A quick web search suggests chalk is alkaline but others here will know better. The suspicious side of my character makes me wonder if someone buried some old drywall or plaster debris or something else down there when grading or backfilling.

Here is a link that might be useful: BBC on garden design and chalk

Oh goodness, anything goes! It was 1948 and there were still shortages from the war. (That's why our houses don't have eaves. Not that you asked ;))

Thanks for the link.

There is only one way to know for sure and that is to have a good, reliable soil test done to see what your soils pH is. Chalk is a form of limestone which could lead on top believe their soil is alkaline, but that may only be in that one area.

Could be just the soil composition in your area. Mine's like that pretty close to the surface. Do some research on the web about your local soil structure.....there is lots of research.

Here is a link that might be useful: Here's some soil distribution maps with soil types

1 could just be a natural layer there in the soil
2 could be caliche, which is a sub soil layer that develops in arid soils. The occasional rains push native mineral salts down into that layer and when the soil drys it makes a hard layer that will be white-ish in color or the color or a light late coffee.
3 could be the C horizon of a well developed soil. Since the soil there has not been disturbed for a long time, it would have developed good "horizons" These will develop in all soils over time if the soil is not disturbed.
Since you describe the layer above it as friable (that is a great word), I'm thinking that is the case here and the friable layer would be the B horizon.
There may have also been a thin layer right on top, anywhere from a quarter inch to as much as 4 inches but usually not more than one inch unless you mulch. That would be the A horizon. The A horizon is where organic mater does it's best work. But constant raking of leaves and such can destroy and rake away the A horizon. A good mulch or just leaving the leaves that fall are actually very good for the plant and developing that A horizon is a big part of why.
Dig down in any forest that isn't just rock, and you will find these layers. (or in my orchard or garden)
Also if this third idea is the case, then your parent soil is the white soil and the upper layers are effected by water, air and organic material which give them their color.
Here is a site that has WAY more information than you probably ever wanted to know, but it has a good picture of the horizons to get an idea of what I'm saying.
http://www.google.com/imgres?imgurl=http://www.soilsensor.com/images/soilhorizons.jpg&imgrefurl=http://www.soilsensor.com/soiltypes.aspx&h=305&w=250&sz=66&tbnid=TIp5Vdf8p28J::&tbnh=116&tbnw=95&prev=/images%3Fq%3Dsoil%2Bhorizon&sa=X&oi=image_result&resnum=2&ct=image&cd=1
You don't need to buy the sensor. You have already seen with your own eyes since you dug down there and that's way better than a sensor.
If you really want to go crazy (ok I'll admit I'm one of those crazy guys) here is another site about soils
http://websoilsurvey.nrcs.usda.gov/app/