Glacial outwash
Sand and gravel sorted by meltwater rivers pouring off a retreating ice sheet — droughty ground, and excellent aquifers.
How it forms
Rivers of meltwater braid away from the ice front, carrying colossal sediment loads. They drop gravel first, then sand, further out, building flat terraces and long ridges where a stream ran in an ice tunnel.
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
- The sorted counterpart to till. Meltwater had energy and sorting power that ice did not, so it separated the material by size and laid it in beds across broad flat plains.
- How to know it
- Well-sorted, rounded, layered sand and gravel, on flat terraces or in long sinuous ridges. Pits dug into it stand in clean horizontal beds.
- How it wears
- Barely — it is loose already, and water moves through it freely.
- What it means above ground
- Droughty, sharply drained, low-fertility ground that warms early in spring. Excellent aquifers beneath, and the flat terraces make the easiest building land in glaciated country.
- Where you stand on it
- Cape Cod and Long Island entirely, the sand plains of Ontario and Quebec, and the outwash aprons along every northern valley.
- What lives on it
- Pitch pine, jack pine and oak barrens, blueberry, and the sandy openings that ground-nesting birds and native bees need.
- What it's used for
- The continent's sand and gravel supply, and a large share of its municipal well water.
- 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 till
the unsorted material the ice dropped directly
River deposits
the same process, running today
Is this what you're standing on?
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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.