A couple of quick comparison pics of Ruby, who arrived here 8 weeks ago. Although she had a reasonable palmar hoof her feet were flat and she'd had a recently bruised sole. She found stony ground a struggle and was clearly feeling her feet.
Today she is much more comfortable and her feet are beginning to build some good concavity. You can faintly see a ridge which extends from the bars round the sole. I've always found this is an indication of thin, weak soles and that as concavity and internal sole depth improves the ridge disappears, as this one is doing now.
Once the whole foot is stronger then this extra support is no longer needed and wears away on its own. As is so often the case, when it comes to hooves, the horse knows best!