When Geology Controls Topography

Unofficial Geology Law #1
Topography doesn't happen for nothing.

Mountains are mountains because they were thrust up in an orogeny, or because they're made of crazy-tough rock that can't be eroded down, or because they are in fact volcanoes.

Rivers twist because they chip off parts of their outer banks, or because the underlying rocks suddenly fault and jog sideways, or because off to one side there's a super-soft rock type that just loves to erode.

Topography happens for a reason.

Here are just a few of my favorite examples:

Soft Rock vs. Hard Rock

Indian Mountain, Lichfield County, on the NW border of Connecticut. The mountain is schist (gray in the map; Middle Ordovician Walloomsac Schist + Cambrian Everett Schist). The valleys on either side are marble (blue in the map; parts of the Lower Ordovician Stockbridge Marble). The black bar is 150 m long. Coordinates: 41°54'56.86"N, 73°28'48.50"W

Another (really cool) example:

Fractures Controlling a River

North Fork of the Shenandoah River, VA. The river's parallel segments suggest that its path is being controlled by subsurface structure, possibly joints running perpendicular to the ridge line. The white bar is 3 miles long. Coordinates: 38°47'8.09"N, 78°24'2.07"W.

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