Earth Science Guy: Shonkin Sag Laccolith

Do you think Charlie knew?
Fifty million years before Charlie Russell painted scenes of central Montana, a volcano dominated the landscape east of Great Falls - The Highwood Mountains are the eroded remains of this ancient volcano. In fact many of the buttes and mountains that form the backdrop in Russell's paintings were formed by ancient volcanic activity. Case in point - In his painting "When the Land Belonged to God", three landforms associated with ancient volcanism appear on the horizon to the right of the bison - Square Butte, Round Butte, and the Highwoods. The chamber that provided the magma for these features is also responsible for the dark cliff shown in the photos on this page. Pressure from magma beneath volcanoes can cause surrounding bedrock to crack, and then magma moves into the cracks or between layers of sedimentary rock, hardening beneath the surface. If a magma-filled fissure cuts across layers of sedimentary rock, the formation is called a dike. If magma forces its way between layers of rock, it is called a sill. If enough moves in to cause layers above to dome up, the formation is referred to as a laccolith (diagram). Square Butte and Round Butte are both laccoliths, and so is the cliff shown above. Stay tuned for more about this famous cut bank later. (geology map of the area).

Does anyone feel a chill?
Fast-forward to 2.6 million years ago (mya) . . . The Highwood Volcano had gone extinct, and erosion had taken it's toll on the cone(s) and the sedimentary layers that the dikes and laccoliths were embedded in, exposing some as ridges and buttes. Also, by around 2.6 mya, the planet had cooled enough for another ice age to get started. From 2.6 mya until 12,000 years ago, Earth experienced several glaciations (popularly called "ice ages"). Over tens of thousands of years, great ice sheets grew southward from Canada into the northern Montana (map). Then those glaciers would melt, replaced by warmer "interglacial periods" that would last for thousands of years before next glacial period started. Over the past 1 million years these glacial periods occurred roughly once every 100,000 years. The most recent one ended 12,000 years ago, marking the end of the Pleistocene epoch.

Here comes that dam glacier!
The formation and flooding of Glacial Lake Missoula (multiple times) is one of the most famous events that occurred during the most recent glaciation. However, a lesser-known incident that shaped the cliff shown in the photos on this page was also quite dramatic. Here is one hypothesis** . . . The strange set pf circumstances began when the Laurentide Ice Sheet grew southward from Canada, reaching the north slopes of the Highwood Mountains. As the ice sheet pinched against the Highwoods, it blocked the flow of the ancient Missouri River, causing the formation of Glacial Lake Great Falls (map). At times the lake was about 500 feet deep where the courthouse sits in Great Falls today, and lake water backed up as far south as the Helena Valley.

The big spill.
The lake eventually grew high enough to spill over the dam near the present-day town of Highwood. At some point the ice dam burst, and water rushed along the southern edge of the ice sheet (along the north side of Highwoods) across existing drainages, carving the impressive channel known today as the Shonkin Sag. Eventually the ice grew back, the lake refilled, and the dam burst again - probably numerous times. Today the wide, deep, dry channel with scattered brackish lakes winds its way between the small towns of Highwood and Square Butte (photo and map).

Not so fast!
Gerald Davidson, a retired research scientist who spent years studying the area, agrees that outburst floods are part of the story, but thinks the area's unique landscape was shaped by a more complicated sequence of events that happened over a longer span of time. According to Davidson, "For the past million years the Earth has spent most of the time locked in ice; so there has been sufficient time for a river to gradually cut a channel around the ice - a channel that could have been scoured and enlarged by several violent overflow events."

Above: Photo by Mark Smith - a closer look at the cliffs of the Shonkin Sag Laccolith

That's some cut bank!
For geologists who visit this remote area to see the Shonkin Sag, the cliff shown in the photos on this page is a "must see". Here, two very different events, separated by tens of millions of years - a period of volcanism and an ice age, teamed up to provide this unique cut bank. This is where ice age torrents of water in the Shonkin Sag cut into the Shonkin Sag Laccolith, exposing the spectacular cross-section represented by the cliff. Such views of plutonic formations are rare - something normally only seen in textbook diagrams. In the photo above, the thick dark part toward the left is eastern edge of the laccolith, and the dark stripes on the right are sills formed as magma from the laccolith squeezed between layers of sandstone (diagram). The photo below, which was taken from the top of Square Butte (4-5 miles away), provides a much different perspective. The laccolith is about 200 feet thick and a mile long. Click here for a close-up of the sills.

Below: Cliffs of the same Shonkin Sag Laccolith - photo taken from top of Square Butte