flat landscape with several narrow incisions caused by the erosion of the main streams.
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