Igneous Rock
Rock formed by the cooling and consolidation of magma.

IHS
International Historic Site.

Imago
The mature or adult stage of an insect.

Inselberg
Prominent steep-sided residual hills and mountains rising abruptly from plains. The residuals are generally bare and rocky, large and small, isolated and in hill and mountain groups, and they are surrounded by lowland surfaces of erosion that are generally true plains, as distinguished from peneplains.

Intaglios
Large designs formed on the ground and usually discernable only from the air. Also known as geoglyphs.

Interbedded
Alternating layers of different materials in a section of bedded rocks.

Intermediate Trail
This type of trail provides longer hikes with more elevation gain and may involve sections where the trail is in poor condition. A round trip hiking time for this type of trail is typically three to seven hours.

Intermontane Basin
A basin situated between or surrounded by mountains, mountain ranges, or mountainous regions.

Internal Frame Pack
An internal frame keeps the load closer to the back, which generally makes the pack more stable and better balanced in difficult terrain.

Introduced
Intentionally or accidentally established in an area by Man, and not native; exotic or foreign.

Intrusive Rock
Rock that cooled within the earth.