An Inverted Landscape in the Sierra Nevada?

One of the most striking features in the Sierra Nevada is a sinuous set of flat-topped ridges in the Stanislaus River watershed. About 28 km (18 miles) long, these mesas rise above a generally flat and featureless plain. 

Table Mountain in the Stanislaus River watershed.

Table Mountain starting from the upper-left corner of the image, passing by New Melones Reservoir, and ending at the bottom-right corner.

The peculiar shape of these ridges is due to their geologic origin. About 10 million years ago, lava erupted from a vent near the crest of the Sierra Nevada and poured down a river bed. As the lava cooled, it assumed the shape of the river. Over time, the material surrounding the hardened lava eroded away, leaving behind the imprint of the river.

A report by J.D. Whitney, published in 1865, was the first to describe the formation of Table Mountain. In this report, Whitney included a cross-section of Table Mountain, at the Buckeye Tunnel Mine, that has since been reproduced in numerous publications and textbooks.

Whitney's geologic cross-section of the Table Mountain. The lava flow (the darkest layer) is capping older volcanic rocks which were deposited on the valley's bottom. The dotted lines represent the walls of the bedrock valley, as imagined by Whitney, that have since eroded away. With the erosion of the surrounding bedrock, the valley floor became a ridge.

Because removal of the valley walls adjacent to the lava flow would have required a significant amount of erosion, Whitney reasoned that this could only have been accomplished if the Sierra Nevada had undergone considerable uplift after the lava had coursed down the valley. Thus was born the theory of recent Sierra Nevada uplift (in the context of the Sierra Nevada, anything happening within the past 10 million years is considered 'recent').

This cartoon illustrates how the landscape is thought to have looked about 10 million years ago. The lava flow (Table Mountain Latite) has cooled along the bottom of a valley.

Sometime after the lava hardened, the surrounding landscape eroded away, leaving behind a ridge capped by volcanic deposits.

While this theory is attractive, like many others regarding the evolution of the Sierran landscape, a closer look at this one tells a more nuanced story. As I demonstrate in a recent paper published in the Geological Society of America Bulletin, there is no evidence that the lava flowed down a bedrock valley. Instead, the evidence indicates that the lava poured down a river that was flowing within large sediment deposits; for example, imagine lava flowing down the Sacramento River.

A cross-section across Table Mountain and the surrounding area. The lava flow capping the Table Mountain was deposited on older volcanic rocks and is higher than much older river sediment that was deposited on the valley floor. This means that the lava flow could not have been confined within a bedrock valley. The dashed line represents the height of the sediment that must have filled the valley when the lava flowed down 10 million years ago.

As shown in the figure above, the lava poured through a channel that was flowing across older sediment, not bedrock. This is a critical departure from Whitney's theory because sediment is much easier to erode than bedrock. Moreover, sediment can be eroded from a specific location simply by throttling the supply of sediment from upstream; this is known as the 'hungry water' effect and can be seen in the downcutting of rivers downstream of dams. Therefore, we don't need to appeal to uplift to explain the erosion of the materials that surrounded the lava flow 10 million years ago: simply reducing the supply of volcanic debris is sufficient. And we know that the supply of volcanic debris has declined because volcanoes are no longer active in this area. In addition, the presence of 40 million year river gravels resting on the floor of the valley in the figure above means that the bedrock topography of this landscape has not changed in a very long time. In other words, if you were to hop in a time-machine and visit this landscape 40 million years ago, it would look very similar to today's (of course, Table Mountain wouldn't be there).

Another surprising observation from this study is that Whitney's geological cross-section, which has been reproduced in many subsequent publications, is fictional! The actual topography is shown below.

In this figure, Whitney's cross-section is embedded in the actual topography at the Buckeye Tunnel Mine. Contrary to Whitney's drawing, the upper surface extends laterally a much greater distance and rises up to a high point. If the bedrock topography had truly been inverted since the lava flow, the top of Table Mountain would be the highest point in the landscape.

The implication that the original bedrock landscape in the Stanislaus River watershed has not been inverted is far-reaching. In the absence of any direct evidence for recent uplift of the Sierra Nevada, geologists have had to rely on indirect evidence, such as signs of significant bedrock erosion in the last few million years. For the Stanislaus River watershed, that evidence doesn't exist.