LiveTopo

LiveTopo gives you full fidelity access to the worlds best topographic and geologic maps. Perfect for hikers, explorers, nature lovers, and anyone who wants to understand more of the world around them.
You can save your favorite locations and hiking tracks without a subscription. When hiking, get realtime metrics like pace, distance and altitude change.
With a small subscription (and a free trial), you get full access to offline maps for selected US public lands: National Parks & Monuments, National Forest lands, and even BLM lands. No limits on downloads (except how much your phone can store).
With version 2, you also get access to the worlds best geologic maps which can be downloaded and used while in the field. These maps get you detailed information about the composition and geologic history of the land around you.