Thick cracked heels are caused by callus building up around the rim of the heel because of friction the foot slides in the shoe. As the callus first builds it protects the foot from the friction but as the layer gets thicker then the skin of the heel loses its elasticity and begins to crack instead of stretching. More callus is formed and can become very thick causing the heel to bleed and leave it vulnerable to infection.
Treatment is simply to remove as much of the overlying callus as possible then the use of a file to smooth the heel back into a heel shape blending the skin into the surrounding good, healthy, flexible skin that surrounds it.
Once the callus is removed then steps must be taken to stop the build of callus or at least slow it down. Moisturising will help keep the skin supple. Friction may cause the build up of hard skin but an underlying problem such as an infection may be causing an excessive or new build up and it is this cause that needs to be dealt with.
As we age then our skin becomes dryer as it produces less oil. The loss of oil means a losing of the flexibility of the skin leaving us more prone to dry cracked skin especially around the heel area.