# Santa Rosa Plateau Topography

## Lifetime of the Santa Rosa Plateau

In the mountain regions of Southern California, it is very rare to find a mesa1. In contrast, on the Colorado Plateau mesas are practically all that are seen if you are not deep inside a gorge!

Even the mesas of the Santa Rosa Plateau are just about gone. We can estimate the lifetime of the Plateau in 2 ways:

1. We can use the present linear size of the Plateaus and divide that by the present-day erosion rate of the edge of the mesas to get a lifetime.

If the average erosion rate of the edge of the mesas in the Santa Rosa Plateau is 1"/year, which is probably within a factor of 10 of the correct rate, the Santa Rosa Plateau will disappear entirely in{(0.53 mi/(1"/year)} = 34,000 years. (0.53 mi represents the farthest distance that any point on the Plateau is from an edge of that Plateau.) Allowing a factor of 10 uncertainty in the erosion rate gives remaining lifetimes ranging from 3,400 to 340,000 years.

2. We can also take the present-day surface area of basalt and divide that by an estimate of the original extent of the basalt to get the percentage of the original basalt remaining. We can convert that into a time by assuming that the uplift that created the present erosional environment happened half its age ago, or 4 million years.
So assume that the basalt once covered at least a circular area with a diameter equal to the present distance between the mesas that are farthest apart. That distance is about 10 miles, giving at least an original surface area of pi times (5 miles)2 = 75 square miles. The current surface area is around 6 square miles. Hence no more than 6/75 of the basalt is still around. Thus in 6/75 of 4 million years, or 320,000 years, the Plateau will be gone. This is an upper limit for two reasons. First, the basalt could have covered a much wider area. Second, as erosion of a circular area proceeds, it accelerates, since at first only the outer radius is involved, but at later times erosion is coming from many different edges as some areas erode more quickly than others. Hence the upper limit to the lifetime is 320,000 years, in quite good agreement with the upper limit derived in the first approach.

It's always reassuring when two completely different calculations give roughly the same answer. The Santa Rosa Plateau will be gone in no more than another 340,000 years, which represents the last 4% of its lifetime. And there won't be any mesas left in Southern California mountains after that, to my knowledge.

The table below gives estimates of the lifetime of each mesa from the erosion-rate calculation, using a size for each mesa which is the maximum distance of any point in its interior from an edge, a parameter called Min{size} and an assumed erosion rate of 1"/year. The lifetime is given in kyr = 1,000 years. Remember that the lifetimes could be a factor of 10 longer, but no more than that.

The Nature Conservancy may be mortified to learn the expected lifetime of the Plateau, since no matter how well they preserve the area from people, defending it from the forces of nature is another matter indeed! Although the Plateau and the Vernal Pools will be gone, it will still be a beautiful area, but the entire area will look like the area around the Visitor Center without the mesas.

## Altitude of the Mesas

Although it appears from a distance that the Santa Rosa Plateau is flat-topped, the mesas are actually each at a different altitude, and that altitude generally increases to the west. This is different from the topography of the rest of the Santa Anas, which reach their highest elevations at the eastern end of the range. The difference is probably due to several causes:

1. Possible smaller uplift of the Santa Anas at the southern end, as the uplift shifted more to the block on the other side of the Elsinore Fault, uplifting the Palomar Range.

2. The extensive dissection that the southern end of the Santa Anas has already experienced. In particular, the Santa Margarita River and De Luz Creek are doing a pretty good job of eating away the southern end of the Santa Anas.

Average elevations:

The Mesas of the Santa Rosa Plateau From East to West, Their Altitudes, Lifetimes and Reference Topo Maps
MesaMedian altitude (')Min{size} (mi)Lifetime (kyr)USGS topo map
Burro19800.5032Murrietta
Punta19600.1510Wildomar and Murrietta
Avenaloca25000.106Fallbrook
Redonda28000.053Fallbrook
Miller29000.010.6Margarita Peak or Sitton Peak

I don't have the quad topo for Miller Mountain, but the Geological Map of California shows that Miller still has a thin coat of basalt over a small area near the peak. From the name "Miller Mountain" and topography as shown on the 1:250,000 map, it looks like there is very little "mesa" left there. The size in the table above comes from Ted St. John, who writes:

It is indeed capped with basalt and the typical "Murrieta stoney clay loam." There are essentially two levels to the mesa, neither very well developed. The lower level is larger and most of it is out of sight from Redonda. You can walk up fairly easy trails to get there- no need to scramble up a layer of basalt. It is more or less level, but the sides slope away. It has MSCL for soil, derived from the local olivene basalt.

The upper layer is pretty much as you guessed- maybe 50 to 100 feet wide (I do not exactly remember, and the way it eroded makes part of it ambiguous) and a couple hundred feet long.

Note that Ted reports that although the top of Miller is public property, access to Miller seems to be only through private property, so is not open to the general public easily.

Even though to a first approximation each mesa seems flat, one quickly discovers by hiking that you can feel the second approximation. Mesa de Burro is highest at its northwestern end. Mesa de la Punta and Mesa de Colorado are highest at their southwestern ends. Avenaloca Mesa is highest at its southeastern end. Redonda is highest at its northwestern end.

## Note

1 Chris Jensen tells me that East Mesa in the Cuyamacas is the closest thing to a mesa in the rest of Southern California. Jerry Schad comments that "East Mesa isn't a mesa in the usual sense, but is a gently inclined bench of broad grasslands interrupted by promontories". (from Afoot and Afield in San Diego County) I can't recall any others. Please email me if you know of another. Coastal or flatland mesas don't count.

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