Helena Area Geology

Helena lies within the Northern Rocky Mountains physiographic province. The Continental Divide, which separates the Columbia and Missouri River drainages, is located 15 miles west of the valley. Quaternary-age sediments (0 to 10,000 years old) up to 6,000 feet thick fill the valley and form a northeast-sloping alluvial plain measuring roughly 64 square miles. The sediments originated from erosion of the mountains surrounding the valley. Lake Helena, the lowest point within valley at 3,650 feet, overlies the thickest zone of these sediments. This sedimentary plain is bounded by broad erosional surfaces called pediments and alluvial fans of the Elkhorn Mountains, the Boulder batholith, the Scratchgravel Hills, and the Big Belt Mountains (including the Spokane Bench).

Boulder Batholith

One of the more interesting geologic features bordering the Helena Valley is the Boulder Batholith, an enormous mass of igneous rock (mostly granite) that intruded the earth's crust between Helena and the Highland Mountains south of Butte. The intrusion extends roughly 70 miles long by 25 miles across.

The occurrence of gold within the Boulder batholith rocks is rather unique. During the batholith's formation roughly 75 million years ago, heat from the molten magma caused the overlying groundwater to circulate and get hot enough - under extreme pressure - to dissolve gold, silver, copper, quartz, and other elements and minerals in the rocks. The circulation process in turn cooled the granite and the water and simultaneously precipitated metallic ore bodies in some of the older sedimentary and metamorphic rocks surrounding the granite, leaving behind the rich vein deposits. Of special note is the Butte portion of the Boulder batholith - one of the world's richest copper-producing zones.

The core of the Scratchgravels represents one of the earliest intrusions of the Boulder batholith at 85 million years old, as does the small knob underlying Carroll College.

Gates of the Mountains

 Just as Lewis and Clark noted over 200 years ago, it's pretty hard to miss the prominent outcrops of the Madison limestone on Upper Holter Lake. The limestone is Mississippian in age, about 320 million years old. The Mighty Mo began flowing across the Gates area roughly three million years ago (during Pliocene time) while the limestone was present but actually buried beneath other soft sedimentary formations. As the processes of erosion lowered the overall elevation of the surrounding topography, the overlying sediments were stripped away, exposing the resistant Madison limestone at the surface. The river carved a deep gorge for itself as its erosive force reduced the surrounding landscape.

Adel Mountains

Another favorite point of rock-head interest is the patch of rugged volcanic hills between Wolf Creek and Cascade. The Adel Mountains are made up of volcanic rubble and mudflow deposits from rain mixed into soft volcanic ashes. These deposits contain large chunks of a fairly unique rock type called shonkinite (named after town of Shonkin near the Highwood Mountains), formed from an alkalic, deep-crustal magma. The large black crystals among the purple-brown matrix are a mineral known as augite, indicating formation deep within the earth's molten mantle. The volcanic mudflows picked up larger rock debris as they flowed down the sides of ancient volcanoes, later hardening into solid rock.

The Adels are only about 50 million years old (Eocene). Montana probably had a relatively dry, semiarid climate during the last 50 million years, but since a more wet climate began over the last two to three million years - especially during glacial periods - the Adels have eroded significantly. Some of the spectacular Adel outcrops are accessible by floating the Missouri downstream from Craig, or along the recreation road between Wolf Creek and Cascade.

Big Belt Mountains

The broad crustal arch of the Big Belt Mountains on the east side of the Helena Valley exposes some of the oldest rocks in the region - Precambrian Belt Supergroup sedimentary rocks. The Big Belt rocks are mostly pale, grey mudstones of the Greyson formation and light-grey limestone of the Newland formation, which are moderately deformed and recrystallized. Patches of Tertiary-age sediments of the Renova formation exist on top of Deep Creek Pass east of Townsend, which indicates that these rocks were once under a valley floor as recently as 25 million years ago, and have been uplifted into mountains since then. That mountain-building episode is revealed in the Lombard thrust fault along the west side of the Big Belts, which marks the easternmost end of a series of linear, parallel faults known as the Northern Rocky Mountains overthrust belt.

Other Belt rocks (mostly Spokane shale) and lower Paleozoic rocks make up the Spokane Hills on the west side of Canyon Ferry. The ancient Belt rocks are also well-exposed upon entering the I-15 canyon west of Wolf Creek. The gently-dipping strata are easily recognized by their burgundy-red, pastel-green, and tan-colored shales that are many thousands of feet thick. In addition, the Belt-series rocks are well exposed throughout all of Northwest Montana, most prominently in Glacier National Park. The Belt rocks are about 1.4 billion years old, and contain no fossil evidence of hard-shelled organisms.

Elkhorn Mountains

South of the Helena Valley, folded and faulted Cretaceous sedimentary rocks, along with granite of the Boulder batholith, are covered up by green-grey andesites and lesser white rhyolites (the volcanic equivalents of granite) of the Elkhorn Mountains. These fine-grained volcanic rocks are 70 to 80 million years old and crystallized from the same magma that formed the Boulder batholith. The main difference is that the Elkhorn volcanics cooled rapidly upon eruption, whereas the Boulder batholith rocks cooled slowly well below the surface of the earth.

Volcanoes from the Elkhorns contributed the ash-fallout sediments that comprise the Two Medicine formation, which contains some of the spectacular dinosaur egg fossils in the badlands west of Choteau. The Elkhorn volcanoes also contributed ash to the Livingston Group volcaniclastic sediments which fill up much of the Crazy Mountains basin to the east. Ash fallout from the Elkhorn volcanoes has even been traced as far north as the Bears Paw Mountains southeast of Havre, and has been detected in core samples drilled deep within the Helena Valley.

Local Geology

One of the Country's largest city parks, Mount Helena, offers a few differences compared to surrounding geologic formations. It is made up of ancient 520 million-year-old Cambrian rocks including limestone, shale, and sandstone. As you descend from the top, you traverse across the following formations:

• Pilgrim Limestone: Light-medium grey limestone and dolomite with minor talc

• Park Shale: Dull greenish-grey shale in saddle between north cliffs and peak

• Meagher Limestone: Light-grey limestone forming the north cliffs

• Wolsey Shale: Pale green, grey, reddish-brown shale; exposed on north slopes and in Reeder's Alley

• Flathead Sandstone: Yellow/reddish-brown/white, cross-bedded sandstone and quartzite; outcrops near the Woolston reservoir

• Helena Dolomite: Pale tan-grey dolomite bedrock

Structurally, the layered rocks of Mount Helena form a syncline, which is basically a trough-shaped fold that in this case plunges down toward the southeast. At the base of Mount Helena, the now inactive Woolston Fault trends north-south under the reservoirs and truncates the east side of the syncline. Using a bit of imagination, you can get a sense of the layout of the rocks while viewing the mountain from the Spring Meadow Lake area. 

The South Hills area of town consists of thick Paleozoic sedimentary rocks, uplifted and tilted down to the south, exposing the Devonian-age Jefferson dolomite, Three Forks shale, and Mississippian Madison limestone atop Mount Ascension.

As you look north from town, you will notice the Sleeping Giant (Beartooth Mountain) keeping his ever-vigilant watch over the valley. The head of the Giant looks similar to the purple-brown shonkinite of the Adel volcanics, but is actually a Cretaceous rhyodacite plug. The prominent chest at 6,800 feet is composed of Precambrian Belt Ravalli formation rocks (quartzite, argillite, and shale) and Pritchard formation rocks (banded slate with interbedded sandstone). Much of the hard yet fissile black slate used as building and landscaping rock around downtown Helena is from Towhead Gulch that forms the gap between the legs and feet of the Giant.

Piecing the Puzzle Together

And now for the tricky part. The Helena Valley is a topographic and structural intermontane basin. Long after all the Precambrian, Paleozoic, and Mesozoic rocks were laid down, the Boulder batholith intruded south of Helena. Other small granitic stocks (up to 80 million years old) also intruded into the Helena dolomite to form resistant mounds such as the one underlying Carroll College. East-west extension then pulled apart the mountains that had formed during late Cretaceous time (65 million years ago) to begin forming the valley about 50 million years ago (Eocene time). As a result of this regional extension, the Precambrian Helena dolomite bedrock was dropped down along steep faults. During middle Miocene time (16 million years ago), renewed extension and basin subsidence continued to form the valley depression.

The Helena Valley fault at the base of North Hills caused the northeast corner of valley to subside. Seismic data indicates that the bedrock floor in this part of the Helena basin is now about 6,000 feet below the surface of Lake Helena. The bedrock floor slopes up and shallows to the southwest. The west side of valley is bounded by the Scratchgravel Hills fault, which runs parallel to Green Meadow Road. Most of the sediments in the valley are overlain by Miocene-age, lake-deposited ash and volcaniclastic sediments (as can be observed near the Spokane Hills).

Tectonic activity in the valley probably ceased about 5 million years ago (during Pliocene time), leaving behind broad erosional surfaces along the south, west, and north sides of the valley. Bedrock eroded along the mountain fronts, depositing huge wedges of coarse gravels along the toe of these pediments. Later, Quaternary streams (Pleistocene and Holocene age, less than 1.6 million years old) deposited channel-fill and alluvial-plain sediments. Small alpine glaciers from 150,000 to 10,000 years ago most likely occurred in the headwaters of Prickly Pear Creek (east of Jefferson City) and Tenmile Creek near Rimini, although no evidence of glaciers actually filling the Helena Valley has been noted. Glacial Lake Great Falls also left some fine-grained deposits in the Helena Valley after its retreat about 15,000 years ago. Since the last ice age, the mountains continue to erode while the valley slowly, slowly continues to fill.

Numerous sources used for this article provide more detailed information, and are available at local bookstores, libraries, and the USGS:
Exploring Mount Helena (geology section by Ray Breuninger, 1997)
Hydrogeology of the Helena Valley-Fill Aquifer System (Dave Briar & James Madison, USGS, 1992)
Profiles of Montana Geology (Montana Bureau of Mines and Geology SP89, 1984)
Roadside Geology of Montana (David Alt, 1986)