The Batholith Meets the Overthrust


WC and Mrs. WC recently concluded a trip into North Idaho, up the Lochsa River over Lolo Pass, then south down the Bitterroot River Valley and up over Lost Trail Pass. We explored part of East Idaho for a bit, and then headed west across southern Idaho with a delightful stop on the Little Wood River. The next few blog posts will mostly focus on that trip. And while the posts will mostly be about birds, we’ll start with some pretty cool geology.

The Lochsa River is a branch of Idaho’s Clearwater River that drains the westerly side of the Bitterroot Mountains. The Bitterroots, in turn, are the continental divide: streams of the westerly side of the mountains drain to the Snake and Columbia Rivers, down to the Pacific Oceans; streams on the easterly side like the Bitterroot River, drain to the Gulf of Mexico via the Missouri and Mississippi Rivers.

The Bitterroot Mountains are still rising: the Lochsa River has a very steep gradient along most of its length, and unlike most older river valleys there are very few terraces, older banks of the river higher up the canyon walls. But what’s most interesting is that a northerly part of the Idaho Batholith – the Bitterroot Lobe – is dissected by the Lochsa River. As you drive upstream, you get a geologic sandwich: the native, Precambrian metamorphic gneisses and schists, then 40 or so miles of batholith and then, near Lolo Pass, more of the Precambrian metamorphics.

As geologists reconstruct events, the Bitterroot Lobe of the Idaho Batholith forced itself up, under the pre-existing metamorphic and sedimentary rock. That lifted the native rock upwards, and melted its underside, at the contact with the granitic batholith. The melted rock was a weak point as the batholith pushed the country rock upwards, and eventually all that upper rock, an area measured in hundreds of square miles, very slowly slid eastward along an eastward sloping fault.

Here’s an analogy: when WC baked his first birthday cake – for his younger brother, in fact – the icing was a little runny. When WC picked up the completed cake to carry it to the table, eight-year old WC carried the cake at an angle, and the topmost layer, greased by that runny icing, slid partially off the cake. Substitute semi-molten rock for the runny icing, and the uplifted native rock for the top cake layer, and you’ve got a kind of analogy for the Bitterroots. That top layer slid – was overthrust – over the course of some 5 – 7 million years some 40 to 80 miles east of its original location. Today, it’s part of the Bitterroots and most of the Sapphire Range on the easterly side of the Bitterroot River Valley. By no means the most dramatic overthrust in America, but still pretty impressive.

The area around Powell Campground and Powell Ranger Station, about 12 miles from Lolo Pass and the continental divide, is in the border zone between the batholith and the metamorphic complex. There are some very nice outcrops.

Outcrop near Powell Ranger Station, Lochsa River, Idaho

On the left is the pale gray granodiorite of the Bitterroot Lobe of the Idaho Batholith. On the right is the metamorphic native rock. The characteristic wavy beds extend up and to the right. At two places the native rock cracked under the pressure and stress, and the granodiorite forced its way into the cracks, creating dikes. The lower dike was fluid enough that it transported a fairly large amount of diorite into the lower right. The dike filled without shifting the joint of the granodiorite and native rock, and the dike extends a considerable distance into the granodiorite. The dike was created after the granodiorite was congealed enough to crack.

Outcrop near Powell Ranger Station, Lochsa River, Idaho

This outcrop shows dikes of different ages. The ones on the left are newer, and they are older as you move to the right, showing increasing distortion as a result of metamorphism. The rightmost band of granodiorite has been so modified by pressure and heat it is almost a gneiss. The dark gray/black rock is some other form of magma, possibly andesite. The gas pocket-riddled area to the left center is beyond WC’s identification skills, but the gas pockets, the vesicles, through a loupe, are lined with crystals of smoky quartz.

Outcrop near Powell Ranger Station, Lochsa River, Idaho

In this last photo, the dikes get older from lower right to upper left. This outcrop is less than five feet from the one in the second photo, yet the layering is reversed. The andesite-like rock is on the lower right. Geology, as WC might have mentioned before, is a messy business.

There’s more information on the Lochsa River and its history at The Clearwater Story, written by a career Forest Service employee, in the early 1960s. It’s a good read. But the detailed geologic history is yet to be teased out.