Mission Gorge EarthCache EarthCache

Stay on existing, designated trails at all times, and do not climb on the dam.  Park in the Old Mission Dam Historic Site lot and take the wheelchair accessible trail at the western / far end of the lot.  To observe this geological EarthCache site you need to go about 700 feet along the designated trail. 

Take time to read the educational signs along the way, particularly the two, 3-D exhibits.   “The San Diego River” (N 32° 50.375” W -117° 02.537”) is about 400 feet distance on the trail, and “The Old Mission Dam” (N 32° 50.374” W -117° 02.545”) is about another 300 feet along the trail.  Here you can see how geological activity has benefitted life in early San Diego history.  There is no container for EarthCaches, follow directions below to log your ‘find’ via email and the geocaching website. Gates are locked at 7PM.

A geomorphologist, William Davis, had a theory that every stream tried to carve its channel down to an imaginary surface called a “base level”.  According to this theory, how the land looked would change over time as the water cut channels into the underlying bedrock and created canyons and then valleys.  Early on the canyons would be V-shaped and have high walls; there would be turbulent water, rapids and waterfalls.  As downward cutting erosion continued, more rocks would fall and expose more surface area for weathering.  Eventually the canyons would widen into valleys as erosion shifted to expending more lateral energy.  Loose material from weathering activities (called regolith) would be deposited on the stream and valley floors.  Do you see evidence that supports this theory?  It may depend upon what season it is when you visit.

The San Diego River is a geological agent, part of earth’s “exterior” drainage system (because its waters end up going into the ocean).  Rivers and streams are responsible for the erosional sculpting of the land; they pick up the particles generated in “weathering” (when rocks disintegrate into cobbles, then pebbles, then sand, then silt, then clay), move them downstream and drop them when the effect of gravity is greater than the effect of the water’s “turbulent” flow.  When the next flood happens the larger particles are picked up and moved downstream again, getting smaller in the process.  Silt- and clay-sized particles are carried more or less continuously in the “laminar” stream flow.

The San Diego River originates in the Cuyamaca Mountains where it is fed by rain and ground water.  Ground water comes from aquifers, rocks that have sufficient porosity (the percentage of a rock or unconsolidated material represented by void space) and permeability (the ability of a rock or unconsolidated material to transmit a fluid to yield water easily).  Flowing water is released from the aquifer and enters the river from below the channel.  When you see moving water it means the water table has risen above a “topographic low”, or the ground level.  The topographic low is effected by seasonal changes and land features (it’s closer to the surface in a valley than on a hill).

What makes this site worthy of being an EarthCache?  Well the geological process shaped this area into something that greatly impacted the ability to sustain life in this area.  In the 1700’s the native people used the natural formations to store water, but storage was significantly impacted by seasonal changes.   The dam and flume were built between 1813 – 1816 by Indian laborers and Franciscan Missionaries to divert waters of the San Diego River a distance of 6 miles to provide a more reliable source of water for Mission San Diego de Alcala.  The reservoir behind the dam was the length of three football fields and was lined with clay tiles made from deposits of weathering particles recovered from the river bed, specifically clay.  It is Registered Historic Landmark. 

Can you see areas here in the gorge that are examples of weathering (where rocks either disintegrate or decompose), exfoliation (where concentric layers of a rock are removed from the surface of the rock), and erosion (where the products of weathering are picked up and carried away)?  Please do post photos of these examples. 

In order to log this cache, send me an email with the answer to the following questions.  Do not include your answers or photos of the 3-D exhibits when you post your log or it will be deleted. 

1. What type of geological feature are you in today (Hint:  title)?
2. What was the geological process or agent that created this feature? 
3.  What type of rock did the geological process cut thorough to create the geological feature? 
4.  What local materials were used to construct the dam? 

Reference material:  The Nature of Earth:  An Introduction to Geology, Professor John Renton, West Virginia University

Mission Trails Regional Park informational trail signs