Geologic Formations

The most famous feature of the park, Devils Tower, is a geologic oddity of epic proportions. We share the story of the Tower's formation on our website.

Sedimentary Rocks

Long before molten rock pushed up to form the Tower, other rocks were forming from different origins. Sand and silt left behind by ancient rivers and shallow seas were buried and compacted. Gypsum deposits formed as water evaporated. Creatures big and small - from dinosaurs to clams - left their marks. Today, the stunning cliffs of red and yellow siltstone and sandstone, interbedded with gray shale or limestone and white gypsum, created a multi-colored landscape dominated by rolling fields and dark green forests.

 

The red cliffs seen in the park are part of a 500 mile ring of red rock that encircles the Black Hills region. This view from the Red Beds Trail explains the name of our 2.8-mile circuit hike.

NPS / Chris Racay

Hiking the Red Beds and Joyner Ridge Trails is the best way to see these other geologic formations. Red Beds is so-named for the Spearfish Formation, comprised of the oldest visible rocks in the park. The red and maroon cliffs along the east and southeast face of the Tower are only part of a sedimentary layer visible around the entire Black Hills region. Lakotas refer to this feature as the "race track," and the 500-mile circumference of rock is part of an oral history detailing a great race between Earth's creatures.

The formation dates to the Triassic period, 225 to 195 million years ago. During this time, a vast area of present-day North America was covered in water. Deposits of sand, silt, and clay mixed with iron-rich minerals, giving the rock its conspicuous red color we see today.

The seas which deposited these sediments ebbed and returned throughout the Jurassic period, 195 to 136 million years ago. This led to substantial deposits of gypsum; evaporating water left this white mineral behind, sometimes even crystallizing in small geometric patterns. White streaks are common towards the top of the red rock layer, evidence of the gypsum deposits.

 

On the connector trail between Joyner Ridge and Red Beds, one can see the gray shale and distinctive yellow sandstone layered above the older, red Spearfish Formation.

NPS / Joe Bruce

The gray rocks above the Gypsum Springs Formation are the oldest part of the Sundance Formation. The Stockade Beaver Member, comprised mainly of shale formed from marine clay, is interbedded with fossiliferous limestone and mudstone. Remains of ancient sea life, such as oysters, clams, mollusks, crinoids, and more, can be found in the fossils present within these rocks. More recent deposits of fine-grained sands resulted in the yellow Hulett Sandstone at the top of the Sundance Formation. The Hulett Sandstone Member is resistant to weathering, creating the beautiful outcrops seen along the tops of ridge lines in the park and around the northwestern Black Hills.

 

The symmetrical ripples visible in this yellow sandstone are evidence of the beach environment which deposited the ancients sands. Today, its exposure to wind and water create a flowing appearance reminiscent of the ocean which helped form the rock.

NPS / Chris Racay

 

The boulder field provides a fun and challenging experience for visitors wanting to scramble over the giant rocks.

NPS photo

Boulder Field

At the base of the Tower, one will see the evidence of the changes it has undergone throughout its geologic history. Massive rocks, some large as a bus, form a 13-acre field. Predominantly around the west and south faces of the Tower, this field of giant rocks was created as pieces of the Tower weathered off and eroded down.

The processes of weathering and erosion are often confused. Weathering is when rock breaks apart; erosion is when rocks move. Forces such as wind and water can contribute to both processes, but the mechanisms are different. The Tower is full of natural cracks created by columnar jointing. If water seeps into these cracks and freezes, the expanding ice can force the rock apart. Over time, this process of frost wedging can cause rock to split and break apart. Plants contribute in a similar wedging process, which occurs as their roots grow down into cracks in the rock. Wind weathering is more visible in the softer sedimentary rocks below the Tower, but affects the boulders from the Tower by smoothing and rounding them over time.

The west and south faces of the Tower are the most exposed to sunlight and storm systems. These forces also contribute to the weathering and erosion process. The sun's heat results in thermal expansion, causing the rocks to shift or crack over time. Rain, hail, and lightning all have gradual impacts to the Tower. Once the rocks have broken off, or weathered, from the Tower, erosion does the work of moving the giant boulders. Gravity is the primary force at work, pulling down loose rock. They may break again when impacting things or rolling, and today pieces of the Tower can be found over a quarter mile from the formation.

 

This boulder near the parking lot, about 6 feet wide and 8 feet long, is mostly buried by soil. It likely fell thousands of years ago (or longer). Despite its long exposure to the elements, the hexagonal column shape is still visible.

NPS / Joe Bruce

How Often Do Rocks Fall?

The short answer to this frequently asked question is "we don't know." Small pieces, baseball or basketball size, fall regularly. Chunks the size of a microwave fall rarely. Yet never in recorded history has anyone seen a massive column fall. Many pieces are covered in soil and plants, indicating they have been in that location for a very long time. Lichens, slow-growing organisms that live on the rock's surface, cover many of the boulders, again an indication of the time that has passed since these rocks fell.