Geology Education and a Scenic Drive in Central Montana

View of square butte

Locals call the area the Big Sag or the Shonkin Sag and it really is a geologic wonder.

On MT Highway 80 between Fort Benton and Stanford, this paved highway takes you through a variety of scenery. It’s wheat country when you leave Fort Benton but as you continue southeast things start to change – the result of a geologic event that happened about 65 million years ago.

Magma traveled between layers of sandstone along shores of what was once a vast inland sea. In the Big Sag area that magma bulged up to form a huge blister (technically a laccolith). The laccolith solidified and formed the flat-topped butte now known as square butte, visible for many miles.

The Shonkin Sag formed when a continental ice sheet spread south from Canada and blocked the Missouri River, backing it up to form Glacial Lake Great Falls. That glacial meltwater moved eastward

Sunset over square butte seen from Hwy 87

through the Highwood Mountains and eroded the landscape to create a dip or sag in it. We call that the Shonkin Sag. If you chat with a geologist they will tell you the Shonkin Sag is one of the most notable meltwater channels in the world.

When you drive this route there are several pullouts for scenic views and we recommend stopping at each one – they are definitely photo-worthy! Small towns sit quietly by the highway that borders the rail line and you’ll see cattle ranches as you travel the route.

The town of Geraldine has preserved their Milwaukee Depot and also remnants of history related to early day rail travel. Geraldine is the largest community in this stretch of road and you’ll find a small grocery store, restaurant, bar, coffee shop and a couple of B&B rooms. Upland game bird hunting (and bird

Geraldine’s Milwaukee Depot

watching) are favorite visitor pastimes. About five miles out of Geraldine is Kingsbury Lake, a national waterfowl production area with grassland birds, raptors and, during migration, shorebirds.

There is a town named Shonkin and only the former school, now a community center, remains. However, it is well worth the drive when it isn’t super muddy. The road is dirt with some gravel. It’s fun to take the Shonkin Road one way, then bring the paved highway back. You’ll see some amazing contrasts in the landscape.

The little town of Square Butte has preserved its old jail, made from stone quarried nearby. The jail is visible from the highway and if you are in town you’ll see a sign that says “Historic Jail 3 blocks”. There aren’t actually blocks in this town but you’ll find the jail!

Between the towns of Geraldine, Square Butte and Shonkin, laccoliths, an ever-present butte and that

Historic Square Butte jail

unique sag, all prominent geologic features dominate the view. Enjoy the scenery as you travel along these rural communities and take time to learn about the landscapes.

If you travel by GPS you’ll find interpretation about the area at Latitide:47,6804, Longitude: 110.3447.

Enjoy your scenic drive as it becomes an outdoor classroom.

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