Do you ever drive through the Southern Interior and admire the landscape of the valley, looking at the layers of stratigraphy in the steep drops cut into the hills along the rivers? Every layer on those hills tells a story, and can even help associate a depth in the ground with a place in time.
The latest major glacial period, the Fraser Glaciation, began before 25,000 Before Present (BP, with present being 1950), reached its maximum extent about 19,000 to 15,000 BP, and ended with rapid ice melting and evaporation around 14,500 BP. During this melting, the Cordilleran Ice Sheet covering British Columbia broke into several large masses which became stranded in lower areas of the interior. Movement of the melting glaciers left behind landforms such as drumlins, which indicate the direction of flow. The glaciers cut into river valleys, incising steep ravines and molding undulating landscapes, while depositing glacial till as it moved and melted. This glacial till is composed of unsorted rocks ranging from small pebbles to large boulders, and soils such as sand or clay. This sediment layer is important in the archaeological procedure.
We know the southern interior was under an ice sheet for millions of years, and evidence of human occupation dates back to at least 14,000 BP, following the end of last ice age. If archaeologists are digging test holes and encounter a layer of glacial till, they can typically deduce that cultural material will not be found below that depth. However, one instance that is not the case is when a landslide transports a wide range of materials, including glacial till, and covers a natural landscape. Another example is where ground disturbance occurs from machinery, displacing sediments and disturbing their natural layers. In these scenarios, an archaeologist takes into account the surrounding landscape and ground disturbance that has occurred. They can also dig a percentage of tests deeper into, or through, the glacial till to confirm a natural deposition rather than an accidental deposit.
When you look at exposed soil in areas such as the Kamloops Bike Ranch, you see different layers of stratigraphy. Some sediments travelled there by water, air, or gravity. Archaeologists let the soils tell them a story of how that landform came to be formed, and from this story, try to figure out a timeline of deposition of cultural material. In some instances, multiple layers of occupation over a range of depths will be found at a single location, which is possible due to sediment displacement from the forementioned processes (water, air, and gravity), which cover the older layers of occupation, making new habitable locations.
Another part of sediment layers that help relate a depth in the ground with a place in time is the presence of volcanic ash- for example, the Mazama eruption, which has been dated to approximately 7600 BP. Volcanic eruptions can provide a chronological framework for archaeologists because they can be dated to a specific time in history. One can assume if artifacts are found below a particular volcanic ash layer, which is a fine white powdery lens, they would have been deposited before that eruption.
Next time you drive from Kamloops to Monte Creek, look at the layers exposed on the hills, and see if you can find the glacial till or the volcanic ash- or just enjoy the view of thousands of years of history. Now that I know more about the temporal aspect of soils, I appreciate the natural landscape with a new perspective, and I hope you do too!