Hi Kate, you didn't say whether you are slashing the top of your loaves. If you don't slash the top, the expanding gasses will find the weakest part of your loaf and escape there, causing the cracking. By purposely slashing your loaves you'll determine where the gases escape. Are you preheating your cast iron skillet? Are you preheating your oven and stone to 475°? If your breads are not reaching the suggested temperature, you'll need to bake them longer. 

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