Glacial till terrain: why material can be far from its original source

Areas that were covered by glaciers during past ice ages, much of the northern United States and Canada, have a genuinely distinctive collecting character because glaciers physically pick up and transport rock, sometimes hundreds of miles, before depositing it as unsorted glacial till when the ice melts. This is exactly why collectors in glaciated regions can find material, like Lake Superior agates, in gravel deposits far from where that material actually originated geologically. A genuinely useful practical consequence, agates and other durable material tend to concentrate in the same gravel bars and beaches as anywhere else, since the same basic water sorting principles apply once the glacially deposited material is reworked by modern streams and shorelines.