Glacial till
Everything a glacier carried, dumped where it melted — clay, sand, gravel and boulders, mixed with no sorting at all.
How it forms
An ice sheet grinds up whatever it crosses and carries the debris frozen into its base. When it melts, all of it drops at once, in place. Where the ice was also pressing down, the result is compacted into a dense layer that has barely loosened since.
Surface deposits — not yet rock
Not rock at all — the loose material lying on top of it, dropped by ice or rivers within the last couple of million years. It is what most people actually dig in, and in a lot of North America it is metres thick.
- What it is
- Sediment released directly from ice rather than from water. Because ice carries everything at the same speed, nothing gets sorted: a boulder sits in clay beside a pebble.
- How to know it
- Unsorted — that is the whole test. Mixed grain sizes with no layering, angular and rounded fragments together, and boulders of rock that does not outcrop anywhere nearby.
- How it wears
- Compacts rather than drains. The fine fraction packs into a hardpan that both roots and water struggle through.
- What it means above ground
- Stony, uneven, often poorly drained ground, with boulders that keep surfacing in ploughed fields. Its chemistry comes from wherever the ice was scraping, so it can be sweet ground sitting on sour bedrock, or the reverse.
- Where you stand on it
- Everywhere north of the last ice limit — all of Canada, New England, the Great Lakes states, and the northern plains.
- What lives on it
- Mixed forest and pasture; the wet hollows in it hold the sedge meadows and alder swamps that much of the north's wildlife depends on.
- What it's used for
- It is what most of northern North America farms and builds on, whether or not anyone chose it.
- What it does to the soil
- Neither sweetens nor sours — the soil above takes its pH from elsewhere.
Next to it in the cycle
Glacial outwash
the part meltwater sorted before dropping
River deposits
the same idea, running water instead of ice
Is this what you're standing on?
Find the rock under your town
Enter a town or postal code and we'll show you the rock beneath it and how old it is, the soil it makes, and the chain of rivers your water joins on the way to the sea.
General descriptions of rock families. Any particular outcrop can differ — rocks grade into one another, and a geological map at this scale describes a district rather than a hillside.