Geology

Geology

Photo credit: Geology.com

The city of Seattle lies within the Puget Sound Lowland, a topographic basin between the cascade and Olympic mountain rages. This area has been impacted by repeated glaciation in the past 2.4 million years. As well as modified by landslides, stream erosion and deposition as well as human activity. This area sits atop a succession of glacial and non-glacial deposits. The glaciation in this region created deep troughs, ravines, and drumlins as they moved over this area.  The subsurface materials from the deepest surface up starts with Olympian beds, which are mad up of sand, clay, and scattered organic material, it is very dense and hard. The next layer is Lawton clay, which is clay with some silt and is gray, and impermeable to water. The top layer is made up of fine to medium sand. Till can also be found and has a wide variation of thickness and with an assortment of different stones, and pebbles. Till was carried by the glaciers and spread out over this region.

An example of what the glaciers left behind are glacial deposits also know as erratic. An erratic is a large granodiorite rock that was carried south by the glacier and left here. One of these Erratics can be found in Ravenna Park. In the picture above you can see Krishna in comparison to the large erratic. Granodiorite is very common in the British Columbia coastal mountains and is not found in this region.