By Darrel Dunn, Ph.D., PG, Geologist
Purpose and Scope of Geology Topics Web Page
The purpose of this web page is to present my comments on geology topics addressed in emails from viewers of this site. These comments are brief statements related to questions and comments in the viewer's emails. They are not comprehensive coverage of the topic.
When a house is located on compressible material such as sand and clay, the amount of settlement depends on the actual compressibility of the material beneath the footings, depth and width of the footings, and the elevation of the water table. If settlement is very small and the structure of the house resists deformation, no cracks or other indications of settlement are noticed. However, in some cases the settlement is noticeable and can produce cracks, tight doors, et cetera. Information on compressibility of geologic materials may be found on the web page titled Aquifer Storativity.
The following table contains my interpretation of the age of events related to the development of the Denver Basin in Colorado, USA, and the adjacent mountains.
Ma Event (Ma = Million Years before present)
0 Post-Laramide uplift continues (Tectonic and rebound due to erosion. Uplift > 5000 feet)
0.0093 Sudden cooling
0.011 End Wisconsin glaciation (Bull Lake, Pinedale, and Holocene Terraces in Denver Area)
0.1 Begin Wisconsin glaciation
Slocum Alluvium deposited (Sangoman/Illinoian)
0.25 End Illinoisian
0.4 Begin Illinoisian
0.64 Verdos gravel deposited (Age based on volcanic ash bed)
0.75 End Kansan glaciation
0.9 Begin Kansan glaciation
1.65 End Nebraskan glaciation
1.8 Begin Nebraskan glaciation
5 End Post-Laramide magmatism (Left Eocene erosional surface buried under flow and ejecta)
5.3 Beginning of Pliocene
7 Alluvial fans began to form along mountain front
10 Rate of exhumation increases
23 Beginning of Miocene
28 Begin Post-Laramide uplift, extension, and rifting
33.5 End of Eocene
36 Begin Post-Laramide magmatism, faulting, northward tilting of Denver Basin
36.7 Deposition of Castle Rock Conglomerate
37 Wall Mountain Tuff deposited
Rocky Mountain erosion surface developed (Low relief surface extends east onto deposits)
40 End of Laramide deformation and beginning of Oligocene
52 Beginning of D2 deposition
54.8 Beginning of Eocene
64 End of D1 deposition
65.5 Beginning of Paleocene
72 Beginning of Laramide deformation
75 End of Laramie Formation deposition (Laramie deposited by retreating Cretaceous sea)
510 Begin advance of Cambrian sea from west.
1100 Begin Pre-Cambrian erosional period.
Related information is contained in the web page titled Palmer Lake Geology.
The Palmer Divide in Colorado is the divide between the Arkansas River watershed to the south and the Platte River watershed to the north. It was formed while the two watersheds were developing. The relevant geologic history starts with the development of the Denver Basin. The Denver Basin began to develop at the same time the Rocky Mountains began to develop about 72 million years ago (Ma) near the end of Cretaceous time. The Denver Basin is a geologic structural basin that downwarped and received sediment carried eastward from the growing Rocky Mountains. This downwarping and sedimentation continued until about 52 Ma (early Eocene), by which time a great thickness of sediment was present in the basin. After that time, the basin ceased to subside, but more sediment was deposited as the mountains continued to erode. The sediment (gravel, sand, silt, and clay) in the basin was buried deeply enough to become lithified to conglomerate, sandstone, siltstone, and claystone. Major deposition ceased by about 34 Ma (end of Eocene), and the deposits began to be eroded by streams flowing from the Rocky Mountains. The courses of the streams evolved until by 5 Ma the ancestral Platte and Arkansas rivers had appeared with a divide in roughly the position of the Palmer Divide near the mountain front. Tributaries to the Platte and Arkansas rivers flowed from the divide to the rivers and eroded the landscape to its present configuration. This evolution of the divide is continuing today.
Spruce Mountain Geology
Spruce Mountain, a mesa located north of Palmer Lake, in Douglas County Colorado, exposes conglomerate and sandstone lithified from gravel and sand deposited over 35 million years ago by streams flowing from granite (Pikes Peak Granite) in mountains immediately west of the area. Some sandy claystone is interbedded with these course grained rocks. These rocks belong to the Dawson Formation. They correlate with similar rocks at Ben Lomand Mountain and Elephant Rock east of Palmer Lake. The sides of Spruce Mountain are covered by colluvial deposits that have washed and rolled downslope on the sides of the mountain. Pleistocene alluvium covers the lower areas around the mountain.