Geomorphology and the Idaho Batholith


If you’re an amateur geologist, this sign is totally unnecessary

WC cautions readers that this post is mostly WC’s speculation, geology ex cathedra WC’s bellybutton, as it were. But it is at least slightly informed speculation. Let’s talk about stream courses in the Idaho Batholith.

Anyone unfamiliar with Idaho who tries to trace the course of the Salmon River on a map will find the task immensely frustrating. And may wind up with sprained digits. The Salmon River, over its comparatively short length, all within the state’s boundaries, manages to flow in every direction the compass offers before debouching into the Snake River in west central Idaho. The sensible reaction of that would-be map reader would be to ask why in the world a river follows such an incredibly twisty course. The Snake River, Idaho’s other big river, is arrow-straight in comparison.

WC suggests it’s all about the plutons.

The Idaho Batholith, the vast mass of some 15,400 square miles of granodiorite that composes most of the central third of the state, is famously formerly molten rock the solidified underground, not erupting as lava. A failed volcano, as it were. Some readers may be aware that it isn’t a single mass, but rather a cluster of plutons, perhaps as many as 30 different plutons, that developed over the course of some 40 million years.1 The tops of those individual plutons today are collectively the surface of the Idaho Batholith.2 

An analogy WC can offer is a cluster of bolete mushrooms, pushing up through the soil. The tops sometimes overlap, and sometimes are spaced a bit apart. The edges of each mushroom have varying amount of dirt on them and around them. The mushrooms themselves are of varying heights and shapes, not identical at all, and of varying ages. 

Substitute individual plutons for the individual mushrooms, and the native rock that was there before the plutons arrived for the soil. Add that the lifetimes of the plutons are measured in geologic aeons and not the mere days for mushrooms, and that the mushrooms never emerged above the surface, but rather the surface eroded down to expose them. Or maybe forget the analogy. 

Where the edges overlap, or where the edges don’t quite touch, there is the older native rock. It was ancient stuff, Precambrian, and the stress and heat from the arrival of the plutons fractured it and at least partially metamorphosed it. In contrast to the solid masses of granodiorite, that altered native rock is weaker, more easily eroded by water. And serve as the preferential channels for streams and rivers, as a result. 

Granodiorite of the batholith on the right; native rock, partially metamorphosed, on the left. In between, a line of shattered rock that Idaho Highways has covered with concrete to try to keep it from falling on the road.

Which leads to WC’s thesis: the rivers that drain Idaho’s batholith are extraordinarily twisty and winding because they are tracking the margins of the plutons, winding around the irregular curves and lobes of those ancient plutons. The streams trace those areas of weakest rock.

There’s supporting evidence for WC’s idea. In the stream channels, you find a lot of metamorphic rock: quartzite (metamorphosed quartz) rich in mica and schist, and gneiss (metamorphosed granite). The surrounding mountains are granodiorite, and of course there are abundant granodiorite cobbles in the stream courses, but the quantity and percentage of metamorphic rock in the streams and rivers is strikingly high. That suggests to WC that the streams are cutting through metamorphic rocks, which occur mostly on the margins of plutons.

The Canyon of the South Fork of the Payette River, Idaho

The exposures in the river an stream canyons show a lot of metamorphic rock, too. Of course, the overwhelming number of outcrops are granodiorite, because the overwhelming percentage of rock is granodiorite. And, because it is comparatively softer and more fractured, most of the native rock has eroded away. 

It’s messier than that, because geology is always messy. Since the plutons arrived over millions of years, it follows that the later arriving molten rock fractured and metamorphosed the earlier arriving plutons, making them weaker and more susceptible to erosion. And this vast a mass of rock is faulted, sometime very impressively. Faults are another weak point in rock, and as a result more easily eroded. 

Admittedly, it’s just an idea, but to borrow a phrase from Barry Hughart, an idea is better than empty headedness, which can only led to more empty headedness, or a career in politics.3 If you have a better explanation, WC invites you to offer it.


1 Small plutons, those having an area of 100 square kilometers or less, are called “stocks,” because geology doesn’t have enough weirdly inappropriate words already.
2 The plutons of the Idaho Batholith are just a small part of an entire series of plutons that extend from Alaska to Patagonia in southern South America. Various parts of the Pacific Ocean have been shoved under the western margins of North and South America for a very long time.
3 Bridge of Birds, Barry Hughart. One of of the best books you’ve never read.