How to counter marine rush as a protoss on maps with huge ramps like Scrap Station?

Assuming that I scout well and find out that my opponent is going for mass marines early on (no gas, reactors), I still seem completely unable to counter marines when they come at me too early. My sentries die immediately, so Guardian Shield isn't up for really long. It's impossible to forcefield the ramp because it's too huge. I usually have not enough stalkers to hold.

I surely do something wrong, but I hope this not micro (I'm terrible at it).

(I hope this is not a duplicate, but I found no reply other than:

• FF your ramp: I'm talking about huge ramps like Scrap Station's
• Collossi : I'm talking about a marine rush, I'd have no way to rush this much to collosus.
• Guardian shield + stalkers is a good answer, but how do I keep my sentries alive? With zealots => less stalkers...

)

EDIT: some good looking answers were provided, but some of them collide. I'll have to try several of them but have not too much time to do so currently. My list looks like this:

• choking the base entrance
• more units from 2 or 3 gateways + cannons (not sure about the order...)
• more sentry-focused army and using FF to create chokes

EDIT: TESTS RESULTS

Ok, I had some time to make some tests this week-end, although not real in-game tests. I just used the UnitTest map.

• zealots alone are good enough in small numbers against a mineral equivalent number of marines, but only in open spaces (not a ramp).
• should you add a sentry (with guardian shield), your army is allowed to be smaller than your opponent's.
• stalkers need an arc and a guardian shield to be really effective without micro.

Anyhow, the good answer is really to just have stuff to defend, it seems.

Solution 1:

A bit of a long answer...

tharibo, the MAIN counter is to have A LARGE ENOUGH army. If he brings 10 marines but you have 2 sentries and 2 zealots, you will die no matter what.

If you detected mass marines - don't worry too much. I play mostly Terran (gold/platinum), and my experience tells me that mass marines is a cheese. If you detected that in time (while reactors are being built), then it's easy for you to get ahead in the early game.

Instead of telling you what to do, I will tell you what NOT to do:

1. Don't expand early (should be obvious, but it's still important to mention);
2. Don't be afraid to build assimilators (you will need gas for sentries/stalkers);
3. Don't build 4 gateways (no time and there is no real value in that);

Now, let's talk about what TO do:

1. Build a forge
2. Build 2 cannons on the choke point (not more, don't over-invest; you need to have a mobile army)
3. Get some zealots and put them close to the cannons. Once the marines began to shoot the cannons, your zealots will attack the marines. Often he will lose much more than 3 marines just to kill the 1st cannon
4. If you managed to build a few sentries, put a FF behind the marines so they won't be able to retreat and will die quickly.
5. Once you fight off your opponent, EXPAND! Don't harass your opponent.

Actually, this 'todo' list is very easy, and is simple to modify or extend. After getting 4-6 zealots, add stalkers, etc...

Don't forget to continue scouting your opponent. If you scouted 2 rax (1 tech lab, another reactored) and believe he is going for mass marines, your opponent can easily switch to hellions, etc...

Solution 2:

First you need to determine how all-in it is. If he is bring SCVs you should definitely pull probes yourself. A few probes to clog up the ramp can help anyway.

I would NOT get more sentries because they take a long time to build and you need as many real units as possible. Guardian Shield is so good against marines that you should try to get a few though. I usually get 1-3 sentries early so they have time to save energy, and if I scout mass marines than I switch to all zealot stalker. I like about 2-1 stalker-zealot ratio.

Make sure you have your zealots in front.

+1 armor is really good against marines. A cannon can also help if you think they are really going all out 1 base marine push.

Finally, stalkers can kite marines (i.e. they are faster and have more range). So if you see the marines coming send out your stalkers and shoot the marines and then run away as they approach you. If it gets desperate you can continue to do this within your base.