British Geological Survey free smartphone apps:

iGeology, for iPhone/iPad, Android and Kindle Fire, gives you an interactive geological map of the UK. Download iGeology from iTunes or Google Play.

iGeology 3D, for Android, ‘paints’ a geological map on the ground around you. It can be used alone or as a companion product to iGeology. Download iGeology 3D from Google Play.

mySoil, for iPhone/iPad and Android devices, gives you an interactive map of the soils of Britain. Download mySoil from iTunes or Google Play.

Geopark Way apps:

Each of the 17 sections of the Geopark Way now has a dedicated app available for smart-phones and tablets. Each app has a map of the relevant section overlain on an interactive geology map. Using GPS you get your position and a feed of data about the geology and other information as you walk. Other information includes photos of the walk with directional guidance, 3D cross-sections, and interpretations of past environments. They can be downloaded from iTunes or Google Play.

If you are in London then try the free London Pavement Geology Mobile App.