The Mehrten Formation is a Miocene river deposit consisting of alternating layers of conglomerate and lahar. Between 10 and 5 million years ago the Sierra Nevada mountains were still in the early stage of uplift. By this time, the processes of weathering and erosion had removed much of the country rock overlying the Sierran “granite” exposing much of the intrusive igneous rocks while leaving small amounts of the metamorphic roof pendant material. Across the surface, streams were busy carrying away the eroded material. Somewhere to the east, near the crest of the new mountain range, volcanoes were occasionally erupting supplying mostly andesite and some basalt to the river, which it would transport as sediment. A number of times these volcanoes were the root cause of volcanic mudflows, called ‘lahars’. When modern lahars occur, these mudflows look and behave much like flowing concrete. They can travel at speeds of up to 60 mph, especially on steeper slopes. They damage or destroy most structures in their path by impact or burial. Their paths are strongly controlled by topography; therefore, areas of low topography (river valleys, plains) close to the volcano are at the highest risk of being affected by these flows.
As you drive up Indian Hill Road from the interstate you pass through metamorphic rocks that have been eroded. If you carefully examine the road cuts near the crest of the hill you will notice that the metamorphics are overlain by large rounded river cobbles of the Mehrten Formation. Keep driving up the hill and notice the pink rock containing large angular clasts (pieces of other rock), that is the lahar. Park near the sidewalk and walk back to ground zero, notice the fence posts are barely in the rock. The lahar is very hard and resistant to erosion and water penetration. Because of this, these former river valleys now stand as ridges. The granite is more easily weathered and eroded; this is called inverse topography. You can see just how much erosion has occurred in the last 5 million years. In this region of northern California, the Mehrten Formation forms long sinuous ridges paralleling I-80 on both sides, noticeable from here all the way down to Rocklin. Before developments invaded the area, the tops of these lahars were the homes of countless of unique species of flowers and animals that lived in vernal pools generated every year by spring rains.
I have a goal for 2014 of hiding caches on days when the month and date match. I hid one cache on 1/1, two caches on 2/2, and three caches on 3/3. With luck and hard work I can keep it up all year. This is one of my eight August hides.
Remember, have fun and write a nice log.