GEOLOGIC SUMMARIES FOR THE LOCATIONS SHOWN IN INDIVIDUAL IMAGES

When viewing an individual image in the Collections section, clicking on the image will enlarge it and display a caption for the image. Many image captions contain a link titled ‘Geologic Summary’ that will display the geologic summary of that image. Not all images have a geologic description. The full list of geologic summaries for images is given below.

Written by John Stockham

This photo shows the north face of Mt. Bachelor.  The photo illustrates the generally symmetrical shape of the mountain, reflecting its relatively young age (18,000 to 11,000 years).  The mountain is the largest and most northern mountain within a chain of subduction-related volcanos that extends southward for 15 miles.

Mt Bachelor post-dated the major glacial advances in the High Cascades, which ended about 20,000 years ago.  However, the north-facing bowl was carved by a mini-glacial advance, likely in the 1700’s. The glacial terminal and lateral moraines around the bowl are well-highlighted in this photo.  The pinnacles exposed at the upper part of the bowl are remnants of resistant conduits that carried volcanic materials to the top of the mountain.

The protrusion on the right-hand side of the photo near Pine Marten Lodge is illustrative of the numerous small vents and cones that appear on the flanks of the mountain.

This photo shows the west and south flanks of Mt. Bachelor as viewed from Sparks Lake.  The symmetrical form and the general shield shape of the mountain capped by a stratovolcano cone are well illustrated in this photo.  There is no evidence of any glaciation on the south side of the mountain.

Sparks Lake in the foreground occupies a glacial basin, which was likely created before Mt. Bachelor was formed.  The lake is one of several shallow lakes that drain internally and are not part of the Deschutes River watershed.

This photo shows North Sister and Middle Sister viewed from a site east of Bend.

North Sister (10,085 feet) is the oldest of the Three Sisters.  Each of the Three Sisters are subduction-related volcanoes created due to plate tectonics.  Unlike the other Three Sisters, the North Sister is a classic stratovolcano, displaying the classic steep-sided peak form typically associated with this type of volcano.   The mountain formed between 120,000 and 50,000 years ago. Repeated glaciation has stripped away more the half of the original bulk of North Sister carving deep cirques in its flanks and exposing much of the volcano’s internal structure.  Constant rockfalls from North Sisters steep slopes make it one Oregon’s most dangerous peaks to climb.

Most of the eruptive activity that formed Middle Sister (10,047 feet) occurred 50,000 to 20,000 years ago.  It has produced a wide range of lava types including basaltic andesite, dacite, and rhyolite. The most recent volcanic activity on Middle Sister occurred about 14,000 years ago.

Collier Glacier, the largest glacier remaining in the Three Sisters, extends from north shoulder of Middle Sister to North Sister’s west flank.  The glacier still has a maximum thickness of 300 feet, but it has shrunk drastically in recent years

Mt Jefferson (10,497 feet) is Oregon’s second tallest mountain and is a visible landmark from either side of the High Cascades.  The mountain is one of the least studied mountains in the High Cascades.

Like the other High Cascades, Mt. Jefferson is a subduction-related volcano that has developed over the past 5,000,000 years.  Beginning about 280,000 years ago, Jefferson erupted in a series of alternating andesite and dacite flows. Beginning about 70,000 years ago the modern mountain erupted with a series of high magma eruptions, some of which surfaced through thick glacial ice cover.  The most recent eruptions on Mt. Jefferson occurred about 20,000 years ago.

Many glacial advances have eroded the flanks of the mountain and lowered the summit by up to 1,000 feet. Today the Whitewater Glacier on the east side and the smaller Mill Creek Glaciers on the west continue to carve into the mountain.  Numerous debris flows caused primarily impounded lakes breaching moraines continually alter the terrain on the flanks of the mountain.

See description for Sister Moon.

This photo shows the southwestern flank of Broken Top, which is an older volcanic mountain that erupted 300,000 to 150,000 years ago.  Broken Top has been significantly modified and eroded by multiple glacial advances, giving the mountain its iconic broken-top appearance.  The original peak was approximately 1,500 feet higher than the present mountain height, which has been eroded and deformed by repeated glaciation.  Many of the towers and pinnacles at top of Broken Top are remnants of resistant dikes and conduits which carried magma to the surface. The softer eruptive material has been eroded and worn away by the repeated glacial advances.

Sparks Lake in the foreground is an internally-draining lake that occupies a basin carved by glaciation.

This photo is of the upper Deschutes River south of Bend. This section of the Deschutes River has been moved and shifted by the formation of Lava Butte, which is a cone that originated on the flanks of Newberry Volcano. Lava Butte, which erupted approximately 6,000 years ago, creating a large basalt lava flow that shifted the river westward.  

The rocks on east side of the river are composed entirely of Newberry Basalts of recent origin.  The rocks on the west side of the river are primarily andesites and tuffs that originated in the Tumalo Volcanic Arc, located along the east side of Broken Top and the Three Sisters.

This photo shows the Deschutes River below Dillon Falls where the river is constricted between the basalts of Newberry Volcano on the east side of the river and basaltic andesites and ash-flow tuffs on the west side of the river.

The Newberry Basalts along the east river bank are the end of a large basalt flow that erupted from the base of Lava Butte approximately 6,000 years ago.  The flow extended approximately five miles westward between Lava Butte and the river. When the basalt flow reached the river upstream from Dillon Falls it entered the channel and forced the river to relocate approximately one-half mile westward from its former location.

The andesites and ash-flow tuff cliffs on the west side of the river originated from Tumalo Volcanic Arc, which is broad band of relatively low volcanic mountains located east of Broken Top and The Three Sisters.

This photo is taken from the west shore Paulina Lake in the Newberry Volcanic National Monument.  Paulina Lake and nearby East Lake occupy the central caldera of Newberry Volcano. The present caldera formed approximately 75,000 years ago when a huge pyroclastic explosion depleted the magma supply and the caused a large subsidence in the center of the volcano.  Subsequent smaller volcanos in the floor of the caldera which occurred around 6,700 years ago formed the isthmus separating Paulina Lake and East Lake.

The peak shown on the horizon in the center of the picture is Paulina Peak (7,897 feet), which is the tallest peak on caldera rim.  The peak towers approximately 1,600 feet above Paulina Lake. The smaller hill on the left edge of the photo is Little Crater, which was one of the relatively recent volcanic eruptions that formed the isthmus between the two lakes.

This photo, taken from the top of Paulina Peak (7,897 feet) shows an overview of the 15-square mile caldera that was formed in the center of Newberry Volcano. The most recent caldera formation was approximately 75,000 year ago.  There have been two previous caldera-forming eruptions of Newberry over the past 600,000 years.

The two lakes in the photo are Paulina Lake and East Lake.  These lakes formed when volcanic eruptions in the floor of the caldera approximately 6,700 years ago created the isthmus separating the two lakes.  Many researchers believe that the caldera floor had formerly been a single larger lake.

The light-colored rock in the lower left-hand portion of the photo is part of the Big Obsidian Flow which erupted approximately 1,300 years ago. The obsidian flow consists of glassy rhyolite, pumice and obsidian.  Clear flow lines of the eruptive material are shown in the photo. Obsidian collected from this flow and other nearby flows was an important resource for Native Americans who stayed in the area on a seasonal basis.  Obsidian objects and implements originating in Newberry Volcano have been found as far south as Central California and as far north as British Columbia.

This photo shows the rocky caldera wall of Paulina Peak (7,897 feet) which towers approximately 1,600 above the lake’s surface.  The photo is taken from the west shoreline of the lake. Paulina Lake is one of two lakes occupying the caldera of Newberry Volcano.  Paulina Peak is the highest peak on the rim of the caldera. The peak and the caldera rim are composed predominantly of rhyolite. The eroded towers of the peak are remnants of rhyolite dikes that were conduits for volcanic material.  The light-colored rock at the base of the peak is talus that has eroded off the rim.

This photo shows Smith Rock which is predominantly welded-tuff wall that is a remnant of the Crooked River Caldera that formed approximately 29,000,000 years ago.  The resistant tuff, which is interspersed with rhyolite dikes has diverted the course of the northward flowing Crooked River.

The resistant walls at Smith Rock has also diverted the basaltic flows of Newberry Volcano and other eruptive volcanos that have followed the general course of the Crooked River.  Those basalt flows were diverted westward and then north into canyon of Billy Chinook Reservoir at the Cove Palisades State Park.

This photo shows the course of Crooked River as it bends around the highly-resistant welded tuffs of Smith Rock.  The prominent towers are predominantly rhyolite dikes that have resisted erosion. The rocks formed about 29,000,000 years ago as part of massive volcanic eruption that created the Crooked River Caldera

This photo is of the 2017 solar eclipse taken at the Balancing Rocks of the Metolius on the Metolius Arm of Billy Chinook Reservoir.  The balanced rocks, also referred to as hoodoos, are caused by differential erosion. These series of stone spires with large slabs of flat rock perched on top are formed by differential erosion.  The rocks are three different volcanic tuffs (i.e. welded volcanic ash pumice) that have different resistance to erosions. Erosion has slowly worn away the softer tuffs leaving boulder shaped stones seemingly stacked on narrow pillars.  The tuffs originated the eruptions in the vicinity of Mt. Jefferson.

This photo shows Wizard Island which is a volcanic island within Crater Lake.  Crater Lake is technically a caldera lake which formed as a result the massive evacuation of magma in the center of the volcano and the subsequent catastrophic collapse of the landmass.  The lake was formed approximately 7,700 years ago by the eruption of Mt. Mazama. Crater Lake is internally drained and has no outfall.

Wizard Island was formed by a cinder cone that erupted on the top of an underwater andesite volcano in the middle of the lake.  The island was likely created within several hundred years of the main Mt Mazama eruption.

The photo is taken from the western rim of the caldera.  The reflected rocks at the top of the photo are the east rim of the caldera.

This photo shows a 90-foot double waterfall that plunges over two distinct layers of Columbia River Basalt. The basalts flowed across the landscape between 17.0 and 14.5 million years ago.  The Columbia River Basalts originated in eastern Oregon and Washington and parts of Idaho and blanketed all the Columbia River valley, extending out into the Pacific Ocean, forming many of the headlands along the northern Oregon coast.  River canyons have cut into the Columbia River Basalts, exposing their successive flow layers, such as those at White River Falls and the Deschutes River Canyon downstream from the falls.

At the base of the falls are remnants of the flumes and equipment from a hydro power plant that was in operation from 1910 to 1960.

This photo shows the layering of Columbia River Basalts along the Deschutes River.  The layers of Columbia River Basalt flowed across the landscape of eastern Oregon and Washington between 17.0 to 14.5 million years ago.  The Columbia River Basalts originated in eastern Oregon and Washington and parts of Idaho and blanketed all the Columbia River valley, extending westward to the Pacific Ocean.

The classic colonnade and entablature seen in many exposed canyon walls has been obscured by erosion and sluffing-off of the canyon walls.

This photo was taken near the Tom McCall State Park at Rowena Gap.  The photo shows successive layers of Columbia River Basalt that flowed down the Columbia River valley approximately 17.0 to 14.5 million years ago.  The Columbia River Basalts originated in eastern Oregon and Washington and parts of Idaho and blanketed all the Columbia River valley. The basalt flows extended out into the Pacific Ocean, forming many of the headlands along the northern Oregon coast.  River canyons have cut into the Columbia River Basalts, exposing their successive flow layers such as those seen in this photo.

This photo is taken above the town of Mosier along the Columbia River.  The foreground of this picture is a high terrace that is perched on Columbia River Basalts.  The High Cascades, which are subduction volcanos, are visible on the horizon.

This photo silhouettes the sea stacks on Bandon Beach, which consist predominantly of Otter Beach sandstone of the Jurassic age (201 million to 145 million years ago).  These rocks have accreted to the Oregon shoreline as a result of plate tectonics.

This photo shows the east shoreline of Abert Lake, which is a highly alkaline body of water that is a remnant of the Lake Chewaucan glacial lake that once covered over 500 square miles of basin floor.  The steep cliff in the background is Abert Rim, which is the western edge of a large fault block that rises 2,400 feet above Abert Lake.

The rim rock at the top of Abert Rim consists primarily of layers of Steens Basalt, which was an early part of the Columbia Basalt Group.  In some areas on the rim top, the basalt layer has been overlain by more recent deposits for volcanic ash deposits and air-flow tuff.

The successive flows of basalt originated approximately 80 mile east of this site near Steens Mountain.  The flows occurred approximately 17.0 to 14.5 million years ago. The faulting and uplift which formed Abert Rim occurred within the past 7 million years, well after the deposition of basalt.

The lichen cover boulders in the foreground are likely boulders of Steens Basalt that cleaved off the rim rock and became rounded in subsequent flows of glacial runoff and lake waters. Many of the boulders contain elongated crystals of plagioclase, which is typical of the Steens Basalts

Abert Rim is representative of many of large fault block ridges in the Basin and Range Physiographic Province.   The tectonic process that created the uplifted fault block of Abert Rim and the down-dropped basin floor of Abert Lake is tectonic crustal extension or spreading of the earth’s crust which causes large faults.  The faulting imparts an asymmetry to many of the ranges: they rise steeply from one valley and they slope gently to the wards the next valley and range.

This photo shows the floor of Alvord Desert, which is located immediately east of Steens Mountain, during wet spring conditions.  In the summer, the desert is totally dry. The Alvord Desert occupies a basin or graben created by the faulting and uplifting of the Basin and Range Physiographic Provence.  Steens Mountain, shown on the left side of the photo, is the east end of a steeply tilted fault block.

Steens Mountain consists of successive layers of Steens Mountain basalt with some more recent overlying ash and ash-flow tuff deposits from eruptions in the Burns area.  The basalt flows of Steens Mountain were deposited approximately 17.0 to 14.5 million years ago. The mountain peak at 9,730 feet towers over a mile above the Alvord Desert floor at 4,060 feet.

The faulting that created Steens Mountain has occurred over a 10-million-year period, which post-dates the deposition of the Steens Mountain basalts.  There has been continuous faulting in this area up to the present.

See above description for Alvord Desert

This photo shows the floor of Summer Lake, which is a dried lakebed that is a remnant of glacial Lake Chewaucan.  During Pleistocene times (2 million to 0.01 million years ago) Lake Chewaucan covered approximately 460 square lies and was up to 300 feet deep.  Today, Summer Lake is a very shallow alkaline waterbody that is seasonally dry at its margins. The margin of the lake displays numerous terraces and evidence of old shorelines

Summer Lake is a basin or graben of the Basin and Range Province. Winter Rim is an uplifted fault block.  The land in the central part of the photo was likely a terrace of the ancient lake. The east face of Winter Rim consists of a combination of stratified basalt flows, tuffs, and ash from nearby rhyolite volcanoes.  The slopes of Winter Rim are notoriously unstable with frequent landslides. The landslides have created the hummocky hills seen along the west side of Summer Lake.

The rim on the far horizon at the op of the photo is Diablo Rim, which consists of Miocene-Pleistocene basalt and pyroclastic rocks that post-date the Columbia River Basalts deposited approximately 16.5 million years ago.

This photo shows the Deschutes River Canyon several miles upstream from where the river enters Billy Chinook Reservoir.  The photo shows alternating strata of basalts and ash-flow tuffs, as well layers of river and flood deposits that accumulated between 8 and 4 million years ago.  Collectively these rocks form the Deschutes Formation.

The lighter-colored materials are generally ash and tuffs which originated from pyroclastic eruptions.  The red coloration of many of layers is the result of the oxidation of the iron in the rocks. The basalt strata are generally in the form on tall polygonal columns, referred to as colonnades.  The mixed rubble both above and below the colonnades, is referred to as entablature. The mixed rubble is caused by the flow of the fresh basalt over older layers.

This photo shows the multi-colored layering of the badlands in the Painted Hills unit of the John day Fossil Beds National Monument.  The badlands shown in the photo are eroded paleosols (i.e. fossilized soils) of the John Day Formation and the underlying Clarno Formation.  The paleosols are red, brown and yellow indicate successive climate conditions during their formation. In general, the red colors indicate deep weathering under warm, wet conditions typical of tropic conditions.  The yellow and brown coloration represents moderate weathering under increasingly dry, cooler conditions, typical template forests.

Much of volcanic material forming Painted Hills originated from volcanic eruptions in the western cascades and various calderas in Central Oregon, including the Crooked River Caldera.

The mountains in the background are likely rhyolite volcanoes with inter-mixed the ash-flow tuff and ash from the Clarno and John Day periods.  Parts of these mountains have layers of Picture Gorge Basalts which are a unit of the Columbia River Basalts dating from 17.0 to 14.5 million years ago.

This photo shows two of the numerous small seasonal lakes that stretch along the west side of the Hart Mountains in the Warner Valley.  Warner Valley is a down-dropped valley, or graben, created by faulting. The steep Jim Poker Ridge and Hart Mountains to the east are the uplifted sections.

The two lakes shown in the photo are remnants of a 500 square mile glacial lake that filled the entire basin. There are numerous beach ridges in the Rabbit Hills and the nearby uplifted areas of Hart Mountain that mark previous shoreline levels.

The Rabbit Hills in the background are relatively recent volcanic eruptions.  The northern portion of the Rabbit Hills produced basalt flows that contain deposits of Sunstone, Oregon’s state gem.  Sunstones occur in plagioclase feldspar found within the basalts.

This photo show portions of the Diablo Sand Dunes east of Summer Lake.  This area lies on the northwest edge of the Basin and Range physiographic province.  The sand deposits consist basalt and tuffaceous sediments that were likely deposited by the ancient glacial Lake Chewaucan, which covered over 500 square miles.  The sand has been redistributed by winds.

The area is part of the Diablo Wilderness Study Area.

This photo shows the canyon of the Little Blitzen River, which flows westward off Steen Mountain and into the Malheur River Basin.  The rocks in the canyon are highly-weathered Steens Basalts, which are a unit of the Columbia River Basalt Group that covers much of eastern Oregon.  The basalt flows were deposited between 17.0 and 14.5 million years ago. The basalt was deposited in numerous flows that can be seen on the canyon walls in the form of multiple colonnades capped by rubble material, referred to entablature.  The rocks at the bottom of the canyon are boulders that have cleaved off the canyon walls. The canyon has been impacted by glaciers that have carved the u-shape valley characteristic of glaciated alpine terrain.

This photo is taken from the rim of Steens Mountain showing the steep east-face side ravines that flow eastward into the Alvord Desert.  Steen Mountain is a classic Basin and Range fault block formation. The mountain falls off steeply on the east-face where the major faulting occurred.  The west side of the mountain gradually tilts westward.

The rock material shown in this photo is predominantly Steens Basalt formed approximately 17.0 to 14.5 million years.  These basalts are a unit of the Columbia River Basalt Group that covers much of eastern Oregon. In some places, there is a cover of ash-flow tuff and ash from more recent volcanoes in the vicinity of Burns.

This photo shows the classic u-shaped valley characteristic of glacially altered alpine terrain.  The photo is taken from the top of Steens Mountain. The rocks on the canyon walls are predominantly Steens Basalt.  These basalts are a unit ofc the Columbia River Basalt Group that covers much of eastern Oregon. In some places, there is a cover of ash-flow tuff and ash from more recent volcanoes in the vicinity of Burns.  The photo illustrated that successive basalt flows and display the alternating colonnades and entablatures consisting of volcanic ruble that are characteristic of basaltic canyons. The basaltic rocks at the bottom of the canyon have been cleaved from the basalt walls.

This photo is taken from the Pueblo Mountains, which are a southward extension of Steens Mountain.  The underlying rocks are predominantly Steens Basalts. In places the Steens Basalts are overlain by ash and ash-flow tuffs deposited by more volcanic activity.

This photo is taken from the rimrocks above the Owyhee River canyon, across from Iron Point, in the southeast corner of Oregon.  The Owyhee canyon is a geologically complex area that has been impacted by a variety of geologic processes.  The canyon is in the Owyhee Uplands Physiographic Province, which is closely related to the High Lava Plains.  The Owyhee area has been influenced by extensive flood basalt flows, graben formation, rhyolite domes and ash-flow tuffs and sedimentation.  

Relatively recent research has identified two calderas in the Owyhee region which were likely associated with the Yellowstone Hot Spot, which has caused a string of volcanic eruptions that extend from the central part of Oregon to Yellowstone Park in Wyoming.  This volcanic activity occurred between 17.0 and 14.5 million years ago, at roughly the same time that Columbia River Basalts flooded much of eastern and central Oregon.

Between 15.0 and 2.0 million years ago there was significant tectonic extension and faulting in the Owyhee basin.  During this period the Oregon-Idaho and Antelope Valley grabens, which are major downward-dropping events, altered the landscape of the area.

In the last 2.0 million years, there have been numerous rhyolite eruptions which have left numerous rhyolite domes and spewed ash-flow tuffs throughout the areas.  During this period, there have also been six major flows of basalt down Owyhee Canyon that have created dams and intermittent ponding and river diversions, resulting in layers of sediments interspersed with the volcanic rocks.  There has also been extensive faulting in the canyon. Major land sliding in the canyon has periodically shifted the course of the river.

The results of these various geologic processes give the Owyhee Canyon its varied terrain and multi-hued vistas.

See description for Dawn Over Iron Point

See description for Dawn Over Iron Point

See description for Dawn Over Iron Point