Triangles: Taurus Virgo Capricorn

Earth triangle: material expression

Each of the four elemental triangles span all three crosses—Cardinal, Fixed and Mutable—the symbolism of which is crucial to an understanding of esoteric astrology. Each elemental triangle represents how the basic principles represented by each cross is reflected in human experience.

Alice Bailey’s own words express the full meaning of this triangle:

The earthly triplicity of Capricorn, Virgo and Taurus form a triangle of material expression … of profound interest … either from the angle of the ordinary round of the zodiac, followed by average and undeveloped humanity, or from the angle of the [consciously developing individual] wherein the path of zodiacal progress is reversed.

The three earth signs are related to each other in a peculiar way in connection with the… Earth. The relation… is… the meeting and fusion of the energies of these three signs upon the Earth and their effect upon the kingdoms in nature […] All these express desire as it merges into aspiration and in the process brings light and life [to the individual and humanity] (EA 259)

To paraphrase: the light of knowledge in Taurus gives place to the light of wisdom in Virgo, yielding finally to the light of initiation in Capricorn. All this… must take place upon “the radiant surface of the earth,” the plane of form…

Incentive behind evolution (Impulse). Desire for experience, for satisfaction. The Light of Knowledge
Incentive behind development (Goal). Desire for expression, spiritual desire. The Light of Wisdom
Incentive behind initiation (Service). Desire for liberation and to give back. The Light of Life

On the ordinary wheel

marks the point of greatest density and concrete expression and shows the [inner] life as deeply imbedded in substance. This is the true state of death as far as the life is concerned; it is captivity in form. Then in…
that life makes its inner pressure felt and the movement—faint yet real—of the hidden life begins to pulsate within the concrete form, producing in…
that reaction to desire and that onward rush and powerful movement which distinguishes the evolutionary progress of the individual, working under the impulse of desire. Forget not, that the first thrill or response of the [soul] is to the pull, urge or suggestion of the form nature wherein it finds itself.

Then later, when all the resources of the form nature (drawn out through desire), are exhausted and the [soul] is exceedingly strong and ready to reveal itself through the death of the Mother, the form, then and not till then is the progress of the wheel arrested and “revolution” takes place with the life aspect reversing itself upon the wheel.

On the reversed wheel

having transmuted desire into aspiration, [the individual] begins [the process of inner development]—objectively and in full consciousness—in the sign Taurus and “on the wings of aspiration” proceeds towards…
and ‘being both the Mother and the Child enters into the House of Labour.’ From that house, the [developing individual] in due time arrives at…
where … matter, form or concrete expression [is transparent to inner] uses and purposes, and thus demonstrates the triumph and the potency of [soul] life. (EA 379)

Also see the other elemental triplicities:

Esoteric Astrology: 379-80