The Medicine Bow Range 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 hornfel 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.
