Geology of Boucherie Mountain Volcanic Complex

Dr. Murray A. Roed

Mount Boucherie is a dominant geologic landmark on the western side of Lake Okanagan in the Kelowna area. The mountain is an erosional remnant of an extensive volcanic complex that developed after a huge chasm split open the Earth’s crust along ancestral Okanagan Valley possibly 60 million years ago, in early Tertiary time. The first major volcanic eruption witnessed the outpouring of yellowish, tan and pinkish volcanic flows composed of rhyolite and andesite derived from a magma chamber in the upper part of the Earth’s crust. These rocks dominate the northern and southern flanks of Boucherie Mountain, and are very distinctive in colour, and contain numerous vesicles (cavities). Some of these cavities have been filled with agate (banded chalcedony, a silicate mineral) and crystalline quartz fillings. This formation is part of the Marron Formation as mapped by Brian N.Church, 1980. However, Boucherie Mountain is best known for its conspicuous cliffs composed of black to dark gray crystalline dacite that forms spectacular columnar structures. Recent geologic mapping has found that the columnar dacite occurs as separate pipes. One large pipe occurs on the southwestern part of the mountain, and another on the northeastern part. Several smaller pipes have been found in the southern and middle part of the mountain. All of these dacite columnar volcanic pipes display intricate and picturesque contorted patterns, mysterious results of the tectonic history of the region. The dacite pipes thus represent individual pipes or volcanic necks that were feeders to surface lava flows, long since eroded. They also represent a major change in the source of magma from a different part of the Earth’s crust.

A diagrammatic structural cross-section (Figure 1) across the mountain reveals that Mt. Boucherie is a composite volcano, built by different lava flows over a long period of time. The familiar dacite columnar lava is actually the last episode of volcanic activity in the mountain since the dacite pipes clearly “intrude” or cut through older volcanic rocks that have an entirely different composition.

This volcanic complex was most active 50 million years ago, or so, and probably towered 2000 meters high, or more. Today, only a mere remnant remains. The rocks visible today represent just one part of an extensive volcanic field that characterizes the Kelowna region. The Okanagan Valley at that time was subject to widespread explosive volcanic eruptions at Summerland, Vernon, Enderby, and Okanagan Falls. Locally, Knox Mountain, Dilworth Mountain, Layer Cake Hill, Hayman Ridge and Black Knight Mountain are other examples of the same volcanic episode near Kelowna.

The remnant composite volcanic complex at Mount Boucheries is roughly circular with a diameter of approximately two kilometers. It now rises only 417 meters above the lake; its highest elevation is 758 meters at its northern edge. Glacial erosion and other geologic events have greatly subdued the conical form of the mountain from its previous towering grandeur.

The dacite volcanic rock of Mount Boucherie is particularly distinctive, as mentioned above, in that it displays a peculiar structure known as columnar structure. This refers to hexagonal geometric forms visible in the rock, formed by contraction during cooling of the lava, similar to the formation of mud cracks as a pond dries. Large boulders of the columns have broken away from the cliffs in places, and have fallen onto the forest floor below where they mysteriously resemble ruins of ancient temples. The columns are contorted into attractive polygonal patterns along steep slopes and at the summit of the mountain where the pipes are exposed.

This former great volcanic prominence has been greatly modified and eroded by several regional geologic events. The first event was the development of a major river that affected the Okanagan Valley near the end of this interval of volcanism. The deposits of this river now consist of stratified rock composed of volcanic conglomerate, other types of conglomerates, a variety of sandstone and siltstone units containing fossil plant fragments, and coal layers that outcrop extensively in the valley. These rocks are at least 1000 meters thick and are known as the White Lake Formation. The White Lake rocks are of course well exposed at White Lake south of Penticton, where they were first named, but they also can be seen along Highway 97 near the Gorman Brothers Mill, in rock cuts near Summerland and along Mission Creek in Gallagher’s Canyon. This formation has partially buried the roots of volcanoes such as Mount Boucherie, and is now found flanking the base of the mountain. Examples nearby include the conspicuous massive cliff of vugular conglomerate and sandstone at the junction of Boucherie Road and Pritchard Drive, and the prominent rock terrace on which Mission Hill Winery is built. Several hoodoos of this rock formation occur just to the east of Gregory Road, Lakeview Place and the Eain Lamont Community Park.

The second geologic event that has substantially affected the mountain is related to regional folding and faulting of the bedrock. During late Tertiary time, the Okanagan was subject to strong mountain building forces, possibly related to the rise of the Rocky Mountains to the East, but also from regional uplift and mountain building that occurred in the whole Province of British Columbia. The rocks were first squeezed from the west and east. These forces formed northerly trending folds. Then, due to uplift and major horizontal forces, the rocks were broken up along faults that not only moved the rock masses up or down, but also displaced the rocks along breaks or faults in a horizontal direction. Strong vertical movements may have been affected also by sudden collapse of an underlying magma chamber, the source of most of the volcanoes.

The last geologic event affecting the mountain involved erosion by thick glaciers and ice sheets that periodically ground their way across the land during the Ice Age that began about two million years ago. The Okanagan and most of British Columbia may have been glaciated at least six times. The latest glacial event is known as the Fraser Glacier that advanced 20 to 25 thousand years ago, and began to melt about 15,000 years ago. This glacier completely engulfed all mountainous terrain. It was southerly-moving, and served to help carve the very steep northern slopes of Mount Boucherie by plucking out large portions of the volcanic rocks. Downstream on the south side of Mount Boucherie the glacier molded and streamlined the terrain, preserving the large triangular terrace underlain largely by the White Lake Formation and other volcanic rocks. Thus, in glacial terminology, Mount Boucherie is an example of a giant-size rock drumlin defined as “an elongated knob of rock with a whale-back form, the long axis of which is orientated parallel to former ice flow, and having a smooth, glacially abraded down flow side, and a steeper glacially plucked up flow side”.

Mount Boucherie overlooks Lake Okanagan to the east, the flat plain of the City of Kelowna, and a broad flanking saddle formed by McDougal and Duncan Creeks to the west. The Kelowna lowland is composed of the deposits from merging of numerous deltas that filled in the valley as a glacial lake drained southward approximately 10,000 years ago. This glacial lake is known as Glacial Lake Penticton. The deposits of silt and clay in this ancient lake now form the gently rolling terraces mantled with orchard and vineyards of Lakeview Heights and, likewise, the terraces flanking the eastern side of Okanagan Lake at Kelowna, and on both sides of the lake near Summerland and Penticton.

The McDougal-Duncan Creek valley is composed largely of glacial out wash sand and gravel (numerous gravel pits now) that represent formerly large glacial streams from melting of the Fraser Glacier. The Fraser Glacier also left behind a lateral moraine composed of glacial till that rims the western, northern and eastern flanks of the mountain. All major geologic features of the mountain are shown on an orthophoto map in Figure 2.

Discovery awaits those who venture to the top of Mt. Boucherie, accessible by a number of trails, some of which are steep and dangerous. Hoodoos of columnar dacite, intricate patterns in rock cliffs, and panoramic views of the Okanagan Valley, Kelowna and Westbank make an outing well worth an afternoon of careful exploration.

Accompanied by:

Figure 1: DIAGRAMMATIC CROSS-SECTION OF MOUNT BOUCHERIE AREA

This is a highly interpretative structural cross-section of the composite volcanic complex of Mount Boucherie. It is not drawn to scale. North is to the right, and South is to the left. The coloured units are deposits from various volcanic eruptions, the last being that represented by dacite pipes with columnar structure. The near vertical lines represent faults along which there has been differential movement; most of the faults are inferred. Dotted sketches are included in the background to provide an impression of the original volcanic scene in the Okanagan Valley.

Figure 2: MAP SHOWING LOCATION OF MAJOR GEOLOGIC FEATURES

#1 COLUMNAR DACITE, DARK COLOUR,
VOLCANIC NECKS, NORTHEAST AND
SOUTHWEST CLIFFS, MT. BOUCHERIE.

#2 ANDESITE, AGATE INFILLING OF SOME
VESICLES; VESICULAR LAVA OF VOLCANO
PREDATING DACITE VOLCANIC NECKS.

#3 OUTCROPS OF PEBBLE-COBBLE CONGLOMERATE
AND SANDSTONE, WHITE LAKE FORMATION.

#4 HOODOOS ERODED FROM WHITE LAKE FORMATION.

#5 LARGE BOULDERS OF DACITE WITH COLUMNAR
STRUCTURE; BOULDERS DERIVED FROM STEEP
EASTERN FACING CLIFF OF MT. BOUCHERIE.

#6 VALLEY MARKS THE TREND OF AN EAST-WEST
FAULT IN THE VOLCANIC BEDROCK;
MOVEMENT COULD BE BOTH LATERAL,
AND UP OR DOWN.

#7 WEST DIPPING SANDSTONE AND THIN COAL
LAYERS IN SEDIMENTS SUSPECTED TO BE
PART OF THE WHITE LAKE FORMATION.

#8 THIN BEDDED RHYOLITIC VOLCANIC ROCKS
DIPPING WEST, PART OF THE OLDER VOLCANIC
ROCKS OF THE KELOWNA VOLCANIC COMPLEX.

#9 PROMINENT LATERAL MORAINE MADE UP OF
BOULDERY GLACIAL TILL. THE MORAINE WAS DEPOSITED
BY THE FRASER GLACIER AND ENCIRCLES THE BASE
OF MT. BOUCHERIE ON THE NORTH, WEST, AND EAST SIDES.

#10 STEEP SLOPE EXPOSING ABUNDANT COLUMNAR
STRUCTURES IN DACITE VOLCANIC ROCK WHICH
HERE OCCURS AS A DISTINCT NECK OR PIPE. THE
VOLCANIC FLOWS APPEAR TO BE DIPPING WESTWARD.

#11 TWO SMALL PIPES OF DACITE WITH COLUMNAR
STRUCTURE “INTRUDE” ALTERED RHYOLITE-ANDESITE.

This account has been kindly reviewed by Dr. John Greenough, Chairman, Department of Earth and Environmental Sciences, Okanagan University College, Kelowna, BC.