After leaving the Badlands, Brett, Tim and I headed west toward the Black Hills. As the Google Earth image above shows, the Black Hills are an isolated mountain range, roughly elliptical in shape with the long axis running northwest-southeast. The hills are rather sharply defined by a valley that runs around them, a portion of which can be seen in the map closeup below. Interstate 90 follows this valley, and most of the towns and cities in the region lie within it.
Rapid City, the largest city in the area, straddles the outer edge of this valley on the northeast side of the mountains. Driving west through town it’s obvious when you’ve reached the edge and started into the valley:
From the floor you can see that the valley is asymmetrical, with a steeper wall on the side away from the Black Hills. The rim on the steep side is usually capped with a resistant sandstone:
In contrast, the floor and lower part of the valley walls are composed of a finer-grained sediment, which is a distinctive bright red color:
If we leave Rapid City and follow I-90 northwest toward Sturgis, we see the same types of rocks:
At Spearfish, at the north end of the Black Hills, we can still see the same deposits:
And again in Sundance, Wyoming, at the northeast corner of the Black Hills region:
If we leave the interstate and head south, we can see these rocks in Newcastle, Wyoming, on the western edge of the Black Hills…
…and again in Hot Springs, South Dakota, at the southern tip of the mountains:
The valley surrounding the Black Hills is known as the Red Valley. If you look again at the map at the top of the page, these red rocks are clearly visible even in satellite images as a pinkish ring surrounding the Black Hills. The walls making up the outer edge of this ring-shaped valley are composed of Jurassic and Cretaceous sediments; it’s often called the Cretaceous Hogback. The red rocks in the floor of the valley are the Spearfish Formation, which is Triassic in age (with possibly some Permian at the base of the unit). The relatively soft and easily eroded Spearfish provides a reasonably flat path around the Black Hills, and its presence controls the locations of most of the towns and major roads in the area.
While Mesozoic sediments make up the outer wall of the Red Valley, I haven’t said anything about the inner wall. In Part 2, we’ll leave the valley and move toward the core of the Black Hills.