Q – I have noticed places around Spokane where there are large accumulations of similarly-size boulders.

They are smooth and generally about a meter in diameter.. The type of stone varies, but none are Columbia Basin basalt.

I assume these are ice-rafted erratics.

My question is how did so many end up in a few locations and why are they all similarly sized?

My guess is that the flood waters ran into a slope and became still enough to drop similarly-sized loads.

Is that right? Or is there another explanation?

The South Hill Park and Ride has 81 scattered around the site, some of which I assume were pushed out of the way for paving. Another site nearby has perhaps 40 used for a retaining wall.

The Spokane Valley YMCA has a lot, as does the parking lot for the Dishman Hills Natural Area. There are also places along Trent Avenue where these stones are found. Thanks for your reply. – James VanderMeer 

A – Most all of the well-rounded boulders that clutter parts of the floor of the Spokane Valley are, indeed, derived from the Ice Age floods, but are probably were not ice-rafted to their present locations.

They were moved by the tremendous power just downstream from the initial Ice Dam failure, rolled or bounced along the bottom of Glacial Lake Columbia and accumulated rapidly in the deeper parts of the lake.

The power of the flood currents dissipated as the waters moved west. Most of the large boulders found father west were probably ice-rafted.

The source for these boulders was most likely glacial moraine and stream sediments from Idaho, Montana, and Canada, thus no local basalt in the mix.

Because they were “current” derived, they are often sorted in size, and derive from a source where there has been much erosion already and well rounded. More information on this can be had in the online Zoom lecture “What Happened When the Dam Burst” by the IAFI on November 24 by Michael Hamilton.