Differential weathering on Hilbre Island

Hilbre Island is one of a line of three odd lumps of sandstone in the middle of the Dee Estuary on The Wirral, between Liverpool and North Wales. I was fascinated by the way that some layers had weathered more than others. The layers are very thin. I wonder what long length of time they represent. They are a nice desert sandstone from when we were as far north as the Sahara is now. Wigton is made of similar sandstone. But look at the layers in the top photograph. Why have some resisted weathering more than others? There was no obvious change in composition. Is it that the "glue" holding the grains together was stronger at that time? I'm guessing that it will be some type of salt holding it but I've now got something to go away and look up.