Rocks

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Across
  1. 3. Glaciers in the high country and running water on the plains carved the modern topography and left behind the large granite boulders for which the city is named.
  2. 5. The oldest rock in the area, dated at roughly 1.7 billion years. This salt-and-pepper colored igneous rock forms the core of the mountains and is visible at the summit of Flagstaff Mountain and in Boulder Canyon. These rocks formed as magma inside the earth slowly moved upward and cooled.
  3. 8. An early mountain-building event uplifted the region, and subsequent erosion created the Fountain Formation (Flatirons). These mountains were formed by tectonic stresses related to the assembly of the supercontinent Pangea, specifically the collision of the North American plate with South America/Africa.
  4. 9. Much of the city of Boulder itself sits on this soft, dark grey shale, which was deposited at the bottom of the Cretaceous Western Interior Seaway.
  5. 10. This hard, erosion-resistant sandstone forms the first major ridge east of the mountains (the Hogback) and was deposited along an ancient sea shoreline 100 million years ago.
Down
  1. 1. These reddish-brown rocks consist of arkosic sandstone and conglomerate deposited approximately 280-300 million years ago. They formed from debris eroded from the Ancestral Rockies.
  2. 2. A fine-grained, salmon-colored rock formed about 270 million years ago from desert sand dunes. It is famously used as the primary building stone for the University of Colorado Boulder,
  3. 4. The modern Rocky Mountains were uplifted. This event tilted the previously flat sedimentary layers (like the Flatirons) to their current angles, often exceeding 50 degrees. Caused by shallow-angle subduction of the Farallon Plate beneath the North American Plate, it produced inland deformation and uplift far from the plate boundary.
  4. 6. A shallow ocean covered the area, leaving behind layers of Pierre Shale and Dakota Sandstone.
  5. 7. SCR