River deposits
Sand, silt and gravel dropped by a river and still being dropped — the youngest ground there is, and the most fertile.
How it forms
A river carries what its current can lift and drops it the moment the current slackens — on the inside of a bend, across a floodplain in flood, or where it meets a lake or the sea. Every flood adds a layer.
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
- Alluvium: loose sediment laid down by moving water on a floodplain, a delta or a bar. Some of it is decades old rather than millions of years.
- How to know it
- Loose, sorted by size into visible layers, rounded grains, and flat ground beside a river. Dig a hole and it does not stop.
- How it wears
- It is already weathered; that is what it is. The river sorts it, moves it, and puts it down somewhere else.
- What it means above ground
- The deepest, most fertile and most level soil in any landscape, which is why floodplains are farmed and towns get built on them. Also the ground that floods, and the ground that liquefies in an earthquake.
- Where you stand on it
- Every river valley, the Mississippi and Fraser deltas, the Central Valley of California, and the flats every prairie town sits on.
- What lives on it
- Silver maple, cottonwood, willow and ostrich fern; the richest foraging ground in most regions, and the first to be ploughed.
- What it's used for
- Farmland, and the sand and gravel that every road and building is made of.
- 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 same sorting, done by meltwater
Glacial till
the unsorted alternative the ice left
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.