Answer:
2002, 2003, and 2012 had the highest reported incidence rates, around one case per 100,000 people, the states that were most affected were Colorado, South Dakota, Wyoming and North Dakota
The slitting of a nucleus
Streams that flow beneath melting glaciers leave long ridges of layered sediments, known as Esker. Eskers are formed within ice-walled tunnels by streams which flowed within and under glaciers. They tended to form around the time of the glacial maximum when the glacier was slow and sluggish and after the retaining ice walls melted away, stream deposits remained as long winding ridges.
The main reason is because they are heavy, and because they don't have to chase their food. They're not predators and their shells give them protection like running away gives protection to other herbivores.