differences. Stratified drift is generally sand or gravel, with but few stones or boulders. It is often as loose and uniform as beach sand and is easy to dig. Till, on the other hand, is a jumble of every size of sediment particle: clay, silt, sand, pebbles, stones, and big boulders all mixed together. If you hit rocks every other shovelful, you are probably digging in till.

Generally speaking, till was deposited directly by glacial ice, either smeared onto the land beneath an advancing ice sheet or dropped out as the ice melted and retreated. Stratified drift, on the other hand, was deposited indirectly by the ice and only during the melting stage. The key difference is that stratified drift is sediment that was picked up and moved by flowing water some distance from where it melted out of the glacier. The movement of water sorts the big and heavy from the small and light. (Prospectors employ this same principle as they slosh water in their pans, seeking heavy gold nuggets.) Consequently, sediment moved by water is sorted by size of particle; a given deposit will have only a narrow range of sediment sizes. Sorting by meltwater from a retreating glacier gave stratified drift its uniform, sandy or gravelly, easy-digging characteristics. In contrast, till sits right where the glacier left it and remains a hodgepodge of every conceivable size of particle, from tiny bits of clay to ten-ton boulders. When you see stratified drift, think ice and flowing water; but when you see till, think ice only.

In the Central Valley, however, the difference between till and stratified drift is not as pronounced as it is elsewhere. Because brownstone erodes so easily, till in the Central Valley is generally free of boulders. When glaciation advanced over Connecticut, burying the land with an ice sheet 6000 to 7000 feet thick, it easily pulverized the weak cement holding brownstone together, grinding the rock back into loose silt and sand. The largest particles in the Valley's till are usually no bigger than fist-sized. Thus, while glaciation is responsible for the stoniness of most of New England (as will be discussed in the next chapter), in the Central Valley glacial erosion was so effective that few boulders were left in the soil to harass the poor farmer. And with few boulders in the soil, the stone walls that line old fields in other parts of New England are virtually absent from the Valley landscape.

Glaciation has been a boon to agriculture in the Central Valley in

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