Point Source

Quaquaversal musings on the geosciences and public information.

AW#30: Hydrogeologically Correct Cake

It’s the Accretionary Wedge #30 Bake Sale! There have been some very creative posts, including a pilaf debris flow, coffee strandlines, stress and strain of spaghetti, and pillow lava bread.

Based on my recent criticism of a geologically incorrect cake — or, more specifically, hydrogeologically incorrect cake — I felt compelled to go forth in search of an edible aquifer.

The Study Area

At first view, the study area seemed simple and orderly:


Aerial view of the study area.

Like so many sites, the aerial view doesn’t always give a clear view of what is going on below.

The Glorious Road Cut

A nearby road cut shows that things are much more interesting — not simple like the pretty diagrams in the textbooks. (You didn’t know dinosaurs had roads, did you?!)


If we look carefully, we can identify three major zones:

Cross section

Cross section with major formations outlined

  • The upper zone is the unsaturated zone.
  • The middle zone is the saturated zone.
  • The lowest zone is bedrock.
Labeled cross section

Diagram indicates location of unsaturated and saturated zones in the overburden and folded bedrock.

Vadose Zone:

  • The unsaturated (vadose) zone appears to be a thin unit, with fairly consistent thickness.
  • The unit is continuous across the site.

Saturated Zone:

  • The saturated and unsaturated zones correspond with different geological units.
  • The upper boundary of the saturated zone is relatively flat.
  • The spaces between the sand and gravel (easily identified by the bright colors) are completely filled by the groundwater, which for some reason looks remarkably similar to chocolate ganache.
  • The distribution of the spaces (pores) between the gravel and sand appears to vary: in other words, the colorful solids and dark fluids are not evenly distributed through the layer. Therefore, we would expect there to be some variation in the permeability, which refers to the ease with which the fluid moves through the formation.

Finally, we inspect the bedrock.

  • The top of the bedrock is very uneven. This could be important if we are investigating the movement of chemicals denser than water that could pool up in the bedrock lows.
  • The bedrock “layers” are not flat.
  • Portions of the same formations are visible on both ends of the road cut but not in the middle. This suggests that the bedrock has been deformed some time in the past.
  • If we look at the angle of the striped formations in the bedrock, it appears that the beds have been folded. In this road cut we can’t see the complete fold, which appears to extend below the bottom of the road cut.
  • The folded layers seem to vanish suddenly at the surface of the bedrock. Could portions of the bedrock have been eroded, leaving behind an angular unconformity?

(**For the how-to on creating the folds, check out Magma Cum Laude’s great post on (edible) rheomorphic tuff.)

Further Analysis Required

Initial sampling has been conducted, but for health and safety reasons only minimal sampling can be conducted on any given day.

Of course, some of the outcrop may be open to different interpretation. Can you see additional or different features worth noting?


Hydrologically Incorrect Cake (and why I care)


National Groundwater Awareness Week

1 Comment

  1. Further research efforts shall have to be directed at differentiating valleys from dinosaur road cuts in deposits. And paleontologists should be on the alert for T-Rex bulldozers. LOL. This was a delight, thank you!

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