Mt. Bonaparte's Icy Past

View looking southeast at Mount Bonaparte. The peak is the high point at top left. 

After visiting Moses Mountain and finding it had been covered by the Cordilleran ice sheet, I moved on to Mount Bonaparte, 29 miles (46 km) to the north-northwest.

Cheryl and I have hiked and explored the geology of the landscape over which Mount Bonaparte presides. We have helped run geology field trips around Mount Bonaparte and met people from the little towns of Wauconda, Havillah, and Chesaw, within Bonaparte's elevated gaze. We have camped and picnicked in the shade of ponderosa pines on the shores of Lake Bonaparte and Lost Lake, which are in glacially eroded troughs east of the mountain.

From time to time, the glaciation of the Okanogan Highlands has come up for discussion and the question of whether the ice sheet covered the top of Mount Bonaparte has been raised, but not answered. Because the answer to that question is important to understanding the nature of the Cordilleran ice sheet, and because it was a matter of discussion among folks who lived in the area, I was motivated to investigate the answer.

To get to the top of Mount Bonaparte, I drove up a Forest Service road to a trailhead on its north side and then walked up a trail about three miles south and 2,500 feet up.

Mount Bonaparte is 7,257 ft (2,212 m) high, nearly 500 feet (150 m) higher than Moses Mountain. For over twenty miles in all directions there is no other peak nearly as high as Mount Bonaparte.

The peak of Mount Bonaparte is at the northeast end of a ridge more than a mile long. The peak is rounded and partly tree-covered.

From a distance, the peak and overall profile of Mount Bonaparte look similar to Moses Mountain and other high points of the Okanogan Highlands, a region known for the rounded and smoothed appearance of its peaks and ridges. These characteristics are commonly ascribed to erosion by the Cordilleran ice sheet.

Up close, however, Mount Bonaparte turned out to have a few distinctive differences from Moses Mountain. These differences revealed themselves step by step in front of me during my visit.

As I hiked up the trail, which used to be a road, I was continuously walking through what I interpreted to be glacial till, as shown by all the loose, randomly oriented boulders of various sizes mixed into the dirt, the partly rounded-off boulders 1-2.5 m (3-8 ft) across sticking up from the ground here and there, and some boulders being stacked on top of each other.

The large boulders of granitic rock I saw on Bonaparte appear to be from the Mount Bonaparte pluton, which forms the bedrock of Mount Bonaparte. 

Among the pieces of cobble in the dirt, I kept finding rocks that did not come from Mount Bonaparte. The most common out-of-place rock type in the cobble and gravel-sized rocks was slightly metamorphosed mudstone.

Depending on which piece I looked at, the rock might qualify as slate or as phyllite, which is a more shiny and wrinkled-looking metamorphic rock than slate. Or I could call it argillite, which is a clay-rich, fine-grained mudstone that got buried deeply enough to get thoroughly compacted and hardened, but not quite metamorphosed. Because an argillite did not have its pre-existing mud-rock minerals replaced with new ones, it still counts as sedimentary rock rather than metamorphic rock. 

The various samples I found span the boundaries of argillite, slate, and phyllite. I'm just going to call it argillite, even though some samples of it might better be classified as slate or phyllite.

The thin, white layers of fine-grained quartz in the argillite may be chert that originated from the siliceous remains of plankton that settled onto an ocean floor which was also accumulating mud. Or they could be from layers of quartz-rich sand that got deposited by moving water, alternating with the dark clay-rich layers that settled out of stagnant water.

These fine-grained seafloor sediments got buried and turned into solid rock. Then they got cooked and squeezed even deeper inside the crust to be metamorphosed. The argillite cobbles match rocks found in the accreted terrane known as Quesnellia, which forms bedrock several miles to the north of Mount Bonaparte.

Thin white layers in the argillite show folding.

The pieces of argillite that nature sprinkled into the dirt on Mount Bonaparte are exotic in two ways.

First, they come from the periphery of a volcanic island arc that was shedding sediment into a marine trench out in the ocean away from the North American continent. Then the lithified sediments were shoved in and accreted to the continent at a convergent tectonic plate boundary, getting heated, squeezed, and recrystallized in the process. That makes these rocks exotic to the continent of North America, qualifying them as part of what is called an accreted terrane.

Second, these pieces of metamorphosed rock were picked up from the exposed bedrock of the Okanogan Highlands, transported to the south several miles, and left on Mt. Bonaparte. The pieces of argillite do not match the local Mount Bonaparte pluton bedrock, making them exotic in that sense as well.

Argillite found atop Mt. Bonaparte.

The most likely way these pieces of metamorphic rock could have gotten picked up and deposited up on Mount Bonaparte is by the ice sheet. It is possible for granitic plutons to have inclusions of metamorphic rock in them, rock that got incorporated into the granitic magma as it intruded upward into the pre-existing solid crust. However, I did not see any inclusions of metamorphic rock in the Mount Bonaparte pluton and the geologic maps and their descriptions of the Mount Bonaparte pluton report no such inclusions.

In addition, the argillite is too low in metamorphic grade to have come from bodies of pre-existing crust that were enveloped by the molten granitic magma of the Mount Bonaparte pluton. The argillite lacks any sign of the contact metamorphism that would have been caused by the heat from granitic magma.

That is why I think it came from several miles north of Mount Bonaparte, where argillite of a similar appearance occurs in bedrock, and why I think it must be the ice sheet that brought the pieces of argillite to Mount Bonaparte, as no other process, such as washing down a stream, makes sense, given the higher elevation of Mount Bonaparte.

The pieces of argillite are like a smoking gun, or rather, like bullets found on a crime scene, pointing to the ice sheet as the culprit. They are pieces of rock that were plucked from the accreted terrane rock north of Mount Bonaparte and mixed with the rest of the rock debris the glacier deposited on the mountain.

I kept finding these pieces of argillite dropped by the ice sheet mixed in with the other rocks in the glacial till all the way to the peak of Mount Bonaparte, which indicated that Mount Bonaparte was not a nunatak. At the time of peak ice, the entire mountain was submerged beneath the moving ice sheet.

Though both mountains were entirely covered by the ice sheet, I was intrigued by how the top of Mount Bonaparte exhibits some glacial features that I did not see on Moses Mountain.

In contrast with the pieces of argillite I found on Mount Bonaparte, in the mixture of loose rocks deposited on Moses Mountain I did not recognize any exotic rocks. However, I must admit that I was not entirely focused on looking for pieces of exotic rocks on Moses Mountain, and I only spent about an hour on top looking around.

This picture is not of Mount Bonaparte!
It is a flashback to the peak of Moses Mountain.

In addition, with regard to Moses Mountain, the bedrock to the north, in the direction the ice sheet came from, is dominated by granitic plutons, including the Moses Mountain pluton, the Mount Bonaparte pluton, and other intrusive bodies of granitic rock. The rocks in other plutons north of Moses Mountain look a lot like the granitic rock in the bedrock of Moses Mountain.

Therefore, it is possible that some of the loose granitic rocks on Moses Mountain came from one of the other bodies of granite that lie to the north of there, were transported southward and deposited on Moses Mountain by the ice sheet, and that I was not sufficiently attuned to the subtle differences between the different plutons in the area to distinguish them from the granite of Moses Mountain.

The bodies of granitic rock underlying Moses Mountain and Mount Bonaparte, and most of the country between, have similar ages. (If you want to know the age, those bodies of granite intruded and turned solid between 60 and 49 million years age. This is according to potassium and argon isotope ratios in the rocks, a method that gives an approximate age of when granitic rocks had solidified and lost some of their initial heat.)

According to the descriptions that accompany published geological maps of the area, and my own experience in the area, the granitic rocks in those plutons look similar to each other. So if there were pieces of out-of-place granite on Moses Mountain that the ice sheet brought in from up to thirty miles away, that is my excuse for not recognizing them as exotic boulders.

The top of Mount Bonaparte, like the top of Moses Mountain, is broad and almost flat. Presumably, the USGS marker is where the surveyors determined the high point.

Just like Moses Mountain, Mount Bonaparte has a lookout tower on top. Unlike the tall metal tower that I saw sticking up into the damp clouds on top of Moses Mountain, the lookout tower on Mount Bonaparte is only about 35 feet tall and is made mostly of wood.

The lookout tower, where John was working, and an old, no-longer-used outhouse on top of Mount Bonaparte. The large pine tree on the left is approximately to the south in this view. The flat, slab-like surfaces of the outcrops dip gently to the north.

I met John the lookout. He told me a few stories about what it was like being up there for weeks at a time, spotting and reporting fires, with only the occasional hiker visiting. While I was there, in fact, John saw a lightning strike set a tree on fire a few miles to the south. He called it in by radio, his first original fire report in a while, which was rather exciting. Later I checked and found out that, whether it was due to being put out or because of the showery, wet weather, the fire never spread, which was good to know.

John pointed out that a branch of the regional Pacific Northwest Trail runs across the northern shoulder of Mount Bonaparte, passing within about a mile of the peak, and this had led to an up-tick in the number of people visiting him because some of the PNT hikers take the side-trip to bag the peak and catch the view.  

The top of Bonaparte is different from Moses not only because of the scattered bits of argillite in the mixture of loose rocks, but also because of the more extensively exposed slabs of Bonaparte Pluton granite. The tops of these slabs consistently slope downward to the north.

On top of Mount Bonaparte. North is approximately to the left, south to the right.

Exposed are large slabs of Bonaparte pluton granite bedrock, most of them still attached to the rest of the rock of Mount Bonaparte, with much smaller, loose boulders here and there. Finer-grained material between boulders includes gravel, sand, and clay. The mixed, loose material is probably glacial till. The ice sheet would have been moving to the right, which is to the south.

The fact that the left-leaning slopes of the slabs are flat and smooth, and the right-learning slopes are abrupt and steep, is consistent with how glaciers tend to smooth the stoss sides (the up-flow-facing sides) of outcrops by abrading them and steepen the lee sides (the down-flow-facing sides) of the outcrops by plucking boulders from them.

The granitic bedrock at the top of Mount Bonaparte is foliated, which means its minerals are arranged in parallel layers. Geologists use the terms strike and dip to describe how planar surfaces are oriented in three-dimensional space. The foliation strikes (horizontal lines on its surface run) in approximately an east-west direction and dips (runs downhill) approximately to the north at a steep angle, as I try to show in the photo below.

By the way, rocks that are foliated are generally classified as metamorphic rocks, because the mineral growth happens through a process of solid-state metamorphic reactions, with no molten rock present. This seems to contradict the idea that the bedrock on Bonaparte is granitic rock, which is solidified magma. In some places, such as at the top of Mount Bonaparte, the granitic rock appears to have been overprinted by metamorphism. In other places, not so much, so just to make it easy to map and name it, it's all called the Mount Bonaparte pluton, even though some of it may have been recrystallized into foliated metamorphic rock after the granite was solid.

The rock became foliated when it was deep in the Earth's crust, undergoing metamorphism. The metamorphosed minerals align parallel to each other in response to the rock being squeezed by tectonic pressure, forcing the minerals to grow long or wide at right-angles to the direction of maximum squeeze. As for why the white and black minerals end up growing in partly separate layers, that has to do with how chemical and physical forces acting on the growing minerals shift them toward their lowest-energy, most "comfortable" arrangement, which is largely by nestling in with other minerals of their ilk.

Now, back to how all joints and foliations in the rock relates to how they crop out, and how that relates to the way the rock has been eroded at the top of Mount Bonaparte. The photo shows a closer view of one of the northward-sloping slabs of bedrock that crop out on the peak of Mount Bonaparte.

North is to the left in this photograph.

In the photo above, alignment of minerals on the surface of the outcrop reveal the foliation within the rock. The red lines trace the parallel rows of black and white minerals across the front and top of the outcrop. I laid the walking stick nearly parallel to the direction the minerals are aligned on top of the outcrop.

The foliations are the planes, or two-dimensional surfaces, of parallel minerals running through the rock. You can see the orientation of the foliation planes in three dimensions by combining the lines on the front side of the outcrop with lines on top of the outcrop. Each intersecting line is the edge of a foliation plane. The foliations dip (slope downhill) very steeply toward the north.

The blue shading indicates that the top of the outcrop is a plane that dips gently to the north.

All the bedrock slabs that I saw cropping out on top of Mount Bonaparte have several things in common. First, they have that foliation striking and dipping the same way as in this outcrop. Second, the roughly planar tops of the outcrops are all approximately parallel to each other, like the one I tried to emphasize with the blue plane in the photo above.

And there is one more thing these slabs of rock all have in common. Beneath their surfaces, they have in common a prominent set of what geologists call joints, which most people would probably call cracks.

It is common for rocks to have sets of joints that are parallel to each other, i.e. with the same strike and dip. The main joint set in the rock on Bonaparte's peak is oriented the same way as the exposed surfaces of the outcrops. It strikes east-west and dips gently toward the north. The joints are spaced 0.3 to 1.2 m (1 to 4 ft) apart and cleave the outcrops into stacks of slabs, as shown in the photo below.

This makes it seem likely that the tops of the slabs are little more than exposed, slightly-eroded joint surfaces.

Side view looking east at bedrock outcrops on mountaintop. The bedrock is parting (splitting apart) on joints (cracks) that slope to the north. The glacier would have been moving from left to right.

When the ice sheet arrived, the bedrock on top of Mt. Bonaparte was already jointed, already splitting apart in this preferred direction. Sloping gently to the north, the joint surfaces faced in what is called the stoss direction, the direction the glacier came from. The base of a moving glacier, its ice laden with grit, dirt, and larger pieces of rock the glacier has picked up, all pressed down by the weight of the total ice thickness above, tends to abrade and smooth stoss-facing slopes.

On the lee side of the peak, the direction the glacial ice departed, the glacial ice would have plucked pieces of the jointed slabs out, leaving near-vertical, south-facing steps in the bedrock. Flowing glacial ice has a propensity for plucking chunks of bedrock from lee-side slopes. Glaciers tend to pluck boulders out of the bedrock beneath them on the lee sides of outcrops. This plucking of lee slopes is due to the differential velocities and pressures in the ice sliding over the rock and then pulling away from the outcrop, and the effect of pressurized ice melting and then refreezing in cracks in the bedrock, which pries the rock apart because frozen water has 9% larger volume than liquid water, combine to favor breaking and lifting of chunks of rock into the moving ice from the lee sides of outcrops.

Looking north at the steep, lee side (down-glacier side) of outcrops on top of the Mt. Bonaparte. 

The shapes and orientation of the slabby, ledgy outcrops on top of Mt. Bonaparte were pre-conditioned by the jointing in the bedrock long before the mountain was glaciated. Given that the pre-existing joints in the granitic bedrock sloped gently in the stoss (north-facing) direction, this is how the erosional effects of a glacier moving north-to-south over the peak of Mount Bonaparte would be expected to have proceeded.

The foliations in the rock, which show up as parallel rows of black and white minerals on eroded surfaces that cut across the foliations, formed at a much earlier stage of the geological history of Mount Bonaparte, when the granite was metamorphosed by heat and pressure while still inside the Earth's crust. The foliations were not created by the glaciation of the top of the mountain, which took place tens of millions of years later at the Earth's surface.

In the photo above, the lines of minerals are the edges of foliations intersecting the surface of the outcrop. The foliation planes strike (run horizontally) east-west and dip (slope downhill) nearly vertically. South is to the right, the direction the ice flowed.

Because it was oriented sideways, at right angles, to the direction the ice flowed, the parallel orientation of minerals in the rock did not have a positive, mutually-reinforcing interaction with the way the base of the glacier eroded the rock.

In places where layers or alignments in the bedrock are oriented parallel to the direction of of glacial ice flow, glacial erosion may proceed more easily in the softer layers and with less frictional resistance overall, forming grooves that follow the contrasting layers in the rock and more easily eroding outcrops into streamlined shapes.

On Mount Bonaparte, because the orientation of the layers of minerals were at right angles to the direction of glacial ice movement, the rock did not lend itself so easily to deep etching or grooving of the rock surfaces.

The fact that the minerals in the rock form parallel lines across the outcrops has nothing to do with the effect of the ice sheet, except that the ice sheet eroded and cleaned the rock down to its current level of exposure, making it easy to see that the rock consists of layers of metamorphic minerals, i.e. is foliated.

As I poked around the top of Mount Bonaparte examining all the joints, foliations, and outcrop shapes, similar pieces of argillite, with thin white layers interspersed in the dark gray rock, kept showing up in the loose, rocky debris. Another piece of argillite from the top of Mount Bonaparte is shown above.

The low amount of weathering of the bedrock surfaces on Mount Bonaparte is consistent with having been scoured off less than 20,000 years ago, during the last glacial maximum of the Pleistocene epoch.

The freshness of the rock exposures, the erosional shapes and patterns of the bedrock outcrops on the peak, the mixed sediment that appears to be glacial till blanketing the mountain all the way up to the rounded peak where it is distributed in thin patches, and the exotic "erratics" of argillite I found in the mixed-rock debris strewn across the top of Mt. Bonaparte, make what seems to me a compelling case: The Cordilleran ice sheet flowed over the top of the mountain. Like Moses Mountain, Mount Bonaparte was not a nunatak, not when the ice sheet was at its maximum thickness.

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OK, now to address the other question. Were there any alpine glaciers on Mount Bonaparte at times when the ice sheet was not there?

The evidence I saw of possible alpine glacial valleys on Mount Bonaparte was from miles away, looking up from below, and from looking at the topography mountain on Google Maps. This cursory analysis has deepened my suspicion that Bonaparte did have alpine glaciers on it at some point.

Every time I have driven on the Havillah-Chesaw Road and looked up, I have wondered about the shape of Mount Bonaparte and the two shallow, possibly U-shaped valleys near its crest. (And then my wife reminds me to keep my eye on the road as I narrowly miss driving into the ditch.) These two swales indent the northwest side of the the ridge that extends southeast from the peak, and it has occurred to me since the first time I swerved on the road while looking at them that they might be due to short-lived cirque glaciers having eroded those valleys.

View looking southeast at Mount Bonaparte. The peak is the high point at top left. Coming down from the ridge to the right of Mount Bonaparte's peak are a couple of shallow valleys, largely in shadow, that might have had alpine glaciers in them. 

High on the north to northeast slopes of Mount Bonaparte are three more shallow valleys, which on topographic maps look like they have amphitheater-shaped upper margins, the type of valley eroded by cirque glaciers.

These possibly alpine-glacier-sculpted valleys on Bonaparte originate near the 6,500 ft (2,000 m) elevation line. The fact that they are all on the north-facing sides of the mountain is consistent with the hypothesis that they had alpine glaciers in them because north-facing slopes receive less direct sunlight, giving them a higher chance of forming a glacier.

Google terrain view (topographic map) with the five valleys pointed out. Valleys a and b are the same ones labeled in the previous photograph. 

The valleys in question lack high, vertical cliffs on the headwalls and do not have closed-contour-line basins in their upper ends. This suggests to me that if there were alpine glaciers in these valleys, they were there a relatively short time and did not erode very deeply into the sides of Mount Bonaparte.

It would require more work, including scrambling through the woods into these valleys to observe their geology as closely as possible, to determine whether there were alpine glaciers on Mount Bonaparte, and if so, when they were there relative to when the ice sheet last flowed over the top of the mountain.

Some Parting Thoughts on My Visits to Moses Mountain and Mount Bonaparte

Assuming my interpretation is correct and both mounts Moses and Bonaparte were completely overridden by the Cordilleran ice sheet, what does that tell us?

For one thing, the ice was not very deep and not moving very fast over the tops of the peaks. That is to be expected, because: 

1. The vertical distance to the top of the maximum ice surface from these high points in the Okanogan Highlands would have probably been less than 1,000 ft (300 m).
2. The ice sheet would have flowed much more slowly across the highest-elevation, mountaintop areas than it did through deep, trough-shaped valleys oriented in its direction of flow, and across low-elevation plains.

The thickness of the Cordilleran ice sheet has been reconstructed (in other studies not cited here) based on the maximum heights it covered on mountains in British Columbia combined with shapes of ice sheets based on today's Greenland and Antarctic ice sheets and theoretical models. The paucity of published measurements of the maximum elevation of the ice sheet on the American side of the border, based on which peaks were nunataks and which were not, is part of what is motivating my research on these peaks. Based on looking at what has been published, estimates for the thickness of the ice sheet in the vicinity of mounts Moses and Bonaparte range from below the tops of the mountains to less than 1,000 feet above the tops of the mountains.

The way the Cordilleran ice sheet flowed across the tops of high-elevation peaks contrasts with how it flowed in the lowlands. The ice sheet is thought to have undergone ice-streaming south through the master valley of the region, the Okanogan Valley, and out across the Cameron Lakes and northern Waterville plateaus at the south end of the Okanogan Valley. Ice streaming occurs where the ice is channeled into troughs or driven across low-relief zones with low-friction surfaces, aided by melt water at the base of the glacier. The rate at which the ice flowed in the low-elevation ice-streaming zones was orders of magnitude higher than the rate at which it moved over top of Moses Mountain and Mount Bonaparte, where the ice sheet was thin and moved slowly.

A related question is whether the glacier was cold-based, which means it was fast-frozen to the underlying rock and dirt, on the high parts of Moses and Bonaparte. It is possible that at the coldest stage of the last glacial maximum it was cold-based. However, the signs of smoothing and plucking of bedrock on these peaks, along with the glacial till, suggests that at some time the ice sheet on these peaks was warm-based. While it was a warm-based glacier it had at least a thin film of meltwater at the base of the ice which helped enable it to slide along the ground and smooth the stoss sides of outcrops, while the freezing and thawing of water in the lee-side cracks in the bedrock helped it pluck out chunks of rock and move them.

The thinness and slow flow rate of ice over the tops of these peaks could account for why most of the slabs of bedrock on top of Moses Mountain appear to have been only shuffled locally, shifted a few feet to a few hundred feet from where they were plucked. It could also explain why in at least one place on the lee side of Moses Mountain there is an old, pre-ice sheet weathering profile more than ten feet (3 m) thick on the granitic bedrock that was not completely scoured away by the glacier.

On both mountains, the thin, slow-moving nature of the ice sheet is why there are no deeply etched and sculpted landforms such as roche moutonnées, whalebacks, and crags-and-tails, which are all subglacial erosional landforms that tend to occur in zones of ice streaming.

The tops of Moses Mountain and Mount Bonaparte would probably have been in the zone of accumulation during the last glacial maximum, which means the snow that fell there each year did not melt away in the summer. Layer by layer, the accumulated annual snow built up and the deeper layers became glacial ice. The rate of glacial ice flow is generally lower the higher one goes in the zone of accumulation, with local variations possible where there is a funneling effect from a constraining valley, and depending on how steeply the glacier slopes downhill.

The slope of the ice sheet surface above Bonaparte and Moses, when the ice was at its thickest, was probably relatively low, tapering gradually from higher elevation in the main zone of ice accumulation across the border in Canada. The shape of the Cordilleran ice sheet, including its thickness and the slope of its surface at each point, has only been roughly estimated. Knowing which, if any, mountain peaks in and around the Okanogan Highlands were nunataks, and which of the high peaks were covered, helps pin down the shape of the ice sheet.

The ice sheet was thin on top of these mountains and in no hurry. It plucked and moved some slabs of granitic bedrock around, tended to smooth the stoss sides of the outcrops, tended to pluck and steepen the lee sides, left some boulders piled randomly on top of each other, and scattered a thin veneer of glacial till. The signs left by the ice sheet on these mountaintops are more subtle than the erosional and depositional calling cards left by ice sheets in the valleys and plains, where the ice was much thicker and flowed much faster.