When a compass needle points towards the North Pole, it is being attracted toward magnetic north by the natural magnetic field of the earth. Today this magnetic north pole is located in northern Canada, off the coast of Ellef Ringnes Island, some 11 degrees away from the actual surveyed north pole where all the lines of longitude meet at the top of the earth. But at different times in the geologic past, the magnetic "north" pole that we see today, in fact lay in the south.
This wouldn't have been discovered, except for the fact that many different types of rock actually "record" the position of the magnetic pole when they are formed. By studying the magnetic directions recorded in rocks of different ages, we have been able to re-create the movement of the continents through time, and we have also been able to construct a time scale from which rocks can be dated from the direction of the magnetic pole that they store within them. In some time intervals, the rocks show that the magnetic pole lay to the north, a "normal" direction from our present-day point of view, and at other times it lay in the south, a direction that we call "reversed."
The study of the magnetic properties of rocks is called Paleomagnetism, literally "ancient magnetism." By looking at the magnetic directions stored in the rocks of the Denver Basin, we hope to be able to calculate their age to a high degree of precision. Our ability to do this is the result of years of research that have shown that the rocks of the Denver Basin were being deposited in a time period when the earth's magnetic field switched at least five times. If we can find these "reversals" when the magnetic pole moved from north to south, and south to north, then we can go back to the time scale where the ages of all these reversals are recorded, and calculate how old the sediments are.
Together with the pollen, leaf fossils, and vertebrates that we find in the rocks, we can construct a history of the Denver Basin that relates how the area changed as the Rockies gradually rose from the latest Cretaceous onward to their present-day height.