machineKey attributes in machine.config & web.config. Are they combined or overriden?

I have a machine.config with this configuration:

<system.web>
     <machineKey compatibilityMode="Framework20SP2" validationKey="123" decryptionKey="456" validation="SHA1" decryption="3DES"/> 

And a web.config with this attribute

 <system.web>
     <machineKey decryption="SHA1"/> 

Does the final configuration do a combination of both?

<machineKey compatibilityMode="Framework20SP2" validationKey="123" decryptionKey="456" validation="SHA1" decryption="SHA1"/> 

or if I have redefined machineKey it will get overriden completely?

 <machineKey decryption="SHA1"/>