Paragneiss
Gneiss that used to be mud and sand. The banding is the ghost of the bedding it started with.
How it forms
A pile of marine sediment is caught in a collision between continents, driven down into the crust, and cooked at depth until every mineral in it has regrown. What was bedding becomes banding; what was clay becomes mica and garnet.
Metamorphic — changed by heat and pressure
Rock taken deep, squeezed and heated until its minerals regrew in new forms, without ever melting. The new minerals line up under the pressure, which is why so many of these split in sheets or come out in stripes.
- What it is
- Gneiss derived from sedimentary rock rather than igneous — sand and mud laid down in a sea, then buried deep enough to recrystallise completely. Geologists tell it from the composition, which still carries the chemistry of the original sediment.
- How to know it
- Banded like any gneiss, but with a wider range of minerals — garnet, sillimanite and graphite turn up in paragneiss and rarely in the igneous-derived kind. The bands differ visibly in hardness and etch out unevenly.
- How it wears
- Slowly and unevenly, since its bands differ in hardness; the softer ones etch out and the rock sheds in slabs.
- What it means above ground
- Thin, acid, stony soil, but more variable than plain granite country — a calcium-rich band left over from a limey bed can leave a sweeter pocket with a quite different flora in the middle of sour ground.
- Where you stand on it
- The Grenville Province — the Gatineau Hills, the Laurentians, eastern Ontario and the Adirondacks — and the metamorphic belts of the Appalachians.
- What lives on it
- Sugar maple and beech on the deeper pockets, hemlock in the ravines, spruce and pine on the thin ground. The sweeter bands grow ferns and orchids.
- What it's used for
- Aggregate and rough building stone; the graphite and garnet in some bands have been mined.
- What it does to the soil
- Sours the soil above it — nothing in it neutralises rain, so the ground stays acid.
Next to it in the cycle
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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.