Potting soil on glacial "clay"

I'm new to this site, which has kept me highly entertained for the past day.  I'm also new to Whidbey Island, just northwest of Seattle, where the soil in the garden of my new house has me perplexed.  It appears to be about a foot (or two feet where the beds are raised) of potting soil on top of a hard-packed white cement-like substance  that can be broken up with nothing short of a pick-axe.  The potting soil doesn't seem very fertile - the weeds growing in the raised beds are sparse, though sections of the garden are dense with quack grass.  The locals call the white substance "clay", but it's nothing like the clay back home in Eugene. 

I planted fava beans in the potting soil back in November, just days after planting the same seed in Eugene.  The Eugene fava beans are growing fine, but the seeds here never came up.  Shall I try again?  I have quite a number left from last season's collection.

Yesterday we sheet-mulched on top of quack grass as the grass was too thick to easily penetrate with a spade.  I've heard from a Eugene friend that sheet-mulching can actually assist quack grass in growing stronger, but it seems like I need to learn some things the hard way.  Intuitively it seemed like a good way to create a garden bed along the sunniest fence line.

Any suggestions for working with this situation to improve fertility, depth and tilth?

Thanks!

is the clay preventing drainage/waterlogging the dirt above it?

Your neck of the woods has a good soil survey (link to the island county survey to read more about your local patterns--use the interactive mapper to see how your property was mapped, but ground truth before jumping to conclusions).  http://www.or.nrcs.usda.gov/pnw_soil/wa_reports.html

Compacted till at 1-3 feet is common over much of Whidbey which is all glacial plateau.  Perched winter water table is common depending on local topography, with water collecting in pockets in the undulating glacial plateau before joining a creek that carves down to puget sound.

The loose stuff is what the glacier left behind... the hard stuff what was overrun by the ice sheet.  Depth to till can vary over short distances.

The garden appears to have good drainage; there's a hill to the west where the water flows.  We're in somewhat of a rain shadow from the Olympic Mountains, and average precipitation is 25 per year.  So compared to Eugene, it's arid around here in the wintertime.

    I read something about deserts having hard set bits of clay, I have found it again, look in google for "arid soils" to find out about the different and strange soils that you can find in arid places.
    The grass you want to get rid of would be the best way to better the clay or the plant that at the moment you know grows on it till you find others .
    The thing is to get the clay full of bits of vegetable roots for exaxmple. it will be hard to do anything with it very quickly except plant on top on raised beds.
    A mulch would keep the water, that did fall in the ground, so it did not evaporate.
       In the island of Lanzarote one of the islands of the Canarie isllands  whcih  is volcanic and the most desert like island, they have a way of farming that uses mulches and is traditional and works.
  They grow vines with less rainfall than you mentions with a mulch of volcanic sand and clay about sixty percent sand and some three or four percent finer bits of volcanic rcck and the rest pebbles of organic rock. The other trick of theirs is to construct semicircles of stones walls to protect each vine forom the drying winds. The volcanic mulch stops moisture loss to an enormouse degree. If i find the article i can mention the degree. agri rose macaskie.
   

I'm curious what the soil survey says! I'm curious what sorts of vegetation grow in abandoned land near you, too, especially heavily-eroded areas.

All that rock flour seems likely to become an amazing resource once there's an efficient way to mix organic matter and maybe stable, coarse particles into it. If the subsoil's natural pH isn't too high to begin with, biochar might be worth looking into as a way to make it seem sandier. I've used potsherds (crushed down below 1cm with an old tree trunk) to good effect on the heavy clay here.

    The list that is somewhere in thesse forums given by someone else has forage plants that seem to be ones that grow on dry soils and even salty soils in the east of America.
As they are dry soil plants they might help you Melanie rios. also it seems it is often the case that land with low rain fall has high salinity as there is not much water to wash salt through th esoil to rivers.
  I have put in a bit of information on them i think it makes them more interesting but i suggest you look them up yourselves. I don't like writting things i have just started reading up about i feel insecure about it.

     the curley leaved mountain mahogony . cercocarpus ledifolius also cercocarpus montana and intricatus. Liked by cattle and sheep, belongs to the east of USA, its leaves are full of proteins, giving the live stock all the protien it needs in winter when there arent to many protiens in grass and it stays green in winter.
     It has medium tolerance to hedging which would keep its leaves at the level of live stock.
     Its wood is very good fire wood and great barbacue wood or charcoal.
does  fix nitrogen in soil.
     As far as i can make out, though it adapts to different soils it likes lose ones and it does not much like clay soils.

the next two plants are from the goosefoot family, chenopodiaceae.
 forage kochia. It is not the same as anual kochia that is invasive, and is not liked by animals. Forage kochia on the other hand makes good forage, competes with cheat grass but is not invasive. Cheat grass so called because it gives forage for such a short time a year.
    If you plant forage kochia the old chaparral bushes start to reappear in the cheat grasssuch as wyoming big sage and thick spike wheat grass when the forage kochia has established.
     It comes from range lands in central eurasia.

    four winged salt bush atriplex chenopodiaceae native [] plant of the east of USA.
      It can live in salt soil and has a salty taste .
 Both its seeds and leaves eaten by native americaans and the seeds used for a drink called pinole.
        The ash was used by native americans as leavening in bread.
 It has bladders cotaining salt on the leaves that  explode covering the leaf with salt.
     Good forage with high protein levels all year round also a dry place forage eaten by all live stock, horses in winter.

 There were other plants on the list but i have lost the list . agri rose macaskie.