We will show you photographs of polished rocks that have been washed down into Flathead Valley from the park by the floodwaters of retreating glaciers, as well as their unpolished counterparts.
About 1.5 billion years ago, large amounts of sediments began to accumulate in most of the western third of the state, as well as adjacent parts of Idaho and British Columbia. This sedimentary accumulation continued for another 600 million years. These sediments are called the Belt formations because they were first studied in the Belt Mountains in central Montana. This formation contains an abundance of plant fossils but no animal fossils.
The Belt formation’s western edge extends along a line just west of Idaho’s western border. During this early period of Earth’s history, hot rock rose through the mantle, splitting the continent, and forcing the cover of Belt rocks apart, allowing the formation of a new ocean between them. Numerous diabase dikes and sills intruded in to the Belt sediments, providing evidence of continental rifting at that time.
The age of the sills has been estimated as .8 to 1.4 billion years, but .8 billion years is the most accepted date, which corresponds to the age of some of the Idaho and British Columbia formations that accumulated along a coastline. The old coast followed a line that starts in northeastern Washington and runs southwest through western Idaho and eastern Oregon in the Sierra Nevada Mountains of California. That line represents the western coast of North America until about 100 million years ago.
The location of this coastline in interesting in light of the series of earthquakes that have recently (November 2001) rocked Spokane, Washington, which was not known to have any major faults.
During most of the Paleozoic, large parts of Montana were shallow seas. The rest were low islands. There were no mountains at that time.
During the Mesozoic, most of Montana remained near sea level. The shorelines of these shallow inland seas shifted as the continental crust rose and sank, flooding all of the eastern two-thirds of the state and most of the western third. By the end of this period, the Rocky Mountains were forming, the continent was rising above sea level, and the inland sea was retreating from eastern Montana.
Shortly before the beginning of the Mesozoic, the lithospheric plates were massed together as a giant continent called Pangea. Later, a new continental rift split this continent. Evidence of this split can be seen in the mid-Atlantic ridge.
As the North American continental plate drifted westward it collided with the floor of the Pacific Ocean. The plate broke along the old western margin of the continent and formed a trench through eastern Washington and Oregon. About 175 million years ago, the floor of the Pacific Ocean began to slide through the new trench and into the mantle. However, most of the events, which formed the Rocky Mountains came later.
Molten basalt and hot steam rose above the oceanic crust and torched the lower part of the continent. This heat melted the ancient basement rock, which allowed the magma to rise.
These small granitic masses cut into the upper part of the continental crust in what is now Idaho. Then, between 90 – 70 million years ago, enormous volumes of magma rose into the upper crust in a northeastern line beginning east of the old coastline in California, and travelling through Idaho and Montana into British Columbia. Those masses formed a series of batholiths.
During this time, the Boulder batholith was also forming between Butte and Helena, Montana. The Boulder batholith is in the wrong place, however. Instead of forming deep in the crust, some of the magma erupted to form the Elkhorn Mountains.
Also during this period, the earth’s surface, which had already been raised by thrust faulting, rose higher as heat caused the expansion of buried rock. As the bulge rose higher, it became unstable. Some of the sedimentary layers broke off in giant slabs. In other places, large blocks of the upper continental crust broke off and traveled eastward.
The Sapphire block is an enormous slab about 10 miles thick, which broke off the Idaho batholith and moved eastward into Montana. It is now found in the Garnet, Flint Creek, Anaconda-Pintlar mountain ranges. As it moved eastward, its leading edge crumpled and pushed the rocks ahead of it into tightly folded outlying ranges. Granite magma the Idaho batholith eventually penetrated the rocks of the Garnet, Flint Creek, and Anaconda-Pintlar ranges. The gap behind the Sapphire block became the Bitterroot Valley.
After 20 million years of this intense mountain building activity, Montana saw 20 million years of relative quiet. At this time, the west coast of North America was where the Okanogan Highlands of Washington is now. Then, 50 million years ago, another small continent collided with North America. This ancient continent is now the Northern Cascades of Washington and the coastal ranges of British Columbia.
50 million-year-old volcanic rocks cover a vast amount of the Absaroka and Gallatin Ranges as well as parts of northern Wyoming. Volcanoes erupted in central Montana and intense volcanic activity occurred around the Boulder batholith over much of south central Idaho. Several large dikes from this time can be found, beginning in south central Idaho and ranging northeast into Southwest Montana.
Central and eastern Montana contains numerous faults and crustal folds, arches and troughs that buckled into the rocks underneath the plains. This region rose several feet above sea level at the same time. About 55 million years ago, the shallow sea had retreated into North Dakota and Saskatchewan.
In “Roadside Geology of Montana”(See note at the bottom), the authors say, “Remember that the continental crust is, in effect, a raft of light rocks floating on the heavier rocks of the earth’s mantle. If small boys find their log raft beginning to sink, they shove more logs underneath to make it thicker, so it will gloat higher. Similarly, if a compressional force shoves the continent, that will thicken the crust by telescoping the rocks together, thus making the continental raft float higher. If the continental crust under Montana was under compression 50 million years ago, that could have thickened the crust and might also have wrinkled the mountain ranges into the plains of central Montana. If such a compressional force were directed from the southwest, it could also have opened the fractures that filled with magma to become the dike swarm of southwestern Montana. If so, then the crust wrinkled to form the mountains of central Montana at the same time central and eastern Montana rose high above sea level and magma filled fractures to form the dikes of southwest Montana.“
Approximately 40 million years ago, Montana’s climate became very dry and stayed dry throughout the Oligocene and early Miocene. This climatic change is evidenced in the sedimentary rocks, which show very thin layers of sediments, due to the lack of water to carry the stream loads.
Due to the lack of ground cover in very dry climates, soil erodes very rapidly, but the streams are too weak to carry away the sediments. Thus, large quantities of sediment accumulated in the broad valleys and plains of Eastern Montana.
These deposits include an assortment of gravel, sand, mud, volcanic ash, limestone, and coal, which is called the Renova Formation in Western Montana, and the White River Oligocene beds in Eastern Montana. The Renova Formation is easily recognized by its tan and gray sands and silts, which are fairly soft and crumble easily.
There is a great amount of ash in the Renova Formation, which cannot be accounted for by the volcanic activity of the time. The ash is not the same type found in the local eruptions. It is possible that this ash came from the violent eruptions of the West Cascades which consist of rhyolite, a light colored, acidic volcanic rock which is only found in violent eruptions.
The Renova Formation contains limestone beds, which were laid down in shallow lakes and coal from peat, which formed in marshes. These lakes and surrounding marshes formed because the lack of water did not allow sufficient drainage of the landscape.
The Renova Formation is also filled with beautiful petrified woods, and fossils of leaves can be found in the coal seams.
Landslides are common when this formation absorbs large amounts of water. The ash breaks down into a very slippery clay. Several towns in Western Montana are built upon benches, which lay upon the Renova Formation.
About 20 million years ago, Montana changed again and became a lush tropical environment. Many places in Montana contain a layer of red laterites, which contain aluminum or iron ores. These laterites can only form in tropical climates. These laterites are found on top of the Renova Formation and are sandwiched between black basaltic lava flows. Similar flows are found on the Columbia Plateau which contain fossil leaves from hardwood trees similar to those found today in Florida and the Caribbean. This tropical period lasted for about 10 million years.
In “Roadside Geology of Montana”, the authors write, “When the dry period of Renova time ended and heavy tropical rains began to fall, streams began to flow again through Montana. Some were large rivers, which flowed through hilly landscapes in Eastern Montana and green mountains cloaked in tropical and subtropical hardwoods in Western Montana. Parts of those valleys survive in the modern landscape in Western Montana. Miocene valleys filled with younger gravel deposits probably exist in Eastern Montana also.”
When this tropical period ended a new dry period began. This was much more arid than the previous one. Montana became much like Death Valley is now and continued as a desert until the ice ages which began about 2.5 million years ago.
The streams dried up and Western Montana was filled with undrained desert basins. An enormous desert plain formed east of the mountains and desert streams spread a thick layer of gravel over a vast area. These sediments are called the Six-Mile Creek Formation in Western Montana and the Flaxville Formation east of the mountains. Both contain coarse gravel which indicate a very arid desert with little ground cover. There is no evidence of volcanic activity during this period. The little rain at this time would flood down the normally dry streambeds, carrying heavy loads of gravel and filling the landscape as the streams dried up. This gravel extends from the Rocky Mountains to central North Dakota. The thickness of the gravel varies from a few inches to several hundred feet.
These gravels contain little petrified wood and no fossil leaves, but the bones and teeth of mammals of all sizes can be found. Among these fossils are found the remains of horses and camels that would have thrived in the desert climate. Because desert soils contain large amounts of nutrients, the sparse desert vegetation is much more nutritious than its counterparts in wetter climates.
The youngest fossils found in these gravels are the remains of animals that lived as the Pliocene ended and the Pleistocene began. Age dates of volcanic ash suggest that the Pleistocene began about 2.5 million years ago.
Large portions of the old Pliocene desert survive almost untouched throughout the eastern two-thirds of the state. They can be seen as flat uplands between streams that are covered by a layer of Flaxville graven and are now filled with wheat fields of the High Plains.
About 2.5 million years ago. Modern streams began to flow in Western Montana. These streams followed old river valleys that survived from the Miocene. Where the modern streams are too small to fill the old river valleys, the old valley remnants can be seen as bedrock benches along the sides of the valley. In many places, movement along faults occurred which raised the Miocene valleys too high, causing newer streams to bypass them. These old Miocene valleys can still be seen today along with their tributary valleys. However, streams no longer flow in them.
As desert sediments filled Western Montana, they buried the older landscape on the valley floor. Modern streams occasionally uncover previously buried hills.
With the coming of the ice ages, Montana’s terrain changed immensely. Roadside Geology of Montana states, “ No one really knows what ice age climates were really like. Most estimates place the drop in mean annual temperature during the last ice age in the range of a few degrees – noticeable, but nothing our wardrobes couldn’t handle.”
“If a new ice age were to start, the immediate effect upon Montana would probably be an increase in snowfall and rainfall accompanied by stormier and cooler weather. Almost every crop now grown in Montana could flourish in wetter and cooler weather, so the change in climate might well increase the agricultural productivity of the state. A new ice age would have to continue for thousands of years before the glaciers could grow large enough to cover any of the places where many people live.”
“Like any carving tool, glaciers make a distinctive cut. Glaciers straighten mountain valleys and deeply gouge them into something approximating a semi-circular profile. The heads of those valleys end in deep hollows, called cirques, that look from a distance as though some mythical giant wielding an ice cream scoop had gouged a helping of rock out of the top of the peak.”
“Where several glaciers gouge their cirques into the top of a mountain, they reduce the peak into a gnarled pinnacle of rock called a horn. Heavy ice age glaciation converted the higher ranges of western and central Montana into serrated rows of craggy horn peaks, each one dropping off into cirques that lead downward into deeply gouged valleys. Many of those glaciated valleys are so straight that you can stand in their lower reaches and look directly at the distant peaks that spawned the glacier. River valleys never provide such long views.”
As mountain and continental glaciers move south, they eventually reach an area where the warmer climate melts the ice as fast as it moves forward. This marks the end of the glacier. The ice thins and deposits its load of sediment (till) which is a mixture of all sizes of sediment. Till can be recognized as an unsorted jumble in contrast to the easily recognized layers of stream deposits. Sometimes large boulders (called erratics) are deposited with the till.
This deposit of till is called a moraine. The different morains record the progress of the glacier, where till was deposited along the sides of the glacier as well as its end. These morains can be seen all over western Montana.
If you are interested in more information on the geology of Montana, I suggest you read Roadside Geology of Montana, by David Alt and Donald Hyndman, published by Press Publishing Company, l986.