While under motor in an anchoring basin this weekend, we were rudely interrupted by an unmarked, uncharted 4' deep sand shoal. I was traveling at about 4knots in 15 feet of water, suddenly the boat stopped, and the bow pitched down into the water. This was followed by my shallow water alarm coming on. I was able to back off the shoal fairly easily. I checked the keel bolts and they were not loose. No water was coming into the bilge. I dove under the boat, but the water was to silty to surmise anything more than the keel was still there and didn't feel loose (like I could move it if it was). Sailing in the SF bay I have gained experience with soft grounding, but I have never felt the boat pitch like this. Would people consider this a hard grounding? Do I need to pull the boat and inspect it? We sailed rest of the weekend in 15-20knots and the boat felt normal. Thanks for your help.