Intoduction to Identification of Rocks

Dr. Murray A. Roed

It is one thing to pick up a rock and try to identify it, it is another to do this with a bit more of an understanding of the various possibilities of what that rock represents. This section attempts to explain a concept to be employed as a framework when trying to identify a rock and speculate on its history and significance. This account is an outline of presentations made to elementary grade students, but it is an initial guide for most non-experts.

Concept of Unconsolidated and Consolidated

One of the first things to learn about geology is that there are a number of “classifications” that have developed throughout the years. The first one is referred to as “the concept of unconsolidated versus consolidated Earth materials”. By consolidation it is meant that the material is hard, for example, any rock. By unconsolidated it is meant that the material is soft, like soil or sand and gravel.

Consolidated Material

Consolidated material is the hard surface of the land composed of solid rock called bedrock. We see bedrock along the mountainsides, and in roadcuts, and bedrock forms the very core of all mountain ranges. But bedrock can be of many different kinds, thus we have another classification, the Classification of Rocks. There are three main types of rocks:

Igneous
Metamorphic
Sedimentary

IGNEOUS ROCKS: Igneous rocks are formed ultimately by melting of the Earth’s interior. Igneous rocks are of two general types:

1. Intrusive, granite for example. These are rocks that have been intruded from hot liquid magma into rocks above from deep within the Earth, but never reached the surface. However, because of erosion, many of these rocks are now uncovered and we can see them in the mountains, and in many places in the landscape.

2. Extrusive, volcanic rocks for example. These rocks form volcanoes of different types, and result from outpouring of magma onto the Earth’s surface when it is then called lava. There are many types of volcanoes, some are explosive like Mt. St Helens in nearby Washington Sate, others are very liquid and flow for miles over nearly flat land like the plateau basalt lava around Kelowna. Kelowna also has very ancient explosive type volcanoes such as Knox Mountain, Mount Boucherie, and Black Knight Mountain amongst others.

METAMORPHIC ROCKS: Metamorphic rocks are rocks formed from pre-existing rock by great pressure and temperature wherein the minerals in the rock are changed and re-formed giving the rock a banded appearance, the bands being composed of different minerals. The rocks are called schist or gneiss and could be granitic, or a great variety of other compositions. Most of the rock in Okanagan Mountain Park is composed of metamorphic rock. Marble is also a metamorphic rock changed from a rock known as limestone.

SEDIMENTARY ROCK: Sedimentary rock is material formed by erosion and depostion by a great variety of processes, and turned into rock by consolidation over long periods of geologic time and a result of deep burial. Thus a sand deposit becomes a sandstone; a gravel deposit becomes a conglomerate. A mud or clay deposit becomes a shale. Sedimentary deposits are also classified, according to their origin: Clastic, sandstone, for example, Chemical, limestone, for example, Organic, coal, for example, and so on.

Unconsolidated Material

All unconsolidated materials have been derived from bedrock as a result of a variety of geologic processes of erosion and deposition. Examples include soil, sand and gravel, sand dunes, river and stream deposits, landslides debris, lake deposits, muskeg or swamp deposits, and most commonly in Canada, glacial deposits formed by melting of glaciers that have covered the land a number of times.

All unconsolidated deposits are called Surficial Deposits, and these are classified further in the following manner:

NONGLACIAL

Nonglacial deposits are those unconsolidated deposits that have not been formed by processes associated with a glacier. The most common include sand, pebbles, cobbles and boulders along creeks and rivers. These materials have all been formed by running water which has eroded bedrock or other material along its path. Another example is sand that has been blown by the wind (Eolian deposits) into sand dunes; sand dunes are particularly common in desert regions, but some occur in the Okanagan Valley. Another excellent example are beach deposits, that is, the pebbles on the beach along the shores of Okanagan Lake. All have been derived from bedrock originally.

GLACIAL

Glacial deposits include all material that has been either deposited directly from melting of glacier ice, forming a material called Till, or deposited by meltwater as the glacier melted, forming material known as Outwash. Outwash is similar to deposits formed in present streams and rivers. Also, if meltwater becomes ponded, or dammed in a valley, then a lake is formed and the deposits in that lake are called glaciolacustrine.

TILL: Till is a deposit formed when a glacier melts and during deposition the material is not modified by meltwater or any other erosional process. This results in a mixture of silt, clay, pebbles and boulders, or whatever was in the glacier ice. This process is similar to the crust of dirt that accumulates on snowbanks in the Springtime. Till is therefore the material that the glacier eroded and carried along as it moved across the bedrock landscape. If there were any surficial deposits along the way, the glacier would like strip these off the land too.

There are many examples of till in the Okanagan Valley. A good one is located along the path on the eastern side of Mission Creek in Mission Creek Park where the till forms a hoodoo.

OUTWASH: When a glacier melts, great volumes of water result. All of the material contained in the ice along the flow of meltwater forms into stream and river deposits similar to those of today. Some of these streams get so big that they form deltas as they enter a glacial lake ponded by the sudden build-up of meltwater. Most of the gravel pits in the Okanagan are developed in this type of Outwash deposit.

GLACIOLACUSTRINE: In the Okanagan, we have one of the best examples of a glacial lake. The familiar silt bluffs along the lake are all glaciolacustrine in origin formed when stagnant blocks of ice blocked meltwater drainage near Okanagan Falls. This glacial lake is known as Glacial Lake Penticton and used to stretch to the north as far as Enderby.

There are many other “classifications” in geology, but this introduction is basic in trying to understand what we see in the landscape today, and helps when we pick up a rock and try to identify it, and ponder what kind of history that rock has experienced.

Examples of Geologic Landforms in the Kelowna Area

WATERFALLS: Erosion by water

1. Crawford Falls

2. Bear Creek Falls and Canyon

VOLCANOES: Mountains formed by volcanic activity, eroded by glaciers

1. Knox Mountain

2. Layer Cake Hill

3. Boucherie Mountain

4. Lakeview Escarpment