The rock in your yard is probably from a gold dredge

If your shovel stops eight inches down on a bed of round, washed rock, you have probably met the tailings from a gold dredge.

A very short history

Gold dredging worked the American River floodplain around Folsom well into living memory. The Natomas No. 6, the last giant gold dredge operating in California, ran here until it shut down on 12 February 1962. Dredging of that era left long ridges of washed tailings across northern California floodplains, and parts of the Folsom and Rancho Cordova corridor still carry the material.

What it means with a shovel in your hand

Three things, mostly:

Digging is slower. An auger stops on rock, so planting holes get dug by hand. On a project with thirty shrubs and three trees that is a real line in the labour estimate, and it is one reason a quote produced from a site walk differs from one produced from a photograph.

Water leaves fast. Coarse washed material drains sharply and holds very little. Irrigation on this ground wants to be more frequent and shorter than on the heavy soils a couple of miles away, and emitters need placing where they will reach a root ball rather than a foot away where the water simply falls through.

Nutrients leave with it. There is not much to hold anything. Organic amendment and a generous mulch layer do more here than fertiliser does, because the problem is retention rather than supply.

The upside nobody mentions

Sharp drainage is exactly what a large part of the plant palette that works in this climate prefers. Manzanita, ceanothus, rosemary, and lavender all do better in fast ground than in the heavy foothill soil that holds water hard through February. Gardeners two streets away are fighting to get drainage into their beds. If you have cobble, you have it already.

What the ground does not suit is anything wanting consistent moisture, and it makes lawn expensive to keep. Which, given the watering schedule, is an argument the ground has already half won.

How to find out what you have

Dig a hole a foot deep. Look at what comes out. Then fill the hole with water, let it drain, fill it again, and time the second drain. Under an hour means fast ground. More than six hours means a drainage problem to solve before planting rather than after. The ground guide has the full version of the test and what each result implies.