Altar
The first altar mentioned in the Word was the one built by Noah after he came out of the ark, after being saved from the great flood. On that altar, he sacrificed clean animals to the Lord.
Mountains represent the Lord because of their height; we need to raise our thoughts above worldly things when "talking" with the Lord. An altar is a small artificial mountain. When it's used in worship, it can call to mind this raising of thought. The fire and smoke that rise from an altar are symbolically being sent to the Lord.
Most altars were made from unhewn stones. Stones represent truths. Unhewn stones - ones that have not been shaped by men - represent truths from the Word, truths that have not been adulterated.
The clean beasts to be sacrificed represent good things, charitable acts done because they are right. The clean birds represent thoughts about doctrine and actions, and about what is right. Presenting these things is an acknowledgment that we have them from the Lord, and a giving thanks to Him for them.
In the Israelitish Tabernacle, the altar of burnt offering represented the acknowledgment of good and the altar of incense that of truth. For this reason this larger altar, which was outside by the door, was made of brass which signifies natural good, while the altar of incense was made of gold, which signifies love to the Lord from whom comes truth.
Joshua 3:11-17
11
Behold, the ark of the covenant of the Lord of all the earth passes over before you into the Jordan.
12
Now therefore take twelve men out of the tribes of Israel, for every tribe a man.
13
It shall come to pass, when the soles of the feet of the priests who bear the ark of Yahweh, the Lord of all the earth, rest in the waters of the Jordan, that the waters of the Jordan will be cut off, even the waters that come down from above; and they shall stand in one heap."
14
It happened, when the people moved from their tents to pass over the Jordan, the priests who bore the ark of the covenant being before the people,
15
and when those who bore the ark had come to the Jordan, and the feet of the priests who bore the ark had dipped in the edge of the water (for the Jordan overflows all its banks all the time of harvest),
16
that the waters which came down from above stood, and rose up in one heap, a great way off, at Adam, the city that is beside Zarethan; and those that went down toward the sea of the Arabah, even the Salt sea, were wholly cut off. Then the people passed over right against Jericho.
17
The priests who bore the ark of the covenant of Yahweh stood firm on dry ground in the middle of the Jordan; and all Israel passed over on dry ground, until all the nation had passed completely over the Jordan.
