By Rita Fontanaro

Abstract:

The principle is to use an existing reference feature, in this case an ancient lake bed preserved by a basalt cap, then attempt to restore the ancient topography associated with the reference feature by filling in the landscape that has been eroded away around the reference feature. One can also note how that landscape might have connected to existing mountain features on the west.

Here are links to glossaries and a time scale that may be helpful when dealing with unfamiliar terms. Please note that geomorphology deals with the surface features of a locality and how they formed.

Landform Glossary

Geologic Glossary

Geologic Time Scale

The Current Geomorphological Situation

Aerial Photo of Golden Area

This is an aerial photograph of the Golden, Colorado area. North Table Mountain is in the center north of the prominent highway that is visible running roughly east and west.

North Table Mountain and its sister to the south, South Table Mountain, are mesas (flat-topped mountains with steep sides) capped by early Tertiary extrusive basalt formations. This material has formed a caprock (rock that is relatively resistant to erosion and therefore typically "caps" underlying rock layers) that has protected the underlying Cretaceous-Tertiary (K-T) strata.

Topographic Map of Golden Area

This is a topographic map of the same region to the same scale. The mesas show considerable relief. "Relief" means they elevated above their immediate surroundings.

They were created when lava extruded into a lake in at least two distinct episodes. The lake was surrounded by a deciduous forest similar to what is encountered in the Southeastern U.S. today.

This is deduced from leaf fossils found in the strata underlying the base of the capstone (caprock) formation. The entire region was very near sea level with a climate not unlike the South of today.

The Hypothetical Ancient Geomorphological Situation

Hypothetical Lake

This is a hypothetical reconstruction of the geomorphological features of this lake and its environs. This reconstruction is based on taking a planar intersection of the foothills on the west and the tops of the mesas. The white area should be visualized as similar to the prairie surface existing to the east today. The white area also represents the planar surface mentioned above that intersects the foothills and mesa tops.

It should be clear that the relief was very much less than it is now.

The ancestral Clear Creek is depicted by the solid aqua line. A creek to the north is presumed to have existed as well at that time. The black dashed lines in the illustration above indicate the hypothetical shoreline. The dashed aqua lines indicate hypothetical stream courses. Because those courses and the shoreline have long since eroded away, they must necessarily remain hypothetical. The solid black lines depict the current elevation line of the mesas with its projection on the foothills. In the case the mesas it also depicts the current rims of those features.

It is possible the lake was larger than the hypothetical map indicates, but it was probably not any smaller. One might suspect the present mesa rims outside of the gap where Clear Creek flows represent a pretty good approximation of the ancient shoreline because an abundance of leaf fossils was found in a condition suggesting they were deposited near the shore. These fossils were recovered from steep slopes just under the caprock.

A seep spring (underground water seeping out of exposed rocks layers) was also observed in one of the fossil recovery locations suggesting that the muds associated with the lakebed still form a sealed layer that forces underground water to migrate horizontally under the capstone formation. One might suspect the source of this water is rain that falls on top of the mesas and percolates down through fissures in the basalt.

Incidently, springs also exist on the top of North Table Mountain near the north rim. They are associated with an ancient indian tool processing site dating from approximately 2000 years ago. The mesa would have afforded not only water but natural fortifications for these people.

Topographic Overlay

This is an overlay of the topographic map onto the hypothetical map to give some clarity.

It should be understood that the relief to the west is hypothetical at best since there has been something on the order of four miles of vertical movement west of the Golden Fault since Cretaceous times.

That fault can be roughly visualized as following the line of the major highway (Colo. 93) depicted as a bold red line running roughly northwest to southwest in the middle of the city of Golden. Where it intersects the road coming out of Golden Gate Canyon, the fault should be following the northwest-southwest strike or trend already established.

The entire area of the town would have been submerged in the debris of the Laramie Orogeny (mountain building episode that uplifted the current Rocky Mountains). That debris has since eroded away due mostly to a regional uplift that has occurred within the last 10-15 million years.

The foothills to the west may be presumed to have eroded at least this much and considerably more due to the aforementioned vertical movement along the fault. However, they still may resemble, above the elevation of the present mesas, the ancestral topography at the time the lake existed.

Conclusion

This is a convenient way of visualizing ancient topography. While not rigorously accurate, it is close enough to give a general impression of what the ancient geomorphological structure or landscape looked like. It also gives a rather graphic impression of how much material has been removed due to mass wasting and erosional processes, in this case primarily the result of the action of Clear Creek and its associated drainage.

Clear Creek is a perennial stream on the average about 12-15 meters across and less than a half a meter deep. It is not exceptional as far as Front Range streams go in either scale or gradient. It feeds into the South Platte River farther east and represents the primary source of drinking water for the city of Golden. Golden is located approximately 16 miles west of Denver, Colorado.

If the reader has any questions or comments, please let the author know. It is hoped this summary will be of interest and will help the reader visualize an example of some very important geological processes of which many people are unaware. Reader comments or critiques are certainly welcome and actively solicited.

--Rita

Sources

Map and Photo Source: MSN Terraserver

Other Sources:

The author's own fossil collecting expeditions to both South and North Table Mountains and the author's own geomorphological surveys of the surrounding area in July and August, 2001.

The Geology of the Golden Area, Colorado, J. Harlan Johnson, Colorado School of Mines Quarterly, Vol. XXV No.3, July, 1930.

Golden Pioneer Museum North Table Mountain Archeological Collection. 923 1Oth Street, Golden, CO 80401.