Chapter 5.1, Problem 51E

Time of the Fourth Ice Age Many scientists believe there have been four ice ages in the past $1$ million years. Before the technique of carbon dating was known, geologists erroneously believed that the retreat of the Fourth Ice Age began about $25,000$ years ago. In $1950$, logs from ancient spruce trees were found under glacial debris near Two Creeks, Wisconsin. Geologists determined that these trees had been crushed by the advance of ice during the Fourth Ice Aga Wood from the spruce trees contained $27\%$ of the level of $C$ found in living trees. Approximately how long ago did the Fourth Ice Age actually occur?