CGS geologists found that there is a bedrock equivalent of swelling soils. They term this phenomenon “heaving bedrock.” Just as certain kinds of clays in soils can swell when they get wet, so the same clays in clay-rich rocks (claystones, shale) can swell also.
Because rock layers just east of the Front Range are tipped up at an angle, the edge of any one rock unit tends to crop out in a more or less linear fashion, parallel to the mountain front. Claystone layers follow this pattern too. Changes in drainage after construction and watering of new grass around a building allow more water than usual to soak into the claystones. The claystones respond by doing what comes naturally: they swell. As long as extra water is provided, the claystones will stay expanded. Rocks on either side of the claystones that contain little clay do not swell. The linear ridges, then, mark the tipped-up edges of claystones that swell more than the surrounding rocks do.
The “roller-coaster road” is the result of uneven swelling and heaving of steeply dipping bedrock layers. Photo credit: Dave Noe.