The Cheakamus Valley basalts are a sequence of basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

ic lava flows at the Cheakamus River

Cheakamus River

The Cheakamus River is a tributary of the Squamish River, beginning on the west slopes of Outlier Peak in Garibaldi Provincial Park upstream from Cheakamus Lake on the southeastern outskirts of the resort area of Whistler. The river flows into Cheakamus Lake before exiting it and flowing...

 in southwestern British Columbia

British Columbia

British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...

, Canada

Canada

Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

. They are part of the Garibaldi

Garibaldi Volcanic Belt

The Garibaldi Volcanic Belt, also called the Canadian Cascade Arc, is a northwest-southeast trending volcanic chain in the Pacific Ranges of the Coast Mountains that extends from Watts Point in the south to the Ha-Iltzuk Icefield in the north. This chain of volcanoes is located in southwestern...

 section of the Cascade Volcanic Arc

Cascade Volcanoes

The Cascade Volcanoes are a number of volcanoes in a volcanic arc in western North America, extending from southwestern British Columbia through Washington and Oregon to Northern California, a distance of well over 700 mi ...

 and were produced during an episode of volcanic eruptions from an unknown vent. The latest basaltic eruption deposited wood in lacustrine sediments under the lava flows that have been dated years old. These youngest lavas form isolated ridges above the older glaciated Cheakamus Valley basalts and were described as "esker-like" by Canadian volcanologist Bill Mathews

Bill Mathews

William Henry Mathews was a Canadian geologist, volcanologist, engineer, and professor. He is considered a pioneer in the study of subglacial eruptions and volcano-ice interactions in North America...

.

Columnar jointing is present all through the most recent basalt lava flow and pillow basalts exist in the lowermost unit, portions of which are underlain by hyaloclastite

Hyaloclastite

Hyaloclastite is a hydrated tuff-like breccia rich in black volcanic glass, formed during volcanic eruptions under water, under ice or where subaerial flows reach the sea or other bodies of water. It has the appearance of angular flat fragments sized between a millimeter to few centimeters...

 breccia. Bill Mathews suggested that these esker-like lava flows were deposited during subglacial eruption

Subglacial eruption

A subglacial eruption is a volcanic eruption that has occurred under ice, or under a glacier. Subglacial eruptions can cause dangerous floods, lahars and create hyaloclastite and pillow lava. Subglacial eruptions sometimes form a subglacial volcano called a tuya. Tuyas in Iceland are called table...

s that traveled away from the vent inside tunnels or trenches melted in the overlying ice sheet. Mathews based his suggestion on the age of the underlying till, the existence of pillow lava close to the bottom of some lavas that indicate subaqueous volcanism, horizontal jointing at the edges of the lavas that indicate rapid cooling, and the absence of apparent paleotopography to explain these features.