The large joint seen in this photo is one of the erosional forces that have shaped the Lower Dells area. They are caused by the cyclical freezing and thawing that occur as winter turns to summer and summer to winter. As these joints separate portions of the sandstone from the larger rock unit, the blocks crash to the floor of the canyon. Eventually, the stream at the base of the canyon erodes away these blocks and the sediment is carried downstream. This in turn widens the valley. The walls of the Lower Dells are irregular and winding, which indicates that it is old, and that erosional forces such as joints have been at work on the canyon walls for quite some time.