Chris Yorath

The bedrock of Butterfield Gardens, on the lower southern slopes of Mt. Newton, is formed of granodiorite, a kind of rock closely akin to granite. The light-coloured crystals are quartz and feldspar, whereas the fewer dark crystals are hornblende and biotite. Hornblende contains the element potassium, a radioactive isotope of which is potassium 40 that decays to argon 40 at a known rate. Samples of hornblende collected from the Mt. Newton granodiorite have been determined to be about 170 million years old, or Early Jurassic in age.

The Mt. Newton granodiorite, together with similar rocks on Bear Hill, belongs to a suite of igneous rocks that form the backbone of Vancouver Island. These rocks, called the "Island Intrusions" formed from molten material which originated from deep within the crust and upper mantle and which was injected, or intruded, into the upper layers of the crust where it slowly cooled to form the large crystals you can see with the unaided eye. Where the magma reached the surface it cooled very quickly to form volcanic lava, the crystals of which are so fine that they can only be seen with a special microscope. Some 110 million years after the granodiorite had solidified within the crust, parts of Vancouver Island were uplifted. The forces of erosion wore away the covering rocks to ultimately expose these granitic rocks. Fifty-five million years later, glaciers invaded the region and helped carve the land forms we see today.

Although bedrock is not well exposed in the gardens, you will notice that outcrops of the granodiorite elsewhere on Mt. Newton are broken by numerous small fractures, steeply inclined to nearly horizontal. These are called "joints" and have many orientations although there is a weak dominance of nearly vertical, northwesterly and northeasterly trending orientations. The combination of these joints, together with the effects of glaciation which occurred here more than 10,000 years ago, resulted in the development of the somewhat terraced physiography of the lower slopes of the mountain.

From several vantage points on Mt. Newton you can look eastward and see the Gulf and San Juan islands. The former are mainly formed of sedimentary rocks that accumulated between 85 and 65 million years ago, or during the latter part of the Cretaceous Period. These rocks, assigned to the Nanaimo group, were later bent into northwesterly trending folds that give the Gulf Islands much of their spectacular scenery. You get a close look at these rocks when you are waiting in ferry lineups at Schwartz Bay. The American San Juan Islands are formed of volcanic and sedimentary rocks that are much older, and have resulted from a different geological history than that of Vancouver Island.

The topography of the Saanich Peninsula is the result of both erosion and deposition. The hills and valleys were carved from granitic, volcanic and sedimentary rocks by glacial ice which receded from this area about 10,000 years ago. Indeed, Saanich Inlet is a glacial fiord that was carved by ice flowing northward out of Finlayson Arm whereas elsewhere on the island, ice-flow directions were dominantly southward. Although bedrock was left exposed on the hills, many of the valleys became the depositional sites for outwash gravel, sand, silt and clay that had been entrapped in the ice and which were freed as the ice melted. The valley of Hagan Creek below the gardens is filled with glacial debris.