The Medicine Bow Range that borders the east side of the park is the result of highly localized movements of the earth's crust as the entire region was thrust upward. Deposition in a huge geosyncline uplifted the Rocky Mountains at the close of the Mesozoic era. Thrust faulting, in which one end of the earth's crust is pushed over the other, profoundly affected State Forest State Park. More than one fault was involved, thus, slices between the faults were exposed. One such slice is the Nokhu Crags. The Nokhu Crags, originally shale but now hornfels, are a hard and resistant metamorphic rock. The shale metamorphosed when magma erupted from below, heating and hardening the shale into hornfels.
Volcanoes erupted and covered the area with lava flows, remnants of which can be seen on Iron Mountain. The lava flows were eroded, exposing the fault slices previously buried. This erosion exposed the near-vertical hornfels layers that formed the Nokhu Crags. A large area of granitic rocks was formed east and south of the crags. Next came glaciers, which carved out the topography we see today. Glaciers formed on the sides of the highest areas and carved out the cirques in which the high mountain lakes are located.
Also of interest, geologically, is the East Sand Dunes Natural Area, the only undisturbed, cold-climate dune in Colorado. This unique site contains one of the two active cold-climate dunes in the state, the other being North Sand Dunes located eight miles north of East Sand Dunes. North Sand Dunes, unlike East Sand Dunes, is open to recreational use by OHV's, and due to resulting disturbance was deemed ineligible for registry as a Colorado Natural Area.
The dunes in North Park differ substantially in structure from Great Sand Dunes National Monument because they contain sedimentary structures unique to moist, cold-climate dunes. The dunes formed when strong, steady winds gathered sand from eroded peaks and carried the sand across North Park. When the winds hit the Medicine Bow Mountains to the east, it lost speed and dumped the sand at North Park's edge, forming dunes. Both East Sand Dunes and North Sand Dunes are part of a larger, predominantly dormant, dune system approximately 25-square miles in size.
The forest climate is typical of the high Rocky Mountain valleys, with an abundance of sunshine, low relative humidity, low precipitation, and wide daily and seasonal temperature variations. Temperature on the Colorado State Forest becomes colder and annual precipitation increases along with the elevation. Precipitation varies across the forest and can average more than 100 inches annually.