Franciscan Sandstone Bedrock and the rock wall I built.

From the lake to our house is a gradual climb from sea level to about 150 feet. Once you get to the 1600 block of Trestle Glen you start to see outcroppings of Franciscan Sandstone. According to map on Wikipedia: By Mikes Clark - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=26589719

Our house would be in the green colored strip. In the photo below you can see a picture of a sandstone outcropping near our house.

Here is another picture for scale of an outcropping under a neighbors deck. 

Rock like this was quarried in Piedmont before he city incorporated in 1907. When I started to dig in the back yard I found some sandstone peeking out. Little did I know at the time that there was a huge outcropping that had been covered up in the 1950's. 

There are two creeks that converge and lead to Trestle Glen but get subverted around Valant in Piedmont. Yesterday it rained and I was able to get a picture of the amazing waterfalls falling over the bedrock around where St. James intersects LaSalle. 

A lady I spoke to said back in the day all the neighbors burned their trash. I guess that was before the Oakland Hills fire and global warming. It made sense though because I found so much trash buried in the dirt. There was a cinder block incinerator in the backyard. I thought this has to go. 

Once I took a sledgehammer to it and removed the wall I knew there was no going back. Curiously there was all these rocks behind the wall. Then one very big rock. As I took more dirt away the rock revealed itself. 

Here is the lone aloe that watched over me the whole time.

Soon my lone Aloe was joined by a few Agave Blue glow and Americana 

Building up the wall with reinforced concrete veneered in sandstone. Unfortunately, the sandstone is too brittle and the wall is too high to dry stack the stones. I made sure to allow lots of escape routes for the water so the wall would stay strong for years to come. 

Building up the wall by the stairs leading up to the garden with all the rocks I dug out from the dirt.

I tried to mimic what was there before. 

Several months had passed by now. I could only spend a few hours on the weekend.

It seemed like I was making progress. 

All the while, I was finding plants on craigslist so you will see as the wall progressed the plants began to fill in the voids. Seems like the deer didn't care much for the Aloe or Agave, but they ate everything else. I made the mistake of planting Agapanthus and they came down every day to eat it. What a nightmare. 

At this point I decided I would extend the wall over and terrace the length of the garden. I began to excavate the hill and eventually moved 10,000 pounds of dirt down three flights of stairs in bags of 40 pounds or less. What was I thinking?
 

Then one day as I was watering two curious deep hole popped up in one of the boulders. What were these? They were deep and strange. Were they made by human hands or an animal or just a feature of the rock? I looked these up and formed two theories, but your guess is as good as mine. Look at the picture below near the green tipped handle of the shovel.

Bedrock mortar sites are also called "milling stations" where local Indians ground up acorns, which were one of the main items of their diet. The mortars they left behind are actually man-made holes, which were ground and pounded into rock slabs and boulders using stone tools called pestles. These are much like the stone mortar bowls and pestles of well-stocked kitchens. Acorns are quite nutritious, but also horribly bitter, so much so that they must be first processed to eat. This is done by grinding the acorn meat into a mash, then pouring water through it many times to leach out the tannins that make it bitter.

You find these all over the East Bay. Perhaps the two holes in our back yard are evidence of the bedrock being thrust up by geologic activity and being underwater at some point. We see these holes near beaches as well. Here is a picture of some bedrock near beaches with the same holes:

According to Don Garlick " these holes are bored by marine “Pholad” clams, chiefly for protection from predators. Holes enlarge with depth because each is excavated by a single growing clam during its lifetime of several years. The clams possess specialized muscles with which to rub and rotate their shells against the rock. 

Could it be that they drilled holes to stick dynamite? That would seem not plausible to me as our house is built on foundation that mimics the grade of the hill.