Detective work...

Walking along the trail I casually picked up a rounded pebble.  Every rock has a story to tell – this one started life as a sandy ocean beach – I knew that because all the grain sizes were uniformly small  like beach sand.  It has been compacted and cemented – then running through it lies a vein of white quartz … so it must have been heated to near melting, fusing the sand grains tightly together, it was cracked, and was injected with the liquid quartz; then permitted to cool.  Forces deep in the earth slowly uplifted the mass of rock to the surface, and fragments eroded into a flowing stream, where tumbling in the river, gave it its rounded shape…  Permitted to continue it would have been tumbled and eroded into sand fragments – and washed again into the sea…

My rock!

 So what can I tell about its age… for that I have to look at the surface geology maps for my area?  In my immediate area different rocks are found.  Our most recent rocks are a mere 2.5 million years old – and are deposits of broken down fragments from earlier rocks – so my rock probably belongs to this group.  But where was it originally formed? We have volcanic rocks that go back to 23 million years old… but my rock shows no sign of volcanic origin.  There are sedimentary rocks of the same time period –but they would be simple – no quartz vein.  We also have complex sedimentary rocks from up to 60 million years ago –but these still would not form the vein. But – bingo -The Franciscan formation sedimentary rocks was metamorphosed by heat and pressure… permitting the origin of the quartz vein – and it goes back to about 250 million years ago.  This pebble was here long before me and will continue its evolution of change long after I am… and for now I stand here and bounce it in my hand…

Surface geology map of Northern Fremont  - each color code denotes rocks of a different origin on the surface of the earth in those locations.

So how can we know its age? To understand how radioactive decay of minerals tells us absolute time – go here for a good explanation in simple language… http://www.kgs.ku.edu/Extension/geotopics/earth_age.html

Northern California is famous for its complex geology – Dominating it all is the vast tilt block of the Sierra Nevada mountains – 400 miles long and about 70 miles wide – the eastern edge is a great earthquake fault that has been tilting slowly upward – over 14000 ft, high in the case of Mt. Whitney. 

Sierra rock - with minerals melted into one mass - this has crystals of quartz, hornblend, and feldspar

The old metamorphic rock that once overlaid the Sierras has been mostly eroded away westward toward the Pacific – except at some locations in the highest mountains it still can be found. The coastal range, also about 400 miles in length, runs parallel to the Sierras – and its mountains are mostly in the 1000-2000 ft range of elevations.  Both of these ranges have been brought about by the continental movement westward caused by continental drift. 
Geologic History of California http://geologycafe.com/geologic_history/index.html
The great pressures and tensions of the drift have also caused the formation of volcanoes in Northern California – most famous today for Mt. Shasta, Lassen, and a whole strong of volcanoes expending into Oregon.  Mt. St. Helens is the most recent serious eruption in this chain.  There are extinct volcanoes in the Bay area that are “almost certainly” inactive.  The dominant mountain above Fremont – Mission Peak – has no volcanic connection (contrary to people who don’t know better) – the evidence is in the clam shell fossils found at all levels of the mountain… true evidence of its sedimentary origin. 

Metamorphic rock - that started as a layered mudstone - then was heated and fused

Fremont lies within the Coastal range zone and part of our region has the famous Franciscan formation of rocks evident on the surface. - These rocks - include very dark color volcanic rocks, some of which have been changed by heat, pressure and added chemicals, thick deposits of microscopic ocean organisms shells made of silicon, sandstones, limestones, shale, and metamorphic rocks… Most of the Franciscan is between 150-190 million years of age. After deposition, these rocks were broken up with earthquake faults, folded over and mixed chaotically!

So we have it all volcanic, sedimentary, high-pressure metamorphic rocks all mixed together with the power of earthquakes and continental drift westward.

this nodule shows the clear print of a fern

Many of our rocks do not have an extensive fossil record – due to the igneous or metamorphic nature of many rocks. Ocean fossils are found in many places… and most more recent mammal fossils (saber cats and dire wolves, etc.) … But there is no record of dinosaurs in northern California – due to the fact that at that time much of the area was below sea level.

Southern California has its own geological stories – some quite different that here in the north…

So many of us Californians live in the midst of a fascinating on going geological laboratory – and I’m not alone in picking up stones and puzzling over their origin.