USGS Logo Geological Survey Circular 838
Guides to Some Volcanic Terrances in Washington, Idaho, Oregon, and Northern California
ROADLOG FOR NEWBERRY VOLCANO, OREGON

Norman S. MacLeod, David R. Sherrod
U.S. Geological Survey, Menlo Park, California 94025
Lawrence A. Chitwood, U.S. Forest Service, Bend, Oregon 97701
and
Edwin H. McKee, U.S. Geological Survey, Menlo Park, California 94025

The route of this field trip (figs. 1, 2) leads southward from Bend across the basalt and basaltic-andesite flows of Newberry's northwest flank and eastward up the west, flank where pyroclastic units are visited. Several stops in the caldera, including an overview stop provided by Paulina Peak, show the diversity of caldera rock units. The trip continues down the east and northeast flanks with stops to see several ash-flow tuffs and returns to Bend across the apron of basalt flows that extend far north of Newberry. Total mileage of the trip is about 118 miles, all on paved or improved gravel roads that are readily traversed by passenger cars or buses. Mileages are approximate, all turns and stops are underlined, and old and new (in parenthesis) U.S. Forest Service road numbers are shown where appropriate. The 14C ages of young basalt flows are mostly from Peterson and Groh (1969).

MILES
0.0 Junction U.S. Highways 97 and 20 in Bend, Oregon. Elevation 3,630. Head south on U.S. Highway 97.
(0.7)
0.7 Diktytaxitic olivine-phyric high-alumina basalts exposed in railroad underpass and intermittently for the next l1 miles to south are typical of Newberry's lower flank flows. Northwest-trending faults that extend in a broad zone through the Bend area offset the flows. A typical analysis of a lower flank flow is shown in table 1, column 1.
(3.2)
3.9 Road 1821 (18) leads eastward to interesting lava tubes (Boyd Cave, 9 miles; Skeleton Cave, 12 miles; Wind Cave, 13 miles; and Arnold Ice Cave, 13 miles)
(2.8)
6.7 Ash-flow tuff exposed on east side of U.S. 97 near draw. Ash flow is widespread around the Bend area and was derived from the Cascade Range west of Bend. Outcrops and float near railroad west of road show a wide range in pumice composition.
(3.3)
10.0 Roadcuts expose cinder plume that extends northeast from Lava Butte. Charcoal from base of cinders here yielded 14C age of 6,160±65 years (Chitwood and others, 1977).
(0.7)
10.7 Northwest-trending fault exposed on east side of highway offsets diktytaxitic olivine basalt. Note young "Gas Line" basaltic-andesite flow south of fault (14C age 5,800±150 to 6,160±65 years, (S. W. Robinson, written commun., 1978).
(0.5)
11.2 Turn west to Lava Lands and drive by and across the Lava Butte flow to top of Lava Butte. Note that the lava flow emerges from the south side of the butte.
(1.8)
13.0 STOP 1 at top of Lava Butte (elevation 4,970 ft). The basaltic andesite flow derived from Lava Butte extends northward more than 5 miles and westward 3 miles to the Deschutes River. A chemical analysis of the flow is shown in table 1, column 2. It is one of many basaltic andesite flows on Newberry that have 14C ages of about 6,100 years. Several, including the "Gas Line" and Mokst Butte flows, can be seem from this stop. Note also the many cinder cones on the north and northwest flank, an area formed mostly of basalt and basaltic-andesite flows and associated vents, moat of which are younger than the pyroclastic units that will be visited on the east and west flanks. A panorama of the Cascade Range, particularly of the Bachelor Butte, Broken Top, and Three Sisters area, can be viewed from here. South of Lava Butte, Pleistocene (and Pliocene?) lacustrine and fluvial sediments of the La Pine basin lie between Newberry Volcano and the Cascade Range. Return to Highway 97.
(1.8)
14.8 Turn south on U.S. Highway 97.
(1.2)
16.0 Road on east leads to Lava River Cave, 1/4 mile from highway. The mile-long lava tube is now a state park and is open to the public (lanterns can be rented at the entrance).
(2.4)
18.4 Sun River Junction. Road on east goes by Camp Abbot Cinder Pit (3/4 mile) to Lava Cast Forest (9-1/2 miles). The cinder pit contains brilliant blue, green, gold, and red cinders. Lava Cast Forest flow (14C ages, 6,380±130 and 6,150±210 years) contains numerous well-developed tree casts. A mile-long paved trail leads past many casts. Other young flows accessible by this road include Forest Road (5,960±100 years), Mokst Butte, and Lava Cascade (5,800±100 years).
(2.4)
20.8 Fall River Junction. Road on east leads to Sugarpine Butte flow (14C age, 5,870±100 years).
(3.2)
24.0 Cross old logging railroad grade that went to the now abandoned town of Shevlin. Many hundreds of miles of roads on Newberry are on or along old railroads developed about 50 years ago when logging in the area was at a peak.
(0.6)
24.6 Approximate boundary of Newberry lavas and sediments of the La Pine basin. Gravel in this area is mostly composed of black lapilli with rhyolitic inclusions like those that form the lapilli tuff at stop 2. These can be seen in this area in several pits, including the garbage dump (Southwest Landfill), east of the highway.
(1.2)
25.8 La Pine State Recreation Area junction. To west 5 miles are excellent exposures of fluvial and lacustrine sediments, including diatomite, along the scenic Deschutes River.
(1.1)
26.9 Turn east (elevation 4,205 ft) on Paulina Lake Road, U.S. Forest Service Road 2129 (21). Road crosses Mazama ash-covered alluvial sediments.
(2.8)
29.7 Cross Paulina Creek. Fed by Paulina Lake, it is the only stream on Newberry.
(0.2)
29.9 Outcrops of Newberry lava flows near road. Farther along the road these flows are locally buried by lapilli tuffs like those at stop 2 as well as by gravel.
(0.5)
30.4 At this site on September 19, 1979, The Cascade Rattlesnake Award was presented to J. P. Lockwood before a cheering audience of 80 geologists.
(3.2)
33.6 Road 2045 provides access to three rhyolite domes at and near McKay Butte (K-Ar age, 0.6±.1 m.y., may be reversely polarized).
(2.9)
36.5 STOP 2 (elevation 5,420) at large scree-covered slope on north side of road. The lapilli tuff at this stop is one of the most widespread tephra units on Newberry's flanks. The unit is rarely exposed but the lapilli in it are distinctive so that float from the unit can be recognized. On the west flank the lapilli tuff covers an area of about 30 square miles, with basalt flows cropping out locally beneath it; higher on the west flank it is overlain by ash-flow tuffs like those at stop 4. The lapilli tuff on the northeast flank occurs higher in the section than the ash-flow tuffs to be seen at stops 12, 13, and 14. It is also a major constituent of gravel deposits on all sides of the volcano. The lapilli tuff is deeply eroded but locally exceeds 200 ft thick, and original volume was probably 5 to 10 cubic miles. It consists of dark gray to black lapilli, and less common blocks and bombs, in an ashy lithic-rich matrix. Sorting is very poor and in observed outcrops the unit is massive. Most lapilli are finely vesicular to frothy, have cauliflowerlike surfaces, and contain small, generally angular, rhyolite. and more mafic inclusions. The normal polarization of the lapilli and matrix suggests that they were above the Curie point temperature when deposited. The poor sorting and massiveness of the unit suggest that it was a hot pyroclastic flow or flows. The large volume of the unit permits speculation that its eruption may have been accompanied by caldera collapse. Continue upslope on road.
(0.4)
36.9 Upper snowmobile parking lot. Rhyolite domes or flows occur north and south of here.
(0.6)
37.5 Contact of lapilli tuff and cinder comes. These vents probably fed flows exposed farther down the road that underlie the lapilli tuff. Cinder cones are buried on their east (caldera) sides by ash-flow tuffs like at stop 4.
(0.7)
38.2 Float of andesitic ash-flow tuff along road.
(0.1)
38.3 Turn south to U.S. Forest Service pit F-17, also known as "Mixture Butte."
(0.1)
38.4 STOP 3 (elevation 5,960 ft) at "Mixture Butte," a cinder cone with rhyolite and pumice inclusions. The pit is on the north side of a horseshoe-shaped cone that is buried by ash-flow tuffs on its east (caldera) side. Rhyolite and pumice, as well as basalt and andesite, occur as inclusions in the cinders and bombs and as accidental fragments in the deposits. Some inclusions have been fused and have flowed. The pumice, in blocks to more than 2 ft wide, commonly shows bands of different color and probably composition. A mile-long rhyolite dome crops out about 1 mile to the southwest, contains phenocrysts similar to those in the pumice and rhyolite inclusions and may extend at depth beneath the cinder cone. Possibly the pumice is derived from a buried pumice ring associated with the done. Many cinder cones and flows on the upper flank of the volcano contain rhyolite inclusions. Rhyolite domes and flows crop out in many places and may be much more extensive at depth. Return to Paulina Lake Road.
(0.1)
38.5 Turn east on Paulina Lake Road. Area between here and stop 4 is almost entirely underlain by andesitic ash-flow tuffs; found as large slabs of float and less commonly as outcrop.
(0.6)
39.1 Turn south on Road 2131 (2121) (elevation 6,020 ft).
(0.9)
40.0 West of road is eroded cinder come with associated flows that are partly buried by ash-flow tuff.
(0.2)
40.2 STOP 4 (elevation 5950). Park at road intersection and walk 500 ft eastward along small side road to exposures of ash-flow tuff along side of ravine. Andesitic ash-flow tuffs crop out over a 5-square-mile area of the upper west flank of the volcano and grade near the rim to pyroclastic deposits that have characteristics more nearly like agglutinate deposits, as at atop 5. The unit consists of ash flows as little as 1 ft thick to as much as 30 ft. The ash flows are pumiceous, rich in lithic fragments and crystals, and reddish brown. Even thin units commonly show welding. At many localities collapsed pumices are brilliant blue and green. The ash flows are very near vent deposits, perhaps from a ring fracture near the west side of the caldera, and have relatively small volume. Ash-flow tuffs similar to these occur at scattered localities on the upper east flank. Return to Paulina Lake Road.
(1.1)
41.3 Turn east on Paulina Lake Road.
(1.0)
42.3 Turn north into Paulina Falls parking lot.
(0.1)
42.4 STOP 5 Paulina Falls (elevation 6,240 ft). Paulina Creek drains Paulina Lake, less than 1/2 mile to the east. The indurated rocks that form the cliffs of the falls are agglutinated andesitic pyroclastic deposits formed of many thin to thick units. Similar rocks occur along the caldera wall for about 1-1/2 miles north of the creek. They grade down the creek into ash flows like those at stop 4 and represent very near vent deposits, perhaps from a ring fracture bordering the west side of the caldera. Most of the deposit is probably co-ignimbrite lag, many units being entirely fall, others stubby agglutinated pyroclastic flows. A chemical analysis for this locality is shown in table 1, column 3. The less indurated rocks below the cliffs are formed of poorly sorted and rudely bedded ash, lapilli, and blocks with abundant accidental lithic fragments. Some beds low in the section contain accretionary and mud-armored lapilli. The contact between the lower and upper unit is gradational. Probably the eruptions first deposited relatively cold material, perhaps from phreatic eruptions, and temperatures increased to the point where all fragments were agglutinated at time of deposition. Lapilli tuffs like those at stop 2 occur at creek level only a few hundred feet downstream from the falls. Return to Paulina Lake Road.
(0.1)
42.5 Continue east on Paulina Lake Road.
(0.3)
42.8 Turn south on road 2160 (500) to Paulina Peak.
(1.3)
44.1 Buried contact between upper west flank pyroclastic flows and Paulina Peak rhyolite dome.
(0.6)
44.7 View northward of gently sloping upper west flank formed mostly of ash-flow tuffs like at stop 4. Note rhyolite domes at and near McKay Butte (N. 60° W., 5-1/2 miles).
(1.1)
45.8 View of numerous cinder cones on south flank and long narrow Surveyor flow (14C age 6080±100 years).
(1.3)
47.1 STOP 6 Paulina Peak (elevation 7,984 ft). Before looking at the caldera, walk to peak for view southward. Note fault-bounded Walker Mountain (S. 42° W., 35 miles). Faults of the Walker Rim zone extend to Newberry's lower south flank. Some of the older rocks, such as the reversely polarized dacite flows on Indian-Amota Butte (S. 30° E., 11 miles), are offset several hundred feet by northeast-trending faults; faults that cut the nearby basalt flows have vertical offsets of less than 100 ft and the faults disappear upslope below younger flows. Other distant features include:

(1) Bald Mountain (S. 12° W., 28 miles), a rhyolite dome on the south side of a poorly preserved 4- or 5-million-year-old caldera with widespread ash-flow tuffs that crop out from Fort Rock Valley east of it, to Walker Mountain.

(2) Cascade Range. From south to north on a clear day you can see Mount Shasta, Mount Scott, Crater Lake, Mount Thielson, Diamond Peak, The Three Sisters, and nearby Bachelor Butte and Broken Top, Mount Washington, Three Fingered Jack, Mount Jefferson, Mount Hood, and Mount Adams.

(3) Fort Rock Valley southeast of Newberry with numerous palagonite tuff rings, cones, and maars. Included in these are Fort Rock in the middle of the valley, and beyond it about 39 miles from here is Table Rock, the site of a field trip by Heiken and Fisher (this vol.).

(4) To the east are China Hat (N. 85° E., 11 miles) and East Butte next to it. They are 0.8- and 0.9-m.y.-old rhyolite domes on the west end of the belt of age transgressive rhyolite vents. Note the 5-m.y.-old rhyolites of the Glass Buttes dome complex on the skyline (S. 80° E., 60 miles). The High Lava Plains, a broad zone of Miocene to Quaternary basalts in addition to rhyolite domes and flows, extends from Newberry beyond Glass Butte.

Now walk to the north along and beyond the fence bordering the cliffs of Paulina Peak to a viewpoint about 50 ft below the elevation of the parking lot. Please use caution in this area—the cliffs are 500 ft high. The generallized geologic map of the caldera (fig. 2) should be referred to for identifying features. From this location you can see the inner and outer (near pumice flats) walls of the south side of the caldera. The caldera wall rocks have been described by Higgins and Waters (1968) and Higgins (1973). The south wall near the Big Obsidian flow consists of platy rhyolite, basaltic andesite flows, scoria and cinder deposits, and an overlying obsidian flow. At the west end of the wall exposures is a thick sequence of near-vent pumice and ash deposits that have been fused near their contact with the obsidian flow. The north and east caldera walls contain rhyolite and basaltic andesite flows and pyroclastic rocks, as well as palagonite tuff; the east wall also contains near vent welded rhyolitic pumice deposits. Basaltic vents and flows occur on the north wall near Paulina Lake (Red Slide) and East Lake (East Lake fissure, Sheeps Rump), and a long fissure vent occurs near the top of the east wall. All of these vents and associated flows are pre-Mazama ash in age, except the East Lake Fissure. It is the southernmost vent of the northwest rift zone sequence of basalt flows and vents, about 6,100 years old, and contains abundant inclusions of obsidian, rhyolite, etc. (Higgins and Waters, 1970).

Young rhyolitic flows, pumice cones, rings, and other tephra deposits are widespread around East Lake. Hydration-rind dating by Friedman (1977) indicates that the East Lake obsidian flows are about 3,500 years old and that many of the other obsidian flows and pumice cones (Interlake flow, Gamehut flow, Central Pumice cone) are 4,500 to 6,700 years old.

The youngest period of volcanism was associated with the vent for the Big Obsidian flow. Eruptions began with a widespread pumice fall that now covers much of Newberry's east flank and which will be seen later at stop 11. Higgins (1969) obtained carbon from beneath the fall that has a 14C age of 1,720±250 years, and we found carbon at another locality with an age of 1,550±120 years (S. W. Robinson, written commun., 1978). Isopachs of the pumice fall clearly show that it was erupted from at or very near the vent for the Big Obsidian flow (Sherrod and MacLeod, 1979). Later eruptions produced the Paulina Lake ash flow (stop 7) 1,300 to 1,400 years mgo. The final event was the eruption of the Big Obsidian flow and the domal protrusion that marks its vent. Slight collapse occurred over a 1/2-mile-wide area around the vent before the flow was erupted.

Paulina Peak dome extends about 3 miles southwest down the flank and is about 1 mile wide. It is marked by large rills parallel to its axis that formed during expansion of the surface of the dome much as cracks form on french bread. The age of the dome is not yet known, but an obsidian flow that occurs on axis with the dome farther down the flank is 0.4 m.y. (E. H. McKee, unpub. data, 1979), and Paulina Peak done may be similar in age. An analysis of rhyolite from the Paulina Peak dome is shown in table 1. Return to Paulina Lake Road.
(4.3)
51.4 Turn east on Paulina Lake Road.
(0.1)
51.5 Paulina Lake campground provides a convenient lunch stop.
(0.3)
51.8 Roadcuts expose Mazama ash covered by a few inches of fine ash from Newberry eruptions. Holes dug through the Mazama ash show that it overlies sand and gravel deposits.
(0.5)
52.3 Road traverses south side of two rhyolite domes, overlain by Mazama ash and bordered on south by landslide deposits.
(0.3)
52.6 Roadcut exposes rudely bedded ash and pumice deposits, perhaps the remnant of a pumice ring associated with the rhyolite domes.
(0.1)
52.7 Western limit of Paulina Lake ash flow.
(0.2)
52.9 STOP 7 Paulina Lake ash flow. Concrete boxes on south side of road preserve collecting site for carbon in ash flow. Originally dated by Libby (1952) as 2,054±230 14C years, subsequent dates indicate a younger age (1,270±60 years, Pierson and others, 1966; and 1,390±200, Meyer Reubin, in Friedman, 1977). The ash flow extends from the Big Obsidian flow to the shore of Paulina Lake. Ridges and furrows on the ash flow are apparently primary features, and their orientation suggests that the vent is located beneath the southern part of the Big Obsidian flow (Higgins, 1973). Although pumice is generally abundant, the ash flow in many places is composed almost entirely of ash. The absence of collapse and welding indicates the deposit had only a moderate temperature when emplaced, but sufficient for oxidation to give the deposit a slightly pinkish color. Continue eastward on road over ash flow.
(0.4)
53.3 Road on north leads to Little Crater Campground and interesting exposures along shoreline of palagonite tuff and silicified pumiceous, fossiliferous, lake deposits.
(0.4)
53.7 Turn Right to parking lot next to Big Obsidian flow.
(0.1)
53.8 STOP 8 (elevation 6,370 ft) Big Obsidian flow. Before walking to the flow, note that the pinkish Paulina Lake ash flow overlies weathered pumice deposits of a pumice ring on the east side of the parking lot. Lost Lake pumice ring is partly covered by the Big Obsidian flow, but its northern part can be seen from the trail ascending the obsidian flow. The trail provides interesting exposures of flow-banded obsidian, pumiceous obsidian, brown streaky obsidian that was formerly pumiceous before it collapsed, and various features indicating the flow behaved in both plastic and brittle manner during its emplacement. Laidley and McKay (1971) did extensive analytical work on the Big Obsidian flow and showed that is is essentially uniform in composition. Their average of 66 chemical analyses is shown in table 1, column 5. Return to parking lot and to main road; note exposures of palagonite tuff of Little Crater north of road.
(0.1)
53.9 Continue eastward on paved road.
(0.4)
54.3 Road on south leads to drill site of U.S. Geological Survey core hole. Note Central Pumice cone to north.
(0.5)
54.8 Game Hut obsidian flow is exposed on north side of road.
(0.5)
55.3 STOP 9 (elevation 6,460 ft). Pumiceous tephra deposits of the East Lake area. Roadcuts expose massive to rudely bedded pumice and ash with large accidental blocks overlain by mud-armored pumice, accretionary lapilli, pumice, and ash beds. Bomb sags occur in the upper bedded part and indicate that the beds were probably wet and cohesive. A hole dug vertically through the deposits indicates that they are about 14 ft thick and overlie Mazama ash, which rests on obsidian. Tephra deposits, similar to the exposures here, occur over most of the eastern part of the caldera except where buried by younger units (East Lake obsidian flows, etc.). The vent or vents for the deposit are not known, and conceivably may lie in East Lake. The obsidian that underlies the deposit is part of an extensive obsidian flow that, mostly on the basis of topographic expression, probably extends to the south (inner) wall of the caldera, its vent apparently being an obsidian dome near the wall. Continue eastward along road.
(0.3)
55.6 Turn left to East Lake Campground and walk eastward along shore.
(0.1)
55.7 STOP 10 (elevation 6,382 ft). Palagonite tuff of East Lake. The palagonite tuff is overlain by Mazama ash and by pumiceous tephra deposits like those at stop 9. Many of the beds in the palagonite tuff are formed of mud-armored lapilli and accretionary lapilli, presumably indicating that the exposed part of the unit was deposited above water. Note also several bomb sags. About halfway along the shoreline exposures is a contact between two palagonite tuff units; the western unit is younger and locally is plastered vertically on the eastern unit. The eastern palagonite tuff contains very abundant and large accidental blocks of palagonite tuff that are more thoroughly palagonitized than is the tuff they occur in. From the shore you can look westward to see the Central Pumice cone, northward at the East Lake fissure, and northeastward at the Sheeps Rump and the obsidian flow that extends to the northeastern shoreline. Return to the main road.
(0.1)
55.8 Turn east on paved road.
(0.6)
56.4 Hot springs occur along shore near headland formed of palagonite tuff. Temperatures as high as 80°C have been measured, but the hot spring water is diluted by lake water.
(0.4)
56.8 Turn east on gravel road, U.S. Forest Service Road 2129 (21). Note exposures of andesite near road intersection. This flow has been traced southeastward to the fissure that occurs at the top of the east wall of the caldera. This fissure contains rocks of highly variable composition and phenocryst content, as does the flow. Cinders, bombs, and spatter associated with the fissure can be seen from several places along the road.
(0.3)
57.1 Pumiceous tephra deposits exposed in roadcuts are similar to those seen at stop 9.
(0.4)
57.5 Road here crosses over andesite flow, but it is buried by Mazama ash and pumiceous tephra like at stop 9.
(0.2)
57.7 Pumice fall derived from vent for Big Obsidian flow forms a thin surficial unit that thickens as we progress up the road toward the south.
(1.1)
58.8 Top of east rim of caldera (elevation 7,008 ft) covered by pumice fall about 8 ft thick.
(0.3)
59.1 STOP 11 (elevation 7,010), Pumice fall derived from vent for Big Obsidian flow. The fall is exposed in a large hole dug in roadcut on north side of road. The fall is about 9-1/2 ft thick here, but the axis of the fall is about 1/2 mile to the south. 14C ages of the fall are 1,720±250 (Higgins, 1969) and 1,550±120 (S. W. Robinson written commun., 1978) years. One hundred and fifty holes dug through the pumice fall (and most not in roadcuts as at this site) show that it forms a very narrow plume (at the 10-in, isopach it is 6 miles wide by 40 miles long) oriented N. 80° E., from the Big Obsidian flow vent. The orientation of the plume is the same as the current prevailing wind as indicated by deformed trees on higher ridges and buttes (for instance, on The Dome, a cinder cone immediately south of here). Pumice lapilli and blocks constitute most of the unit, but accidental fragments of basalt, rhyolite, obsidian, etc., are common. Ash forms a conspicuously small part of the unit. Sizes of the pumice and lithic fragments decrease regularly eastward. Within about 6 miles of the vent the upper part of the unit contains a few thin ash beds, generally 1 in. or less thick. They may be products of small groundsurges, ash washed from the eruption cloud, or decline of eruption pulses. Continue east.
(0.9)
60.0 View back to west of The Dome, a cinder cone partly covered by pumice fall. Road down east flank traverses the pumice fall. At the intersection of the road with Road 1821 (18) the pumice fall is about 3.3 ft thick.
(2.0)
62.0 Note Red Hill, a young cinder cone to north; pumice fall once covered it but now is eroded.
(4.7)
66.7 Turn south on Road 1821 (18) (elevation about 5,250 ft) which passes on west side of China Hat, an 0.8-m.y.-old rhyolite dome.
(0.9)
67.6 Road intersection (Road 2228), keep to left on 1821 (18).
(1.5)
69.1 Note young basalt flow on west. It is overlain by Mazama ash and Newberry pumice fall.
(0.8)
69.9 Ash-flow tuff crops out beside road.
(0.5)
70.4 Section line 23-26 marker on tree.
(0.1)
70.5 STOP 12. Ash-flow tuff (elevation about 5,070 ft). Walk eastward on small dirt road to outcrops of ash-flow tuff. Note the exposures in roadcuts above the small cliffs. This ash flow crops out over a north-south distance of 5 miles but is overlain in places by alluvial deposits and basalt flows. It is buried on the west by similar rocks. The base of the ash flow is not exposed and the top is eroded, but it occurs in outcrops as much as 25 ft high. The ash flow is dark colored and contains abundant gray to black pumice in an ashy lithic-rich matrix. This ash flow may correlate with an ash flow that crops out locally for a distance of about 10 miles northward from this locality, and higher in the section than the ash flows to be visited at stops 13 and 14. Turn around and go back (northward) on Road 1821 (18).
(3.4)
73.9 Cut on east side of road exposes pumice fall over Mazama ash. Old railroad grade to east leads to road on west side of China Hat where pumice falls and ash flows are plastered on the side of the rhyolite dome.
(0.4)
74.3 Road 2129 (21) on west, continue northward on 1821 (18).
(4.4)
78.7 Road on left leads to site of Brooks-Scanlon Camp No. 3, used in the heyday of logging in the 1930's. Exposure of alluvium on left. The alluvium extends up the slope to the caldera rim.
(0.9)
79.6 Ravine. South of the ravine are exposures of basalt with local windows of rhyolitic ash-flow tuff. North of draw rhyolitic ash-flow tuff and a thin veneer of gravel form the surficial rocks to and beyond the next turn at 82.2 miles.
(2.5)
82.1 Brooks-Scanlon Camp No. 2. Former logging site was east of road.
(0.1)
82.2 Turn west on Road 2113 (1835) (elevation 4,885 ft) Road is along broad ridge covered by gravel deposits that overlie ash-flow tuffs.
(2.1)
84.3 STOP 13. Ash-flow tuff (elevation 5,170 ft). Park in wide area on south side of road and walk southward to outcrops along side of ravine. This dark-colored ash-flow tuff, characterized by porphyritic obsidian as a common accidental lithic constituent, is formed of black to dark-gray porphyritic pumice in an ashy lithic- and crystal-rich matrix. It crops out along ravine walls at this general elevation for a distance of about 4 miles, beyond which it is buried by basalt flows. Farther upslope it is buried beneath alluvial deposits and basalt flows. The unit is similar to the ash flow visited at stop 12 but has a different lithic component and is much more crystal rich. Ash flows, like those at stop 12, occur further north; some occur plastered on the sides of cinder cones (e.g., Orphan Butte). A thick sequence of lapilli tuff, identical to that at stop 2 on the west flank, occurs about 2 miles northeast of this locality and higher in the section. Turn around and return eastward on road 2113 (1835).
(2.1)
86.4 Continue on road 2113 across road 1821 (18). Road traverses area of rhyolitic ash-flow tuff like at stop 14, overlain by thin gravel veneer. Exposures of ash-flow tuff can be seen along many small ravines north and south of road.
(1.5)
87.9 Kipuka of basalt projects through rhyolitic ash-flow tuff south of road.
(1.2)
89.1 Turn north on Road 2013 (13) (elevation 4,665).
(0.7)
89.8 Basalt flow on east was derived from a vent east of China Hat.
(0.6)
90.4 STOP 14 (elevation 4,540 ft). Teepee Draw ash flow. Walk west from the border of the National Forest to outcrops of ash-flow tuff on sides of ravines. Note that basalt flows crop out on east side of nearest ravine and on west side of the far ravine immediately north of the ash-flow tuff. This rhyolitic ash-flow tuff covers a large area on the lower eastern and northeastern flank and is buried to the northwest and south by basalt flows. Toward the caldera gravel on the surface of the ash flow thickens to the point where float and outcrops of the ash flow can no longer be found. This is the oldest known tephra unit on the volcano. Plagioclase separated from pumice in it yielded an age of 0.7 m.y. (McKee and others, 1976), but unfortunately a high atmospheric argon content resulted in an uncertainty as large as the age. The ash flow and overlying units are normally polarized. At this locality the lower part of the ash flow is uncollapsed and consists of large pumices in an ashy lithic- and crystal-rich matrix. The pumices show progressive collapse upward and the top of the unit is welded. Note the scattered fossil fumaroles in the cliff face. Toward the caldera the unit becomes progressively more densely welded. The base of the unit is exposed only where kipukas of basalt project through it and the top is eroded. Along some ravines, however, the unit is more than 70 ft thick with the base not exposed. This ash-flow tuff probably was associated with an early, if not the earliest, period of caldera collapse. It and the ash-flow tuff at stop 13 are older than the tephra deposits of the west flank and may occur at depth entirely around the volcano. Look to the east to see Pine Mountain, a complex of flows and intrusions mostly of dacitic to rhyolitic composition that are about 21 m.y. old and that probably correlate in part with the John Day Formation. Continue north on road.
(0.9)
91.3 Turn northwestward on road 2043 (23).
(2.0)
93.3 Outcrop of ash-flow tuff west of road.
(0.4)
93.7 Outcrop of ash-flow tuff on subdued hill east of road is the most northeasterly known outcrop. Eroded alluvial fan deposits derived from Newberry and Pine Mountain cover the distal part of the ash flow and it is also buried on the west by basalt flows from Newberry.
(1.1)
94.8 To the west is Horse Ridge, a highly faulted sequence of older basalt flows and near-vent deposits. The faults are part of the Brothers fault zone.
(2.5)
97.3 Junction with U.S. Highway 20. Turn west.
(1.7)
99.0 Viewpoint. Dry river canyon is 800 ft wide and 300 ft deep at this point. For next 0.8 mile roadcuts and canyon walls expose lavas of Horse Ridge. The basalt flows, with interbedded tuff and breccia, have yielded a 7-m.y. K-Ar age.
(2.0)
101.0 U.S. Highway 20 from here to Bend crosses diktytaxitic high-alumina basalts derived from Newberry that cover much of the low-lying region to the north.
(16.4)
117.4 Road on north leads to Pilot Butte. Although young in appearance, this cinder cone is probably many hundreds of thousands years old, as it is locally overlain by an early Pleistocene pumice fall.
(0.7)
118.1 End of trip at Junction of U.S. Highways 20 and 97 in Bend, Oreg.

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Last Updated: 28-Mar-2006