When used as a verb, till means to work the soil for farming, as by plowing or harrowing. When used as a noun, till is the geologists’ name for the sediment left behind by a glacier—a mixture of clay, sand, gravel, and boulders. Unlike sediment dropped by moving water, these materials were not sorted by size and weight, which is why those mining the sandand-gravel pits dotted across the northern Midwest in deposits of glacial till must do the sorting mechanically. Wherever the lobe of a glacier broke off from the main ice sheet and slowly released its sediment over an extensive area, as across central Illinois and Indiana, it formed a till plain. If farmers don’t actually till the till, they do cultivate the rich soil formed on top of it by windblown loess (another gift of the glaciers) and humus (a gift of the prairies and forests that succeeded the glaciers).