Throughout the geologic past the appearance and disappearance of seas on
the continents have been frequent events. In fact such changes are
slowly taking place even today. Hudson Bay and the Baltic and North Seas
are examples of shallow seas situated on the continents. The area around
Hudson Bay is rising; as attested by the fact that some of the fish
weirs constructed in water along the shore during the past several
hundred years are now a considerable distance inland. We know also that
our Atlantic coast has been subsiding for a number of years at an annual
rate of about 0.02 feet. To be sure, these movements are slow, but if
continued over a long period they might conceivably make some rather
profound changes, even as the birth and death of the Belt sea.
Within Glacier National Park the Belt series is divided on the basis of
lithologic differences into six distinct formations. Because each has a
characteristic color, these formations can easily be identified, often
from distances of several miles. Usually two, sometimes three or four,
of them comprise a single mountain, the oldest always at the mountain
base and the youngest on the summit, this being the relative position in
which they were deposited in the form of sediment.
[Illustration: MUD CRACKS ON A LAYER OF THE APPEKUNNY FORMATION
(PHOTO BY C. L. FENTON. OUR AMAZING EARTH. DOUBLEDAY AND CO.)]
[Illustration: RIPPLE MARKS ON A LAYER OF THE SHEPARD FORMATION NEAR
LOGAN PASS. THEIR ASSYMMETRICAL FORM INDICATES FORMATION BY CURRENTS IN
SHALLOW WATER.
(DYSON PHOTO)]