Cakes: What causes the line near the edge of cake layers?

Scott recently e-mailed me to ask what causes cake layers to have a line about 3/4- to 1-inch from the sides of the pan. He said, “I bake square cakes and rectangular cakes. For some reason the finished product has a square inside the square. “  I passed his e-mail along to Joanne Lamb Hayes, who helped with the dessert section of my cookbook, and she explained that when cake layers bake, heat penetrates them from the top, bottom, and sides and makes them rise and become firm. Heat sets the outer 3/4- to 1-inch  of batter well before the center and it stops rising. As the center continues to rise until it too gets hot enough to set, it leaves a line in the surface of the cake  at the point where the edge stopped rising. This is more pronounced in square or rectangular cakes because the corners are exposed to heat from two sides and set even faster than the edge of a round layer.  There are two easy solutions. You can buy insulated pans or special fabric strips to moisten and fasten around the outside of your pans to keep the edge of the cake from heating quickly.