The Sierra Beyond Yosemite: Donnell Vista and Sonora Pass

It's gloomy and foggy, and I haven't seen the sun for days. It's a few more weeks before field season (Death Valley!), but I can't help exploring the sunnier places from warmer times. I'm looking back at the pics from our fall semester where we explored a lot of places in the Sierra Nevada that aren't Yosemite.

That's the thing. Say "Sierra Nevada", and a lot of people will immediately think of the beautiful valley of the Ah-wah-nee, John Muir's favorite place on the planet, and in many ways mine as well. But the floor of Yosemite Valley is about 7 square miles. The national park covers 1,190 square miles (3,081 square kilometers). But the Sierra Nevada? It covers 39,612 sq miles (102,594 km²).  You could hide more than 30 Yosemite parks in the rest of the range. It is in fact the largest single range in the lower 48 states (large mountain systems like the Rockies and Appalachians are made up of numerous smaller sub-ranges).

So we are off onto a short exploration of some of the wonderful corners of the Sierra Nevada that aren't Yosemite Valley. We are following a week's worth of our trips last fall that took us over the range at Sonora Pass and down the east side of the range as far as Lone Pine and Mt. Whitney. We'll also explore the other national parks of the range, Sequoia and Kings Canyon, which we visited on a second trip.

We began our journey in some serious smoke from a series of fires burning through the western slopes of the Sierra Nevada. The drought and the fires have been catastrophic. We made a stop at the Twain Harte Lake exfoliation site. I wrote about it back then, and it was picked up on Reddit and IFLS, links that led to the post being the most read ever on Geotripper (12,400 hits and counting). We emerged from the smoke and climbed the upper reaches of the Stanislaus River, approaching Sonora Pass, which after Tioga is the highest paved highway over the Sierra Nevada at 9,624 feet (2,933 meters). Tioga Pass in Yosemite is 9,943 ft. (3,031 m.).

In a series of ice ages, glaciers covered about 30% of the range, reaching as low as 3,000 feet or so in some of the deeper canyons. Yosemite is simply the most famous of the glacially carved gorges, and many others are of incredible and spectacular beauty. This was not always fully appreciated, and some of these wonderful wild canyons were dammed for irrigation storage and domestic use. Hetch Hetchy is the most familiar, but the canyon below Donnell Vista on Highway 108 has also been inundated. Still, the glacial heritage of Middle Fork of the Stanislaus is evident from the viewpoint. The steep canyon walls of granitic rock and the overall U-shape of the valleys are the result first of ancient river erosion and then modification by thick rivers of ice.

If you look at the second picture above, you can see some unusual looking mountain peaks. Their blocky flat aspect indicates they are composed of something different than the "expected" granitic rock. They are the remains of lava flows, ash flows and volcanic cones that once covered this part of the Sierra Nevada. Indeed, until 9 or 10 million years ago, the Sierra looked far more like today's Cascades Range than the lofty glacial peaks we see today. There were a number of snow-covered stratovolcanoes, but much of the remainder of the range was composed of lower hills. The upper reaches of Highway 108 where it crosses Sonora Pass cut through some of the volcanic rocks.

We stopped a mile or two short of the pass to get a detailed look at the granitic rocks. Depending on the relative proportions of plagioclase and orthoclase feldspar and quartz, rocks may be identified as granite, granodiorite, tonalite, diorite, or monzonite. The rock exposed just below the pass is called the granodiorite of Topaz Lake, dated about 89 to 83 million years ago, during the Cretaceous era. It was intruded in the deep crust about 4 or 5 miles down where it cooled slowly, forming visible crystals of feldspar, quartz and dark minerals like biotite mica and hornblende.

Glaciers scoured the surface of the granodiorite, polishing it and providing a nice view of the structure of the rocks. Some of the orthoclase (potassium feldspar) has formed huge blocky crystals easily visible in the shot below. Even better, during the intrusion process, blocks of the surrounding rock broke off and sank into the magma. Composed of minerals that had higher melting points, it didn't melt, but instead persisted as an alien mass in the granitic rock. Such inclusions are called xenoliths. They provide a peek at what existed here before the intrusion of the magmas.

We drove over Sonora Pass and headed into the barren lands beyond. Our destination was the site of a gold rush, but not the one that Californians are familiar with.

Northern Convergence: The Olympics, Where a Trench Became Sky-Piercing Peaks

Our journey through Western Canada and the Pacific Northwest began as we met with our students in the Seattle area one evening in late July. After a complicated couple of hours of meetings and negotiating van rentals (reservations three months earlier are only the preliminaries), we settled in for the night, and prepared to hit the road at 6:30 AM. We had a long day ahead that wouldn't end until we rode the last ferry across the Strait of Juan de Fuca, arriving in Victoria at 11 PM. It wasn't the kind of itinerary I like, but in the end it worked out (as noted in one of the few posts I was able to complete during the trip itself).

We wove through the morning traffic of Tacoma and Olympia (luckily most everyone was headed the opposite direction). We reached the village of Blyn near Port Angeles by 9 AM or so where we stopped at a roadside rest to have our first introductory presentations on the geology and anthropology of the Puget Sound and Olympic Peninsula. The skies were somewhat overcast, which was worrisome because a storm was rolling in soon, and we had hopes of having a clear view of the Olympic Mountains from Hurricane Ridge.

Mrs. Geotripper and I had paid a visit to the ridge a few days earlier and were treated to spectacular views, and I would have been heartbroken if we had showed up with the students to a fogged-in viewpoint. It's happened before; you can ask my students of their memories of seeing Mt. St. Helens in 2011 and get blank stares. We spent most of a day at St. Helens looking at fog, rain, and a few downed trees next to the highway. I didn't want such a thing to happen again, so I was tense as we started the climb out of Port Angeles towards Hurricane Ridge. In fog.

To my great relief, we broke through the clouds and drove into bright sunlight. We climbed the winding highway through the thick forest with increasingly far-ranging views. But nothing quite prepares anyone for the view from the end of the paved road at Hurricane Ridge. It is simply astounding.

The Olympic Mountains are geologically distinctive, to say the least. The mountains rise from sea level to nearly 8,000 feet and are extremely rugged. They capture prodigious amounts of rain and snow on the western flanks, so much so that temperate rainforests coat the western slopes. It would have been a nightmare for geologists who were trying to unravel the geologic history.

Subduction is the story of the Pacific Northwest. For most of 200 million years a convergent boundary has been active in the region, as the crust of the Pacific Ocean basin has been sinking against the edge of the North American Continent. In some places, for instance California, the subduction zone has been replaced by a transform boundary (the San Andreas fault). But in Northern California, Oregon, Washington, and part of British Columbia, the subduction zone is still active, still producing earthquakes, and still raising mountains. It's called the Cascadia Subduction Zone

Source: Geological Society of America

In a "normal" subduction zone, there are four parts: the trench, an accretionary wedge, a forearc basin, and a magmatic arc. The trench is the deepest part of the ocean floor where the oceanic crust sinks back into the mantle. The accretionary wedge is a collection of seafloor sediments and crust that has been scraped off the subducting plate and added to the edge of the continent. The forearc basin is a shallow sea that may develop inland of the accretionary wedge (California's Great Valley originated in this fashion). The magmatic arc is a system of volcanoes and intrusive plutons resulting from the melting of rocks in the lower crust and upper mantle above the descending slab (water released from the slab lowers the melting point of the rock, leading to the formation of the molten rock). Other features may develop, depending on the angle of subduction or the geometry of the plate boundary. There will be more on those later in the series.

Looking at the thickly forested slopes below Hurricane Ridge, I cannot envy the geologists who originally mapped the Olympic Mountains. Simply finding an exposure of rock must have been challenging at times. What these geologists did was to take the rare rock exposures and extrapolate them into a semi-coherent map that reveals the structure of the Olympic Mountains. They did the equivalent of taking a few pieces of a jigsaw puzzle, putting them in the right location relative to the others, and then drawing in the remainder of the puzzle from scratch. I've been way too spoiled by the naked rock exposures of places like Death Valley and the Mojave Desert!

The geologic map reveals the basic structure of the Olympics. A "horseshoe" of basalt and sedimentary rocks (the Peripheral Rocks, or Crescent Formation) partially surrounds the "Core Rocks", an assemblage of lightly metamorphosed sandstone and shale layers. The Core Rocks are characteristic of the types of deposits that form from underwater landslides ("turbidity currents") within the trench and accretionary wedge of a subduction zone. The fact that these rocks are now thousands of feet above sea level is the interesting conundrum. Accretionary wedges are generally below sea level, or exist as small islands. They can be pushed higher. For instance, the rocks of the Cascadia accretionary wedge are exposed in the Coast Ranges of  Washington, Oregon and California, but nowhere are the exposures as spectacular as the Olympic Mountains.

Convergent boundaries can be exceedingly complex places. Bits and pieces of continents and island arcs may randomly arrive at the subduction zone, mucking up the subduction process the way too many sheets of paper at once can muck up a paper-shredder. In the case of the Olympics, there was a mass of land north (Vancouver Island) and an accreted terrane to the south (the North Cascades), and a bend in the subduction zone itself. In essence, too much material was being stuffed into the subduction zone, so the excess material went the only way it could, which was up (see my earlier post on this subject, "Sorry, this trench is full..."). The mountains have been rising for around 15 million years. They would be higher, but the incredible amount of precipitation tears the mountains down at a roughly equivalent rate.

The basalt is exposed along the Hurricane Ridge Road and along trails near the viewpoint. The slopes below Hurricane Ridge also include exposures the intensely folded shale layers.

When we first visited Hurricane Ridge the prior week, the highest peaks were obscured by clouds. When the class arrived, the mountains were clear and we could easily observe the glaciers that scour the upper reaches of the mountains. Glaciers technically shouldn't exist here. Although we were at a high enough latitude, the nearby Pacific Ocean moderates the climate, keeping things warmer than they would otherwise be (the Olympics are at the same latitude as Great Falls, Montana, or St. Paul, Minnesota). But temperature isn't the only factor in glacier development. The sheer amount of snowfall in combination with temperatures that are just cold enough allows glaciers to exist at these unusually low elevations.

We had a good introduction to the basic features of alpine glaciation as we gazed across the valley to Mt. Olympus. We could clearly see bergshrunds (the cracks that develop at the top of glaciers where they pull away from cliffs), and crevasse fields in the lower parts where the glaciers flowed over obstructions. There were horns, aretes, and cirques as well. Glaciers were going to be a big part of the story of British Columbia, and Hurricane Ridge provided a spectacular setting for the first discussion of how they worked.

We gave the students some time to wander the network of trails around the visitor center, and I set off to Sunrise Point to get a panoramic view. I was so relieved that the storm had not yet arrived, but of course it was still out there, and there would be a few consequences for our trip. But not on this most beautiful of days.

We headed down to Port Angeles for lunch and to find to road to the other major locale the day: Neah Bay and lands of the Makah people.

The Other California: Clark Fork of the Stanislaus, a quiet version of Yosemite

It's not really fair, of course. No place is like Yosemite, and I shouldn't compare Clark Fork to Yosemite. I doubt there is anywhere in the world that can compare to Yosemite Valley with the high waterfalls, sheer cliffs and strangely shaped domes. But there are some things that are not so nice about Yosemite. It's been used and abused for the last century and a half, and today something like four million people crowd into the few square miles of valley floor. It can be noisy, smoky, and crowded. It's possible to find lonely places on the valley floor where nature still dominates, but it's not easy.

Clark Fork on the Stanislaus River is a world apart from Yosemite Valley. Like Yosemite, it was carved by river erosion, followed by glacial scouring, although not to the same depths. There are small waterfalls and cascades, but nothing like the stunning leaps of Yosemite Falls or the others.

What is there is in Clark Fork is peaceful and uncrowded serenity. A single paved road diverges from Highway 108 and follows the valley for nine miles, passing just a few organizational camps and three campgrounds. No resorts, no stores, and no crowds. We were there on Memorial Day weekend, and there were still campsites available on Saturday evening. The road ends at beautiful Iceberg Meadow, which at this time of year was filled with Water Plantain Buttercups (to the best of my guessing)

Brooding over the meadow is the unique granitic monolith called simply The Iceberg. It's 8,350 feet high. From the perspective of the meadow it looks like a prominent peak, but a look at the topographic shows it to be the last knob of a long divide between Disaster Creek and Clark Fork. Just two miles away, Disaster Peak rises 1,600 feet higher.

The peak is composed of the granodiorite of Topaz Lake, which at about 84 million years is one of the youngest granitic intrusions in the Sierra Nevada batholith (granodiorite is similar in appearance to granite, but contains more plagioclase feldspar and mafic minerals that makes it slightly darker in color). The rock is broken up into joints that are caused by pressure release when the rock is exposed by erosion. This explains the general lack in Clark Fork of bold cliffs like El Capitan or Half Dome, which are relatively less affected by jointing. The rocks of peaks like the Iceberg are more readily eroded by glaciers, which can pluck out the jointed blocks or rock.

Photo by Mrs. Geotripper

Very much unlike Yosemite, there is an abundance of volcanic rocks on the higher ridges above the valley. They are visible from various points along the road and can be accessed by trails that lead up Clark Fork and side canyons like Arnot and Disaster Creeks. Similar volcanics are found along Highway 108 near the summit of Sonora Pass. As noted in the previous post, these rocks were erupted between 10 and 12 million years from a volcanic center along the Sierra crest, similar in appearance to the Mt. Lassen volcanic complex further to the north.

We didn't have a lot of time on our visit last Saturday, but we walked around Iceberg Meadow and saw not a single other person. Cattle are still allowed to graze in the region, but the meadow itself is fenced off to preserve it from damage (deer have no problems jumping the fences). Despite the drought, the meadow is a bright emerald green in places, though in some parts it looked bluish from the impending bloom of some irises or other swamp flowers. It's only May, and it should be still partly covered in snowbanks, but we'll take the short spring over none at all. I hope the drought breaks this year...

Photo by Mrs. Geotripper

Clark Fork is a wonderful part of the Sierra Nevada, one of those largely ignored corners of a beautiful mountain that would probably be a national park in any other setting. It is the kind of place that John Muir wrote about in the Mountains of California when he said “Climb the mountains and get their good tidings. Nature's peace will flow into you as sunshine flows into trees. The winds will blow their own freshness into you, and the storms their energy, while cares will drop away from you like the leaves of Autumn.”

Photo by Mrs. Geotripper

This is one of the irregular entries in my blog series called "The Other California" which existed long before any Toyota commercials with a similar slogan. It is an exploration those places in my fair state that don't always show up on the tourist postcards, but which have incredible geology and are scenic to boot.

The Other California: Highway 108 and Donnell Vista - like Yosemite, only with Volcanoes!

Those of you who've followed my blog over the years may remember that I produced a blog series called "The Other California" which existed long before any Toyota commercials with a similar slogan. I come back to it time and again when I find (or rediscover) those places that don't always show up on the tourist postcards, but which have incredible geology and are scenic to boot.

Today we are exploring a few corners of the Upper Stanislaus River drainage in the country north of Yosemite National Park. People from outside the region can be forgiven if they think that the Sierra Nevada is just Yosemite Valley, Sequoia National Park, and Lake Tahoe. The range is 400 miles long, and pretty much all of it is spectacular, except Yosemite and Sequoia are just a little more so. I cannot doubt that if Yosemite Valley didn't exist, the upper Stanislaus would have been one of California's national parks. In some ways, Sonora Pass and the upper Stanislaus are even more interesting in the geologic sense. The reason? Volcanoes.

Donnell Vista Point on Highway 108 about 18 miles above Pinecrest Lake and Strawberry is the site of today's exploration. The parking lot and quarter-mile trail to the viewpoint were recently renovated with funds provided by the America Recovery and Reinvestment Act. The old worn-out trail was resurfaced, and a new ADA compliant trail winds along the western part of the slope, providing some new views to the west.

The first thing one notices from the overlook at Donnell Vista is the deep U-shaped canyon occupied by the lake behind Donnell Reservoir. During the Pleistocene Ice Ages, rivers of ice repeatedly scoured the gorge below, with the last glacial stage ending only about 12,000 years ago. The ice exposed the underlying granite, forming steep cliffs and numerous small rounded asymmetical domes called roches moutonnées. Some nice examples of glacial polish can be found in the region.

The ice in most places removed a cover of volcanic lava flows and any soils were stripped away as well. As a result, wide areas of granitic rock are exposed, and the forest grows only in fractures and joints where bits of soil can accumulate.

Glaciers also scour out basins where lakes can subsequently form, but the lake below the view point is clearly not natural. It is Donnell Reservoir, constructed in 1957 by the Oakdale-South San Joaquin Irrigation district for storage of water for agricultural irrigation and power generation. It stores a bit more than 60,000 acre-feet of water which today is only used to generate electrical power. Storage for irrigation now takes place downstream in the much larger (and more controversial) New Melones Reservoir.

The high peaks to the north and east of Donnell Vista preserve the evidence of the volcanic activity that took place here 10-12 million years ago. The few remnants not stripped away by the ice form the mesa-like peaks on the skyline. When the volcanoes were active, the region would have looked a great deal like the Mt. Lassen volcanic center, which remains active today.

The lava flows filled a fault basin near the present-day Sierra Crest in the vicinity of Sonora Pass, and at times spilled over into the adjacent river valleys. One lava flow traveled nearly sixty miles to Knights Ferry in the Sierra foothills above Oakdale. The landscape surrounding the lava flow eroded away during the uplift of the Sierra Nevada block, leaving the lava flow as a prominent ridge called an inverted stream.

The upper drainage and headwaters of the Stanislaus River up to and over Sonora Pass offers up some fascinating geology. I helped edit a field guide of the geology of the region for a meeting of the National Association of Geoscience Teachers in 2012. It's for sale through Sunbelt Publications and the proceeds support scholarships for geology majors in California, Nevada and Hawaii. Get more information about the book here.

PS: Ron Schott has a great gigapan shot of the view from Donnell Vista at  http://www.gigapan.com/gigapans/114032

The Other California: Springtime along the Great Western Divide

Lupines along the Kaweah River gorge (Moro Rock and Alta Peak in the distance)

The Great Western Divide? Where's that?

My off and on blog series on the Other California is an exploration of the little-known places in my fair state with interesting, even fascinating geological features. Sequoia National Park might seem too familiar a place to be included as part of the "Other California", given that my own definition of the series is that it should include those places that don't normally show up on postcards, and Sequoia National Park certainly does.

So why include Sequoia? The primary reason is that it actually is less known than other parts of the Sierra Nevada. Ask folks where they go in the Sierra, and Yosemite Valley or Lake Tahoe are often the first places mentioned. And people often come to the park not so much for the geology, but for the biology, mainly to see the trees after which the park is named. But the park has a rich geological heritage as well.

Moro Rock from the Kaweah River gorge

The shape of the Sierra Nevada is often described as a tilted block of granitic rock and metamorphic rocks. That's true in the Sierra Nevada north of Sequoia. Driving to Yosemite National Park from the west involves a gradual climb up the western slope that continues to the Sierra Crest before dropping precipitously into the Owens Valley or Mono Lake on the east. Sequoia and the Southern Sierra Nevada is quite different. When one arrives from the west (which actually is the only way in which the park can be approached by roads), the road climbs steeply to a plateau, and the slope continues to a high crest. But not of the Sierra Nevada. It's an entirely separate mountain ridge called the Great Western Divide. The actual crest of the Sierra Nevada lies farther east, across the deep gorge of the Big Arroyo and the Kern River.

The Great Western Divide is a spectacular mountain range. It rises nearly to the height of the actual Sierra Crest, with several peaks exceeding 12,000 feet in elevation (A dozen or so peaks on the Sierra Crest reach 14,000 feet). Despite being far to the south, the peaks were high enough to be scoured by the glaciers of the Pleistocene Ice Ages.

A few weeks back we had the chance to pay an early springtime visit to Sequoia National Park. We came in from the west, up Highway 198 along the Kaweah River through the town of Three Rivers, and then up the Generals Highway to the Sequoia groves for which the park is justly famous. The road is notably curvy, and climbs through a rugged canyon choked with giant boulders that have tumbled from the cliffs above.

We soon passed one of those kitschy things that Civilian Conservation Corps workers in the 1930s seemed especially fond of constructing: a drive-through rock (to go along with "drive-through" trees). They enlarged the opening under the immense boulder, and put the road through it.

If one wonders why the road today circumvents the rock tunnel, one need only look at the underside of the boulder. People driving in scenic national parks are not known for paying close attention to their driving, and in a time of massive recreational vehicles, this kind of thing just doesn't cut it anymore.

A hat tip to the arrival of spring in the Sierra Nevada...the redbuds were in full bloom up the canyon, providing a splash of vivid color on the dark green slopes.

Although not as colorful as redbud, the ceanothus shrubs added a wonderful fragrance to the air. The bees and other pollinating insects were in heaven.

As we went further up the canyon, Moro Rock loomed ever higher above us. The granite rock of the dome expanded as the rock was exposed at the surface. The plutonic rock tended to fracture parallel to the surface, which had the effect of removing corners and edges from the rock outcrop, eventually leading to the formation of the dramatic dome. The process is called exfoliation. It would be so cool to climb the dome, but we knew that most of the park access roads would be closed because of the winter snowpack.

Except that they weren't. When we reached Giant Forest, there were a few snow patches here and there, but the snowpack is currently at half the normal level. All the roads were open, so we headed over to the Moro Rock trailhead. As dramatic as the dome is, trail access is easy because the CCC put in a stairwell to the summit. Easy that is, if you don't have problems climbing 300-400 steps. The views in the clear spring air were stunning.

Immediately across the Kaweah Gorge were the Castle Rocks (9,000+). The spires and towers of granitic rock exhibit the other result of rock expansion: jointing. When the fractures that result from pressure release are vertical, they allow water to get into the narrow spaces. If the water freezes it expands, wedging the rock apart. The water also aids in the chemical weathering of the rock, so as time goes on the cracks widen, forming the prominent spires.

Moro Rock is best known for the wonderful perspective it provides on the Great Western Divide. The mountains are often hidden from view by other high ridges or thick forest, but Moro Rock stands out from the mountainside, and the view is tremendous.

With the covering of winter snow, one can imagine the glaciers that carved the horns, aretes, and cirques that are so well exposed here. Cirques are the bowl shaped basins on upper ridges where the glaciers accumulated. Aretes are the knife-edged ridges that divide glacially carved valley, and horns are the sharp pointed peaks that result when glaciers pluck rocks from the base of the cliffs in the cirques and aretes. The Matterhorn in the Alps is a familiar example, but there are many horns to choose from on the Great Western Divide.

I have not yet had the privilege, but the High Sierra Trail winds its way from Crescent Meadow in Sequoia to the summit of Mt. Whitney and the Whitney Portal trailhead. It is 60+ miles long, and crosses two major passes, and it must be a marvelous adventure. It is second only to the John Muir/Pacific Crest Trail in popularity.

The picture below illustrates the difference between glacial erosion (the cirques and horns in the upper part of the photo), and the weathering and exfoliation that happens at the lower elevations (the domes both right and left of center).

Moro Rock also provides a wonderful view west towards the Sierra Nevada foothills and the usually invisible Central Valley. You are looking at the most polluted air in the United States: Bakersfield and Fresno. It's not entirely their fault, as they don't necessarily produce more pollution per capita, but the towns are surrounded by high mountains, so they can't blow their pollution into someone else's area the way other municipalities are able to. The photo below is as clear as I've ever seen it from Moro Rock.

Next, we took a look at some trees with a unique geologic history...

Vagabonding across the 39th Parallel: In the Former Realm of Glaciers...

In the last post, we saw Rocky Mountain National Park turn black, white, and shades of gray during an intense thunderstorm. It was another day on our vagabonding journey across the 39th parallel, a trip that explored the geology across Nevada, Utah and Colorado last July. I've been expounding on our discoveries off and on ever since. We managed to score a campsite for a second night in Rocky Mountain National Park, which gave us time for a short hike into the realm of the glaciers.

Our campsite the first night was in a rocky gorge just downstream of the glacial terminus in Fall River Valley at Aspenglen. The previous day we had experienced a bit of magic on a walk around Bear Lake, one of the most popular stops in the park, and we decided we wanted to explore a bit higher, closer to the origins of the glaciers that once existed in the park.

The short hike to Nymph and Dream Lakes looked like it would do the trick. It shared a trailhead with Bear Lake, but climbed a few hundred feet in a mile to a pair of tarns, rocky basins scoured out by glaciers during the Pleistocene Ice Ages. It didn't take long to arrive at the lily-filled Nymph Lake.

An entire corner of the lake was covered with some kind of water-loving flower with strange stringy petals, whose identity I leave to you, the reader. This is because I am a geologist, who despite knowing hundreds of mineral and rock names, cannot remember the names of more than a dozen flowers or so...

The skyline above the lake was dominated by the tower of Hallett Peak (12,713 feet; 3875 meters), a reasonable example of an almost-horn. A glacial horn is a spiky kind of mountain peak that is surrounded by cliffs caused when glaciers plucked away at the base of the mountain. The Matterhorn in Italy/Switzerland is a famous example. Hallett Peak has indeed been plucked, but a considerable area of the original unglaciated surface still remains on the highest part of the mountain.

From Dream Lake a little higher up the trail, the profile of Hallett was clearer. One can see the u-shaped valley to the right of the peak. Rivers only erode the bottom of a valley, and mass wasting adjusts the valley walls into an V-shape. Glaciers scour the valley walls and the valley bottom, giving the valley a more circular aspect.

I was entranced by another flower (yellow this time; identification welcome!) along the trail.

Walking down the trail, I was more aware of the sweeping view we had to the south towards Longs Peak (14,259 feet; 4346 meters), the highest peak in the park. Glaciers actually had a challenge with the rocks of Rocky Mountain National Park. They consist mainly of hard granite and metamorphic rocks of Proterozoic eon, mostly between 1.7 and 1.4 billion years. The glaciers repeatedly tore away at these rocks during a series of glaciations starting about 2 million years ago and ending only about 13,000 years ago. There is clear evidence of three episodes, but independent climate records (on the ocean floor, for instance) suggest there were many more. A mere handful of glaciers remain, covering only a few acres.

We hiked back down the trail and saw the storm clouds gathering again. We were starting to appreciate just how sunny our hike had been as a second huge thunderstorm enveloped the region. This one was actually more fierce than our storm the previous day, and lasted for hours. We found that our tent had a slight vulnerability to heavy rain, and we watched helplessly as the tent filled with water. Finally, around midnight the storm let up (we had retreated to the car), so I grabbed a towel and cleaned things up as much as I could. In the morning we discovered that our campsite had a stunning view of Longs Peak that we couldn't have seen during the storm of the previous night. Strange lenticular clouds drifted around the summit of the mountain in the early morning.

It was time to cross the Continental Divide and start our homeward journey...in the next post we tackle the Trail Ridge Road.