34 NORTH AMERICA/Continental Interior Figure 15 Cross section of the Nemaha Anticline, a major structural feature showing the result of exaggeration (upper) to at scale (lower) Exaggeration is used by geologists to show relationships better in flat lying stratigraphic units (from Lee, 1956, Kansas Geological Survey Bulletin 121) Table Feature of plains type folds (Modified from Powers (1925) Geological Society of America Bulletin, v 36.) Structural noses are dominant structure at shallow depth (ie in surface units) Most folds are small in areal extent and amount of structural closure Closure increases downward and structure becomes sharply defined Amount of closure is proportional to size in many structures centuries and were utilized by the Indians Copper was also produced at Ducktown, Tennessee from Cambrian metamophosed sediments The occurrences of lead and zinc deposits in south-eastern Missouri were known from the eighteenth century The Tri-State District centred on Joplin, Missouri, described as the greatest zinc district on Earth, occurs in Mississippian carbonates The rich Precambrian iron-ore deposits of the Mesabi Range in Minnesota and in the adjacent states of Wisconsin, and Michigan have been utilised for a century Sudbury, Ontario, Canada is a major world source of nickel that occurs in a Precambrian complex Extensive sub-bituminous and bituminous coal deposits occur in the Midcontinent area and is mined both underground and in strip pits where shallow Petroleum seeps in Miami County, Kansas were used by the Indians for medicinal purposes, and pioneers