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QUEBEC

South-central part of Eastern Townships (Estrie)

Spillitic meta-basalts associated with accretion during the Taconic Orogeny (closure of Iapetus Ocean)

Fieldwork by Sir William Logan

THE SCIENCE

The fundamental tenets of field mapping had been established by the early 1800’s.

However, unlike so many of the amateur “gentlemen” geologists/naturalists of this period, Logan had come to geology from the pragmatic perspective of a businessman, where the need for careful and quantifiable measurements were of utmost importance. As a consequence Logan brought a rigor to geological field work that would have been unrivalled in his day.

The geological “bible” in midcentury would have been Lyell’s Principles of Geology (1832). Geological traverses on compass bearings were made by walking, with distances determined by counting paces or with the use of an optical device known as Rochon’s micrometer. Logan describes field conditions as “living the life of a savage, sleeping on the beach in a blanket sack with my feet to the fire, seldom taking my clothes off, eating salt pork and ships biscuit, occasionally tormented by mosquitoes.”

In the field, Logan usually started at dawn and continued into the evening until he could no longer see distinctly. By the light of the campfire he would work until a late hour to ink-over the penciled notes and figures made during the day and plot on field maps all the measurements and dimensions which had been recorded.

His geological observations were authenticated by pen and ink sketches which clearly reveal his artistic talents. His note books, held by the Geological Survey, are exemplary models for recording field data.

The dominant element in the Krieghoff painting is Owl’s Head Mountain. The bulk of the mountain is made up of volcanic rocks (solidified lava) and this rock type also forms the cores of the other “mountains peaks” that are aligned with the “Owl” along the western margin of Lake Memphremagog, including Jay, Bear, Sugar Loaf and Hogs Back. Due to the more resistant nature of the volcanics, this north-south chain of mountains stand proud of the surrounding area which is underlain by softer more easily eroded rocks composed of folded and metamorphosed shales and siltstones. The topography of these ancient volcanoes has been sculpted by glaciers within the past 100,000 years, but the volcanoes themselves formed in the Ordovician period, about 480 millions years ago.

The Owl’s Head is a strikingly picturesque hill but in reality falls short of the dramatic mountain depicted by Krieghoff, rising a mere 539m from lake level to summit. Logan geologically mapped and studied the Eastern Townships over the course of at least 12 field seasons and most certainly would have traversed the mountain and collected samples from its outcrops.

It is therefore most curious to view the manuscript compilation map of the Eastern Townships produced in 1866, drawn at a scale of 1” to 4 miles (1:63,360), as it shows no distinctive map unit (unique rock type) associated with those areas occupied by the Owl and other mountains in the same chain.

The first geological map that correctly depicts this mountain chain as having a common rock type was produced in 1896 by the Survey geologist Robert Wheelock Ells. He described the Owl rocks as being igneous in origin, but rather than the extrusive volcanics that we recognize today, he classified the
rocks as diorite, a medium crystalline intrusive rock comprised of plagioclase (andesine), hornblende, with minor amounts of quartz and potassium feldspar. Ordovician Period He thought their age was likely to be Silurian based on field relationships with surrounding rocks. More recent mapping published by Lamonthe in 1980 (Quebec Dept Natural Resources) and by Rickard in 1991 (Geological Survey of Canada) refer to the Owl’s Head rocks as the Bolton Volcanics (and more precisely, as spilitic meta-basalts).

Further work on the nature and genesis of the Owl’s Head volcanics has been conducted by Melancon et al. (1997), and based on the chemical composition of the rocks these scientists believe that they formed part of the seabed (oceanic crust) of an ancient ocean (called Iapetus) that was present off the eastern margin of North America, before the existence of the Atlantic.

These seabed volcanics were slowly squeezed up and onto the North American craton by immense pressures during plate collision about 440 million years ago (an event called the Taconic Orogeny).

THE GEOLOGIST

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