Geology of the Dishman Hills

Within the Deep Ravine, you will come across many interesting geologic features including steep rock walls, gneiss outcroppings, and ancient bedrock.

These features are a result of the Ice Age Floods (also known as the Missoula Floods) that occurred 18,000 to 12,000 years ago.

Galcial Lake Missoula was a huge and ancient lake dammed up by glaciers in what is now Montana. Periodically, the lake would build up behind the ice dams and eventually burst through, sending massive amounts of water rushing over the Pacific Northwest landscape. These flood profoundly and permanently changed the landscape of the region, resulting in what we see today.

The Dishman Hills, though a relatively small area, was directly in the path of the floods. The inundation of water over the landscape carved out the softer sedimentary rocks, revealing the harder bedrock that we can still today. The water created a series of cliffs and valleys in the Dishman Hills that you may be able to observe. If you look closely at the land within the Natural Area, you’ll notice a pattern of rock walls, followed by a canyon, followed by another rock wall. The water would carve out a valley, fill it up, then spill over and create another valley.

These features helped create the variety of micro-climates that exist in the Dishman Hills. The valley tend to be wetter and shadier, creating a different ecosystem than the high parts which tend to be dry and sunny.

Fun Fact: The bedrock in the Dishman Hills is some of the oldest rock in Washington State, over 1.5 billion years old

As you hike through the Deep Ravine, you should come across this particular outcrop of rock along the trail.

This is an example of exposed bedrock, composed of several different types of rock. The darker color indicates the solid bedrock, made of gneiss (pronounced like “nice”) an ancient metamorphic rock. The lighter veins within it are made of 70 million-year-old igneous granite that filled into the cracks over time.

Fun Fact: The waves from the Missoula Floods reached 98 feet high and were spaced about 300 feet apart. In total, the floods released 9 cubic miles of water per hour that traveled at speeds up to 50 mph.