Geology: The Columbia Basin is that large, triangular mountainous
region in southeastern British Columbia that is drained by the
Columbia and Kootenay rivers. It includes the Main Ranges of the
Rocky Mountains to the east, the Purcell Mountains, the Selkirk
Mountains and, to the west, the Monashee Mountains. The Columbia
Basin does not correspond to a coherent geologic region -- in fact,
it is crossed by a fundamental geologic boundary that separates
Laurentia, ancient North America, from exotic terranes that
collided with Laurentia during the Mesozoic.The strata making up
the Rocky Mountains are largely limestones and shales of Palaeozoic
age, deposited as a thick apron on the seaward side of Laurentia.
Even though these rocks are faulted and upthrust, they have not
been significantly altered or metamorphosed and their contained
fossils are generally well preserved. The rocks of the Purcell and
Selkirk mountains are mainly sandstones and grits of late
Cryptozoic (late Precambrian) age. These rocks are commonly highly
deformed and metamorphosed, and locally they are intruded by
younger granites. They were deposited on the margin of ancient
North America. The Monashees and related mountain ranges in the
western Columbia Basin are made up of a crazy quilt of rocks
including exoticterranes of Late Palaeozoic and Mesozoic volcanics
and sedimentary rocks, as well as large masses of granites.During
the Jurassic, Laurentia, with its thick packages of late Cryptozoic
and Palaeozoic strata, collided with large exotic terranes that had
formed out in the palaeo-Pacific Ocean during the late Palaeozoic.
This collision pushed the entire package of Palaeozoic rock
eastward in a series of overlapping faults to form the Rocky
Mountains. The late Cryptozoic rocks were squeezed and faulted up
to form the structures now seen in the Purcell and Selkirk
mountains. The colliding terranes on the west were shoved onto the
granitic basement, making the crust much thicker, altering and
heating it, and causing the melted rocks to be intruded as large
granitic masses. Lower Cambrian Trilobites
Brown and grey shales that make up the Lower Cambrian Eager
Formation in the vicinity of Cranbrook and Fort Steele contain
numerous fragments and many complete specimens of olenellid
trilobites. These trilobites are invariably the oldest trilobites
found at localities on all continents, but they are fully formed
trilobites and not obviously primitive. These fossils have been
avidly collected by amateurs and professionals since 1921, when
they were first made known to palaeontologists by Colonel C.H.
Pollen of Cranbrook.
The most abundant trilobite in the Eager Formation is Olenellus --
which has a large, semi-circular head, a body of 15 segments (of
which the third is much longer than the others), a long spine on
the 15th segment, and a minute tail. A less common olenellid
trilobite is Wanneria -- which has a large head lacking conspicuous
furrows, and a broad body without an expanded third segment.
The shales of the Eager Formation are widely exposed in the
valleys of the Kootenay and Columbia rivers but, by far, the most
fossiliferous site is located on Crown Lands leased to the
Cranbrook Rifle Club, located just outside Fort Steele. Abundant
collections from this site proved critical to the recently
completed Ph.D. thesis by Lisa Bohach, who was able to place them
in the upper part of the Lower Cambrian, but not the uppermost.
Bohach also used these rich collections to establish a new
classification of olenellid trilobites based on the different
growth history (ontogeny) of major groups.
The Rifle Range Site has recently been subjected to very intense
collecting pressure by both amateur fossil collectors and by
professional fossil dealers. A number of websites offering fossils
for sale include specimens of Olenellus and Wanneria from Crown
Lands leased by the Cranbrook Rifle Club.
In order to log this cache please include a picture of yourself or
your GPS at the cache site and email me the answer to the
following: How high do you estimate the shale deposit is at its
highest point?