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.
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.